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XMODEM/YMODEM PROTOCOL REFERENCE
A compendium of documents describing the
XMODEM and YMODEM
File Transfer Protocols
This document was formatted 10-14-88.
Edited by Chuck Forsberg
This file may be redistributed without restriction
provided the text is not altered.
Please distribute as widely as possible.
Questions to Chuck Forsberg
Omen Technology Inc
The High Reliability Software
17505-V Sauvie Island Road
Portland Oregon 97231
VOICE: 503-621-3406 :VOICE
TeleGodzilla BBS: 503-621-3746 Speed 19200(Telebit PEP),2400,1200,300
CompuServe: 70007,2304
GEnie: CAF
UUCP: ...!tektronix!reed!omen!caf
- 2 -
1. TOWER OF BABEL
A "YMODEM Tower of Babel" has descended on the microcomputing community
bringing with it confusion, frustration, bloated phone bills, and wasted
man hours. Sadly, I (Chuck Forsberg) am partly to blame for this mess.
As author of the early 1980s batch and 1k XMODEM extensions, I assumed
readers of earlier versions of this document would implement as much of
the YMODEM protocol as their programming skills and computing environments
would permit. This proved a rather naive assumption as programmers
motivated by competitive pressure implemented as little of YMODEM as
possible. Some have taken whatever parts of YMODEM that appealed to them,
applied them to MODEM7 Batch, Telink, XMODEM or whatever, and called the
result YMODEM.
Jeff Garbers (Crosstalk package development director) said it all: "With
protocols in the public domain, anyone who wants to dink around with them
can go ahead." [1]
Documents containing altered examples derived from YMODEM.DOC have added
to the confusion. In one instance, some self styled rewriter of history
altered the heading in YMODEM.DOC's Figure 1 from "1024 byte Packets" to
"YMODEM/CRC File Transfer Protocol". None of the XMODEM and YMODEM
examples shown in that document were correct.
To put an end to this confusion, we must make "perfectly clear" what
YMODEM stands for, as Ward Christensen defined it in his 1985 coining of
the term.
To the majority of you who read, understood, and respected Ward's
definition of YMODEM, I apologize for the inconvenience.
1.1 Definitions
ARC ARC is a program that compresses one or more files into an archive
and extracts files from such archives.
XMODEM refers to the file transfer etiquette introduced by Ward
Christensen's 1977 MODEM.ASM program. The name XMODEM comes from
Keith Petersen's XMODEM.ASM program, an adaptation of MODEM.ASM
for Remote CP/M (RCPM) systems. It's also called the MODEM or
MODEM2 protocol. Some who are unaware of MODEM7's unusual batch
file mode call it MODEM7. Other aliases include "CP/M Users'
Group" and "TERM II FTP 3". The name XMODEM caught on partly
because it is distinctive and partly because of media interest in
__________
1. Page C/12, PC-WEEK July 12, 1987
Chapter 1
X/YMODEM Protocol Reference June 18 1988 3
bulletin board and RCPM systems where it was accessed with an
"XMODEM" command. This protocol is supported by every serious
communications program because of its universality, simplicity,
and reasonable performance.
XMODEM/CRC replaces XMODEM's 1 byte checksum with a two byte Cyclical
Redundancy Check (CRC-16), giving modern error detection
protection.
XMODEM-1k Refers to the XMODEM/CRC protocol with 1024 byte data blocks.
YMODEM Refers to the XMODEM/CRC (optional 1k blocks) protocol with batch
transmission as described below. In a nutshell, YMODEM means
BATCH.
YMODEM-g Refers to the streaming YMODEM variation described below.
True YMODEM(TM) In an attempt to sort out the YMODEM Tower of Babel, Omen
Technology has trademarked the term True YMODEM(TM) to represent
the complete YMODEM protocol described in this document, including
pathname, length, and modification date transmitted in block 0.
Please contact Omen Technology about certifying programs for True
YMODEM(TM) compliance.
ZMODEM uses familiar XMODEM/CRC and YMODEM technology in a new protocol
that provides reliability, throughput, file management, and user
amenities appropriate to contemporary data communications.
ZOO Like ARC, ZOO is a program that compresses one or more files into
a "zoo archive". ZOO supports many different operating systems
including Unix and VMS.
Chapter 1
X/YMODEM Protocol Reference June 18 1988 4
2. YMODEM MINIMUM REQUIREMENTS
All programs claiming to support YMODEM must meet the following minimum
requirements:
+ The sending program shall send the pathname (file name) in block 0.
+ The pathname shall be a null terminated ASCII string as described
below.
For those who are too lazy to read the entire document:
+ Unless specifically requested, only the file name portion is
sent.
+ No drive letter is sent.
+ Systems that do not distinguish between upper and lower case
letters in filenames shall send the pathname in lower case only.
+ The receiving program shall use this pathname for the received file
name, unless explicitly overridden.
+ When the receiving program receives this block and successfully
opened the output file, it shall acknowledge this block with an ACK
character and then proceed with a normal XMODEM file transfer
beginning with a "C" or NAK tranmsitted by the receiver.
+ The sending program shall use CRC-16 in response to a "C" pathname
nak, otherwise use 8 bit checksum.
+ The receiving program must accept any mixture of 128 and 1024 byte
blocks within each file it receives. Sending programs may
arbitrarily switch between 1024 and 128 byte blocks.
+ The sending program must not change the length of an unacknowledged
block.
+ At the end of each file, the sending program shall send EOT up to ten
times until it receives an ACK character. (This is part of the
XMODEM spec.)
+ The end of a transfer session shall be signified by a null (empty)
pathname, this pathname block shall be acknowledged the same as other
pathname blocks.
Programs not meeting all of these requirements are not YMODEM compatible,
and shall not be described as supporting YMODEM.
Meeting these MINIMUM requirements does not guarantee reliable file
Chapter 2
X/YMODEM Protocol Reference June 18 1988 5
transfers under stress. Particular attention is called to XMODEM's single
character supervisory messages that are easily corrupted by transmission
errors.
Chapter 2
X/YMODEM Protocol Reference June 18 1988 6
3. WHY YMODEM?
Since its development half a decade ago, the Ward Christensen modem
protocol has enabled a wide variety of computer systems to interchange
data. There is hardly a communications program that doesn't at least
claim to support this protocol.
Advances in computing, modems and networking have revealed a number of
weaknesses in the original protocol:
+ The short block length caused throughput to suffer when used with
timesharing systems, packet switched networks, satellite circuits,
and buffered (error correcting) modems.
+ The 8 bit arithmetic checksum and other aspects allowed line
impairments to interfere with dependable, accurate transfers.
+ Only one file could be sent per command. The file name had to be
given twice, first to the sending program and then again to the
receiving program.
+ The transmitted file could accumulate as many as 127 extraneous
bytes.
+ The modification date of the file was lost.
A number of other protocols have been developed over the years, but none
have displaced XMODEM to date:
+ Lack of public domain documentation and example programs have kept
proprietary protocols such as Blast, Relay, and others tightly bound
to the fortunes of their suppliers.
+ Complexity discourages the widespread application of BISYNC, SDLC,
HDLC, X.25, and X.PC protocols.
+ Performance compromises and complexity have limited the popularity of
the Kermit protocol, which was developed to allow file transfers in
environments hostile to XMODEM.
The XMODEM protocol extensions and YMODEM Batch address some of these
weaknesses while maintaining most of XMODEM's simplicity.
YMODEM is supported by the public domain programs YAM (CP/M),
YAM(CP/M-86), YAM(CCPM-86), IMP (CP/M), KMD (CP/M), rz/sz (Unix, Xenix,
VMS, Berkeley Unix, Venix, Xenix, Coherent, IDRIS, Regulus). Commercial
implementations include MIRROR, and Professional-YAM.[1] Communications
Chapter 3
X/YMODEM Protocol Reference June 18 1988 7
programs supporting these extensions have been in use since 1981.
The 1k block length (XMODEM-1k) described below may be used in conjunction
with YMODEM Batch Protocol, or with single file transfers identical to the
XMODEM/CRC protocol except for minimal changes to support 1k blocks.
Another extension is the YMODEM-g protocol. YMODEM-g provides batch
transfers with maximum throughput when used with end to end error
correcting media, such as X.PC and error correcting modems, including 9600
bps units by TeleBit, U.S.Robotics, Hayes, Electronic Vaults, Data Race,
and others.
To complete this tome, edited versions of Ward Christensen's original
protocol document and John Byrns's CRC-16 document are included for
reference.
References to the MODEM or MODEM7 protocol have been changed to XMODEM to
accommodate the vernacular. In Australia, it is properly called the
Christensen Protocol.
3.1 Some Messages from the Pioneer
#: 130940 S0/Communications 25-Apr-85 18:38:47
Sb: my protocol
Fm: Ward Christensen 76703,302 [2]
To: all
Be aware the article[3] DID quote me correctly in terms of the phrases
like "not robust", etc.
It was a quick hack I threw together, very unplanned (like everything I
do), to satisfy a personal need to communicate with "some other" people.
ONLY the fact that it was done in 8/77, and that I put it in the public
domain immediately, made it become the standard that it is.
__________________________________________________________________________
1. Available for IBM PC,XT,AT, Unix and Xenix
2. Edited for typesetting appearance
3. Infoworld April 29 p. 16
Chapter 3
X/YMODEM Protocol Reference June 18 1988 8
I think its time for me to
(1) document it; (people call me and say "my product is going to include
it - what can I 'reference'", or "I'm writing a paper on it, what do I put
in the bibliography") and
(2) propose an "incremental extension" to it, which might take "exactly"
the form of Chuck Forsberg's YAM protocol. He wrote YAM in C for CP/M and
put it in the public domain, and wrote a batch protocol for Unix[4] called
rb and sb (receive batch, send batch), which was basically XMODEM with
(a) a record 0 containing filename date time and size
(b) a 1K block size option
(c) CRC-16.
He did some clever programming to detect false ACK or EOT, but basically
left them the same.
People who suggest I make SIGNIFICANT changes to the protocol, such as
"full duplex", "multiple outstanding blocks", "multiple destinations", etc
etc don't understand that the incredible simplicity of the protocol is one
of the reasons it survived to this day in as many machines and programs as
it may be found in!
Consider the PC-NET group back in '77 or so - documenting to beat the band
- THEY had a protocol, but it was "extremely complex", because it tried to
be "all things to all people" - i.e. send binary files on a 7-bit system,
etc. I was not that "benevolent". I (emphasize > I < ) had an 8-bit UART,
so "my protocol was an 8-bit protocol", and I would just say "sorry" to
people who were held back by 7-bit limitations. ...
Block size: Chuck Forsberg created an extension of my protocol, called
YAM, which is also supported via his public domain programs for UNIX
called rb and sb - receive batch and send batch. They cleverly send a
"block 0" which contains the filename, date, time, and size.
Unfortunately, its UNIX style, and is a bit weird[5] - octal numbers, etc.
BUT, it is a nice way to overcome the kludgy "echo the chars of the name"
introduced with MODEM7. Further, chuck uses CRC-16 and optional 1K
blocks. Thus the record 0, 1K, and CRC, make it a "pretty slick new
protocol" which is not significantly different from my own.
Also, there is a catchy name - YMODEM. That means to some that it is the
"next thing after XMODEM", and to others that it is the Y(am)MODEM
__________
4. VAX/VMS versions of these programs are also available.
5. The file length, time, and file mode are optional. The pathname and
file length may be sent alone if desired.
Chapter 3
X/YMODEM Protocol Reference June 18 1988 9
protocol. I don't want to emphasize that too much - out of fear that
other mfgrs might think it is a "competitive" protocol, rather than an
"unaffiliated" protocol. Chuck is currently selling a much-enhanced
version of his CP/M-80 C program YAM, calling it Professional Yam, and its
for the PC - I'm using it right now. VERY slick! 32K capture buffer,
script, scrolling, previously captured text search, plus built-in commands
for just about everything - directory (sorted every which way), XMODEM,
YMODEM, KERMIT, and ASCII file upload/download, etc. You can program it
to "behave" with most any system - for example when trying a number for
CIS it detects the "busy" string back from the modem and substitutes a
diff phone # into the dialing string and branches back to try it.
Chapter 3
X/YMODEM Protocol Reference June 18 1988 10
4. XMODEM PROTOCOL ENHANCEMENTS
This chapter discusses the protocol extensions to Ward Christensen's 1982
XMODEM protocol description document.
The original document recommends the user be asked whether to continue
trying or abort after 10 retries. Most programs no longer ask the
operator whether he wishes to keep retrying. Virtually all correctable
errors are corrected within the first few retransmissions. If the line is
so bad that ten attempts are insufficient, there is a significant danger
of undetected errors. If the connection is that bad, it's better to
redial for a better connection, or mail a floppy disk.
4.1 Graceful Abort
The YAM and Professional-YAM X/YMODEM routines recognize a sequence of two
consecutive CAN (Hex 18) characters without modem errors (overrun,
framing, etc.) as a transfer abort command. This sequence is recognized
when is waiting for the beginning of a block or for an acknowledgement to
a block that has been sent. The check for two consecutive CAN characters
reduces the number of transfers aborted by line hits. YAM sends eight CAN
characters when it aborts an XMODEM, YMODEM, or ZMODEM protocol file
transfer. Pro-YAM then sends eight backspaces to delete the CAN
characters from the remote's keyboard input buffer, in case the remote had
already aborted the transfer and was awaiting a keyboarded command.
4.2 CRC-16 Option
The XMODEM protocol uses an optional two character CRC-16 instead of the
one character arithmetic checksum used by the original protocol and by
most commercial implementations. CRC-16 guarantees detection of all
single and double bit errors, all errors with an odd number of error
bits, all burst errors of length 16 or less, 99.9969% of all 17-bit error
bursts, and 99.9984 per cent of all possible longer error bursts. By
contrast, a double bit error, or a burst error of 9 bits or more can sneak
past the XMODEM protocol arithmetic checksum.
The XMODEM/CRC protocol is similar to the XMODEM protocol, except that the
receiver specifies CRC-16 by sending C (Hex 43) instead of NAK when
requesting the FIRST block. A two byte CRC is sent in place of the one
byte arithmetic checksum.
YAM's c option to the r command enables CRC-16 in single file reception,
corresponding to the original implementation in the MODEM7 series
programs. This remains the default because many commercial communications
programs and bulletin board systems still do not support CRC-16,
especially those written in Basic or Pascal.
XMODEM protocol with CRC is accurate provided both sender and receiver
Chapter 4 XMODEM Protocol Enhancements
X/YMODEM Protocol Reference June 18 1988 11
both report a successful transmission. The protocol is robust in the
presence of characters lost by buffer overloading on timesharing systems.
The single character ACK/NAK responses generated by the receiving program
adapt well to split speed modems, where the reverse channel is limited to
ten per cent or less of the main channel's speed.
XMODEM and YMODEM are half duplex protocols which do not attempt to
transmit information and control signals in both directions at the same
time. This avoids buffer overrun problems that have been reported by
users attempting to exploit full duplex asynchronous file transfer
protocols such as Blast.
Professional-YAM adds several proprietary logic enhancements to XMODEM's
error detection and recovery. These compatible enhancements eliminate
most of the bad file transfers other programs make when using the XMODEM
protocol under less than ideal conditions.
4.3 XMODEM-1k 1024 Byte Block
Disappointing throughput downloading from Unix with YMODEM[1] lead to the
development of 1024 byte blocks in 1982. 1024 byte blocks reduce the
effect of delays from timesharing systems, modems, and packet switched
networks on throughput by 87.5 per cent in addition to decreasing XMODEM's
3 per cent overhead (block number, CRC, etc.).
Some environments cannot accept 1024 byte bursts, including some networks
and minicomputer ports. The longer block length should be an option.
The choice to use 1024 byte blocks is expressed to the sending program on
its command line or selection menu.[2] 1024 byte blocks improve throughput
in many applications.
An STX (02) replaces the SOH (01) at the beginning of the transmitted
block to notify the receiver of the longer block length. The transmitted
block contains 1024 bytes of data. The receiver should be able to accept
any mixture of 128 and 1024 byte blocks. The block number (in the second
and third bytes of the block) is incremented by one for each block
regardless of the block length.
The sender must not change between 128 and 1024 byte block lengths if it
has not received a valid ACK for the current block. Failure to observe
__________
1. The name hadn't been coined yet, but the protocol was the same.
2. See "KMD/IMP Exceptions to YMODEM" below.
Chapter 4 XMODEM Protocol Enhancements
X/YMODEM Protocol Reference June 18 1988 12
this restriction allows transmission errors to pass undetected.
If 1024 byte blocks are being used, it is possible for a file to "grow" up
to the next multiple of 1024 bytes. This does not waste disk space if the
allocation granularity is 1k or greater. With YMODEM batch transmission,
the optional file length transmitted in the file name block allows the
receiver to discard the padding, preserving the exact file length and
contents.
1024 byte blocks may be used with batch file transmission or with single
file transmission. CRC-16 should be used with the k option to preserve
data integrity over phone lines. If a program wishes to enforce this
recommendation, it should cancel the transfer, then issue an informative
diagnostic message if the receiver requests checksum instead of CRC-16.
Under no circumstances may a sending program use CRC-16 unless the
receiver commands CRC-16.
Figure 1. XMODEM-1k Blocks
SENDER RECEIVER
"sx -k foo.bar"
"foo.bar open x.x minutes"
C
STX 01 FE Data[1024] CRC CRC
ACK
STX 02 FD Data[1024] CRC CRC
ACK
STX 03 FC Data[1000] CPMEOF[24] CRC CRC
ACK
EOT
ACK
Figure 2. Mixed 1024 and 128 byte Blocks
SENDER RECEIVER
"sx -k foo.bar"
"foo.bar open x.x minutes"
C
STX 01 FE Data[1024] CRC CRC
ACK
STX 02 FD Data[1024] CRC CRC
ACK
SOH 03 FC Data[128] CRC CRC
ACK
SOH 04 FB Data[100] CPMEOF[28] CRC CRC
ACK
EOT
ACK
Chapter 4 XMODEM Protocol Enhancements
X/YMODEM Protocol Reference June 18 1988 13
5. YMODEM Batch File Transmission
The YMODEM Batch protocol is an extension to the XMODEM/CRC protocol that
allows 0 or more files to be transmitted with a single command. (Zero
files may be sent if none of the requested files is accessible.) The
design approach of the YMODEM Batch protocol is to use the normal routines
for sending and receiving XMODEM blocks in a layered fashion similar to
packet switching methods.
Why was it necessary to design a new batch protocol when one already
existed in MODEM7?[1] The batch file mode used by MODEM7 is unsuitable
because it does not permit full pathnames, file length, file date, or
other attribute information to be transmitted. Such a restrictive design,
hastily implemented with only CP/M in mind, would not have permitted
extensions to current areas of personal computing such as Unix, DOS, and
object oriented systems. In addition, the MODEM7 batch file mode is
somewhat susceptible to transmission impairments.
As in the case of single a file transfer, the receiver initiates batch
file transmission by sending a "C" character (for CRC-16).
The sender opens the first file and sends block number 0 with the
following information.[2]
Only the pathname (file name) part is required for batch transfers.
To maintain upwards compatibility, all unused bytes in block 0 must be set
to null.
Pathname The pathname (conventionally, the file name) is sent as a null
terminated ASCII string. This is the filename format used by the
handle oriented MSDOS(TM) functions and C library fopen functions.
An assembly language example follows:
DB 'foo.bar',0
No spaces are included in the pathname. Normally only the file name
stem (no directory prefix) is transmitted unless the sender has
selected YAM's f option to send the full pathname. The source drive
(A:, B:, etc.) is not sent.
Filename Considerations:
__________
1. The MODEM7 batch protocol transmitted CP/M FCB bytes f1...f8 and
t1...t3 one character at a time. The receiver echoed these bytes as
received, one at a time.
2. Only the data part of the block is described here.
Chapter 5 XMODEM Protocol Enhancements
X/YMODEM Protocol Reference June 18 1988 14
+ File names are forced to lower case unless the sending system
supports upper/lower case file names. This is a convenience for
users of systems (such as Unix) which store filenames in upper
and lower case.
+ The receiver should accommodate file names in lower and upper
case.
+ When transmitting files between different operating systems,
file names must be acceptable to both the sender and receiving
operating systems.
If directories are included, they are delimited by /; i.e.,
"subdir/foo" is acceptable, "subdir\foo" is not.
Length The file length and each of the succeeding fields are optional.[3]
The length field is stored in the block as a decimal string counting
the number of data bytes in the file. The file length does not
include any CPMEOF (^Z) or other garbage characters used to pad the
last block.
If the file being transmitted is growing during transmission, the
length field should be set to at least the final expected file
length, or not sent.
The receiver stores the specified number of characters, discarding
any padding added by the sender to fill up the last block.
Modification Date The mod date is optional, and the filename and length
may be sent without requiring the mod date to be sent.
Iff the modification date is sent, a single space separates the
modification date from the file length.
The mod date is sent as an octal number giving the time the contents
of the file were last changed, measured in seconds from Jan 1 1970
Universal Coordinated Time (GMT). A date of 0 implies the
modification date is unknown and should be left as the date the file
is received.
This standard format was chosen to eliminate ambiguities arising from
transfers between different time zones.
__________
3. Fields may not be skipped.
Chapter 5 XMODEM Protocol Enhancements
X/YMODEM Protocol Reference June 18 1988 15
Mode Iff the file mode is sent, a single space separates the file mode
from the modification date. The file mode is stored as an octal
string. Unless the file originated from a Unix system, the file mode
is set to 0. rb(1) checks the file mode for the 0x8000 bit which
indicates a Unix type regular file. Files with the 0x8000 bit set
are assumed to have been sent from another Unix (or similar) system
which uses the same file conventions. Such files are not translated
in any way.
Serial Number Iff the serial number is sent, a single space separates the
serial number from the file mode. The serial number of the
transmitting program is stored as an octal string. Programs which do
not have a serial number should omit this field, or set it to 0. The
receiver's use of this field is optional.
Other Fields YMODEM was designed to allow additional header fields to be
added as above without creating compatibility problems with older
YMODEM programs. Please contact Omen Technology if other fields are
needed for special application requirements.
The rest of the block is set to nulls. This is essential to preserve
upward compatibility.[4]
If the filename block is received with a CRC or other error, a
retransmission is requested. After the filename block has been received,
it is ACK'ed if the write open is successful. If the file cannot be
opened for writing, the receiver cancels the transfer with CAN characters
as described above.
The receiver then initiates transfer of the file contents with a "C"
character, according to the standard XMODEM/CRC protocol.
After the file contents and XMODEM EOT have been transmitted and
acknowledged, the receiver again asks for the next pathname.
Transmission of a null pathname terminates batch file transmission.
Note that transmission of no files is not necessarily an error. This is
possible if none of the files requested of the sender could be opened for
reading.
__________
4. If, perchance, this information extends beyond 128 bytes (possible
with Unix 4.2 BSD extended file names), the block should be sent as a
1k block as described above.
Chapter 5 XMODEM Protocol Enhancements
X/YMODEM Protocol Reference June 18 1988 16
Most YMODEM receivers request CRC-16 by default.
The Unix programs sz(1) and rz(1) included in the source code file
RZSZ.ZOO should answer other questions about YMODEM batch protocol.
Figure 3. YMODEM Batch Transmission Session (1 file)
SENDER RECEIVER
"sb foo.*<CR>"
"sending in batch mode etc."
C (command:rb)
SOH 00 FF foo.c NUL[123] CRC CRC
ACK
C
SOH 01 FE Data[128] CRC CRC
ACK
SOH 02 FC Data[128] CRC CRC
ACK
SOH 03 FB Data[100] CPMEOF[28] CRC CRC
ACK
EOT
NAK
EOT
ACK
C
SOH 00 FF NUL[128] CRC CRC
ACK
Figure 7. YMODEM Header Information and Features
_____________________________________________________________
| Program | Length | Date | Mode | S/N | 1k-Blk | YMODEM-g |
|___________|________|______|______|_____|________|__________|
|Unix rz/sz | yes | yes | yes | no | yes | sb only |
|___________|________|______|______|_____|________|__________|
|VMS rb/sb | yes | no | no | no | yes | no |
|___________|________|______|______|_____|________|__________|
|Pro-YAM | yes | yes | no | yes | yes | yes |
|___________|________|______|______|_____|________|__________|
|CP/M YAM | no | no | no | no | yes | no |
|___________|________|______|______|_____|________|__________|
|KMD/IMP | ? | no | no | no | yes | no |
|___________|________|______|______|_____|________|__________|
5.1 KMD/IMP Exceptions to YMODEM
KMD and IMP use a "CK" character sequence emitted by the receiver to
trigger the use of 1024 byte blocks as an alternative to specifying this
option to the sending program. This two character sequence generally
works well on single process micros in direct communication, provided the
programs rigorously adhere to all the XMODEM recommendations included
Chapter 5 XMODEM Protocol Enhancements
X/YMODEM Protocol Reference June 18 1988 17
Figure 4. YMODEM Batch Transmission Session (2 files)
SENDER RECEIVER
"sb foo.c baz.c<CR>"
"sending in batch mode etc."
C (command:rb)
SOH 00 FF foo.c NUL[123] CRC CRC
ACK
C
SOH 01 FE Data[128] CRC CRC
ACK
SOH 02 FC Data[128] CRC CRC
ACK
SOH 03 FB Data[100] CPMEOF[28] CRC CRC
ACK
EOT
NAK
EOT
ACK
C
SOH 00 FF baz.c NUL[123] CRC CRC
ACK
C
SOH 01 FB Data[100] CPMEOF[28] CRC CRC
ACK
EOT
NAK
EOT
ACK
C
SOH 00 FF NUL[128] CRC CRC
ACK
Figure 5. YMODEM Batch Transmission Session-1k Blocks
SENDER RECEIVER
"sb -k foo.*<CR>"
"sending in batch mode etc."
C (command:rb)
SOH 00 FF foo.c NUL[123] CRC CRC
ACK
C
STX 01 FD Data[1024] CRC CRC
ACK
SOH 02 FC Data[128] CRC CRC
ACK
SOH 03 FB Data[100] CPMEOF[28] CRC CRC
ACK
EOT
NAK
EOT
Chapter 5 XMODEM Protocol Enhancements
X/YMODEM Protocol Reference June 18 1988 18
ACK
C
SOH 00 FF NUL[128] CRC CRC
ACK
Figure 6. YMODEM Filename block transmitted by sz
-rw-r--r-- 6347 Jun 17 1984 20:34 bbcsched.txt
00 0100FF62 62637363 6865642E 74787400 |...bbcsched.txt.|
10 36333437 20333331 34373432 35313320 |6347 3314742513 |
20 31303036 34340000 00000000 00000000 |100644..........|
30 00000000 00000000 00000000 00000000
40 00000000 00000000 00000000 00000000
50 00000000 00000000 00000000 00000000
60 00000000 00000000 00000000 00000000
70 00000000 00000000 00000000 00000000
80 000000CA 56
herein. Programs with marginal XMODEM implementations do not fare so
well. Timesharing systems and packet switched networks can separate the
successive characters, rendering this method unreliable.
Sending programs may detect the CK sequence if the operating enviornment
does not preclude reliable implementation.
Instead of the standard YMODEM file length in decimal, KMD and IMP
transmit the CP/M record count in the last two bytes of the header block.
6. YMODEM-g File Transmission
Developing technology is providing phone line data transmission at ever
higher speeds using very specialized techniques. These high speed modems,
as well as session protocols such as X.PC, provide high speed, nearly
error free communications at the expense of considerably increased delay
time.
This delay time is moderate compared to human interactions, but it
cripples the throughput of most error correcting protocols.
The g option to YMODEM has proven effective under these circumstances.
The g option is driven by the receiver, which initiates the batch transfer
by transmitting a G instead of C. When the sender recognizes the G, it
bypasses the usual wait for an ACK to each transmitted block, sending
succeeding blocks at full speed, subject to XOFF/XON or other flow control
exerted by the medium.
The sender expects an inital G to initiate the transmission of a
particular file, and also expects an ACK on the EOT sent at the end of
each file. This synchronization allows the receiver time to open and
Chapter 6 XMODEM Protocol Enhancements
X/YMODEM Protocol Reference June 18 1988 19
close files as necessary.
If an error is detected in a YMODEM-g transfer, the receiver aborts the
transfer with the multiple CAN abort sequence. The ZMODEM protocol should
be used in applications that require both streaming throughput and error
recovery.
Figure 8. YMODEM-g Transmission Session
SENDER RECEIVER
"sb foo.*<CR>"
"sending in batch mode etc..."
G (command:rb -g)
SOH 00 FF foo.c NUL[123] CRC CRC
G
SOH 01 FE Data[128] CRC CRC
STX 02 FD Data[1024] CRC CRC
SOH 03 FC Data[128] CRC CRC
SOH 04 FB Data[100] CPMEOF[28] CRC CRC
EOT
ACK
G
SOH 00 FF NUL[128] CRC CRC
Chapter 6 XMODEM Protocol Enhancements
X/YMODEM Protocol Reference June 18 1988 20
7. XMODEM PROTOCOL OVERVIEW
8/9/82 by Ward Christensen.
I will maintain a master copy of this. Please pass on changes or
suggestions via CBBS/Chicago at (312) 545-8086, CBBS/CPMUG (312) 849-1132
or by voice at (312) 849-6279.
7.1 Definitions
<soh> 01H
<eot> 04H
<ack> 06H
<nak> 15H
<can> 18H
<C> 43H
7.2 Transmission Medium Level Protocol
Asynchronous, 8 data bits, no parity, one stop bit.
The protocol imposes no restrictions on the contents of the data being
transmitted. No control characters are looked for in the 128-byte data
messages. Absolutely any kind of data may be sent - binary, ASCII, etc.
The protocol has not formally been adopted to a 7-bit environment for the
transmission of ASCII-only (or unpacked-hex) data , although it could be
simply by having both ends agree to AND the protocol-dependent data with
7F hex before validating it. I specifically am referring to the checksum,
and the block numbers and their ones- complement.
Those wishing to maintain compatibility of the CP/M file structure, i.e.
to allow modemming ASCII files to or from CP/M systems should follow this
data format:
+ ASCII tabs used (09H); tabs set every 8.
+ Lines terminated by CR/LF (0DH 0AH)
+ End-of-file indicated by ^Z, 1AH. (one or more)
+ Data is variable length, i.e. should be considered a continuous
stream of data bytes, broken into 128-byte chunks purely for the
purpose of transmission.
+ A CP/M "peculiarity": If the data ends exactly on a 128-byte
boundary, i.e. CR in 127, and LF in 128, a subsequent sector
containing the ^Z EOF character(s) is optional, but is preferred.
Some utilities or user programs still do not handle EOF without ^Zs.
Chapter 7 Xmodem Protocol Overview
X/YMODEM Protocol Reference June 18 1988 21
+ The last block sent is no different from others, i.e. there is no
"short block".
Figure 9. XMODEM Message Block Level Protocol
Each block of the transfer looks like:
<SOH><blk #><255-blk #><--128 data bytes--><cksum>
in which:
<SOH> = 01 hex
<blk #> = binary number, starts at 01 increments by 1, and
wraps 0FFH to 00H (not to 01)
<255-blk #> = blk # after going thru 8080 "CMA" instr, i.e.
each bit complemented in the 8-bit block number.
Formally, this is the "ones complement".
<cksum> = the sum of the data bytes only. Toss any carry.
7.3 File Level Protocol
7.3.1 Common_to_Both_Sender_and_Receiver
All errors are retried 10 times. For versions running with an operator
(i.e. NOT with XMODEM), a message is typed after 10 errors asking the
operator whether to "retry or quit".
Some versions of the protocol use <can>, ASCII ^X, to cancel transmission.
This was never adopted as a standard, as having a single "abort" character
makes the transmission susceptible to false termination due to an <ack>
<nak> or <soh> being corrupted into a <can> and aborting transmission.
The protocol may be considered "receiver driven", that is, the sender need
not automatically re-transmit, although it does in the current
implementations.
7.3.2 Receive_Program_Considerations
The receiver has a 10-second timeout. It sends a <nak> every time it
times out. The receiver's first timeout, which sends a <nak>, signals the
transmitter to start. Optionally, the receiver could send a <nak>
immediately, in case the sender was ready. This would save the initial 10
second timeout. However, the receiver MUST continue to timeout every 10
seconds in case the sender wasn't ready.
Once into a receiving a block, the receiver goes into a one-second timeout
for each character and the checksum. If the receiver wishes to <nak> a
block for any reason (invalid header, timeout receiving data), it must
wait for the line to clear. See "programming tips" for ideas
Synchronizing: If a valid block number is received, it will be: 1) the
expected one, in which case everything is fine; or 2) a repeat of the
previously received block. This should be considered OK, and only
indicates that the receivers <ack> got glitched, and the sender re-
transmitted; 3) any other block number indicates a fatal loss of
synchronization, such as the rare case of the sender getting a line-glitch
Chapter 7 Xmodem Protocol Overview
X/YMODEM Protocol Reference June 18 1988 22
that looked like an <ack>. Abort the transmission, sending a <can>
7.3.3 Sending_program_considerations
While waiting for transmission to begin, the sender has only a single very
long timeout, say one minute. In the current protocol, the sender has a
10 second timeout before retrying. I suggest NOT doing this, and letting
the protocol be completely receiver-driven. This will be compatible with
existing programs.
When the sender has no more data, it sends an <eot>, and awaits an <ack>,
resending the <eot> if it doesn't get one. Again, the protocol could be
receiver-driven, with the sender only having the high-level 1-minute
timeout to abort.
Here is a sample of the data flow, sending a 3-block message. It includes
the two most common line hits - a garbaged block, and an <ack> reply
getting garbaged. <xx> represents the checksum byte.
Figure 10. Data flow including Error Recovery
SENDER RECEIVER
times out after 10 seconds,
<--- <nak>
<soh> 01 FE -data- <xx> --->
<--- <ack>
<soh> 02 FD -data- xx ---> (data gets line hit)
<--- <nak>
<soh> 02 FD -data- xx --->
<--- <ack>
<soh> 03 FC -data- xx --->
(ack gets garbaged) <--- <ack>
<soh> 03 FC -data- xx ---> <ack>
<eot> --->
<--- <anything except ack>
<eot> --->
<--- <ack>
(finished)
7.4 Programming Tips
+ The character-receive subroutine should be called with a parameter
specifying the number of seconds to wait. The receiver should first
call it with a time of 10, then <nak> and try again, 10 times.
After receiving the <soh>, the receiver should call the character
receive subroutine with a 1-second timeout, for the remainder of the
message and the <cksum>. Since they are sent as a continuous stream,
timing out of this implies a serious like glitch that caused, say,
127 characters to be seen instead of 128.
Chapter 7 Xmodem Protocol Overview
X/YMODEM Protocol Reference June 18 1988 23
+ When the receiver wishes to <nak>, it should call a "PURGE"
subroutine, to wait for the line to clear. Recall the sender tosses
any characters in its UART buffer immediately upon completing sending
a block, to ensure no glitches were mis- interpreted.
The most common technique is for "PURGE" to call the character
receive subroutine, specifying a 1-second timeout,[1] and looping
back to PURGE until a timeout occurs. The <nak> is then sent,
ensuring the other end will see it.
+ You may wish to add code recommended by John Mahr to your character
receive routine - to set an error flag if the UART shows framing
error, or overrun. This will help catch a few more glitches - the
most common of which is a hit in the high bits of the byte in two
consecutive bytes. The <cksum> comes out OK since counting in 1-byte
produces the same result of adding 80H + 80H as with adding 00H +
00H.
__________
1. These times should be adjusted for use with timesharing systems.
Chapter 7 Xmodem Protocol Overview
X/YMODEM Protocol Reference June 18 1988 24
8. XMODEM/CRC Overview
Original 1/13/85 by John Byrns -- CRC option.
Please pass on any reports of errors in this document or suggestions for
improvement to me via Ward's/CBBS at (312) 849-1132, or by voice at (312)
885-1105.
The CRC used in the Modem Protocol is an alternate form of block check
which provides more robust error detection than the original checksum.
Andrew S. Tanenbaum says in his book, Computer Networks, that the CRC-
CCITT used by the Modem Protocol will detect all single and double bit
errors, all errors with an odd number of bits, all burst errors of length
16 or less, 99.997% of 17-bit error bursts, and 99.998% of 18-bit and
longer bursts.[1]
The changes to the Modem Protocol to replace the checksum with the CRC are
straight forward. If that were all that we did we would not be able to
communicate between a program using the old checksum protocol and one
using the new CRC protocol. An initial handshake was added to solve this
problem. The handshake allows a receiving program with CRC capability to
determine whether the sending program supports the CRC option, and to
switch it to CRC mode if it does. This handshake is designed so that it
will work properly with programs which implement only the original
protocol. A description of this handshake is presented in section 10.
Figure 11. Message Block Level Protocol, CRC mode
Each block of the transfer in CRC mode looks like:
<SOH><blk #><255-blk #><--128 data bytes--><CRC hi><CRC lo>
in which:
<SOH> = 01 hex
<blk #> = binary number, starts at 01 increments by 1, and
wraps 0FFH to 00H (not to 01)
<255-blk #> = ones complement of blk #.
<CRC hi> = byte containing the 8 hi order coefficients of the CRC.
<CRC lo> = byte containing the 8 lo order coefficients of the CRC.
8.1 CRC Calculation
8.1.1 Formal_Definition
To calculate the 16 bit CRC the message bits are considered to be the
coefficients of a polynomial. This message polynomial is first multiplied
by X^16 and then divided by the generator polynomial (X^16 + X^12 + X^5 +
__________
1. This reliability figure is misleading because XMODEM's critical
supervisory functions are not protected by this CRC.
Chapter 8 Xmodem Protocol Overview
X/YMODEM Protocol Reference June 18 1988 25
1) using modulo two arithmetic. The remainder left after the division is
the desired CRC. Since a message block in the Modem Protocol is 128 bytes
or 1024 bits, the message polynomial will be of order X^1023. The hi order
bit of the first byte of the message block is the coefficient of X^1023 in
the message polynomial. The lo order bit of the last byte of the message
block is the coefficient of X^0 in the message polynomial.
Figure 12. Example of CRC Calculation written in C
The following XMODEM crc routine is taken from "rbsb.c". Please refer to
the source code for these programs (contained in RZSZ.ZOO) for usage. A
fast table driven version is also included in this file.
/* update CRC */
unsigned short
updcrc(c, crc)
register c;
register unsigned crc;
{
register count;
for (count=8; --count>=0;) {
if (crc & 0x8000) {
crc <<= 1;
crc += (((c<<=1) & 0400) != 0);
crc ^= 0x1021;
}
else {
crc <<= 1;
crc += (((c<<=1) & 0400) != 0);
}
}
return crc;
}
8.2 CRC File Level Protocol Changes
8.2.1 Common_to_Both_Sender_and_Receiver
The only change to the File Level Protocol for the CRC option is the
initial handshake which is used to determine if both the sending and the
receiving programs support the CRC mode. All Modem Programs should support
the checksum mode for compatibility with older versions. A receiving
program that wishes to receive in CRC mode implements the mode setting
handshake by sending a <C> in place of the initial <nak>. If the sending
program supports CRC mode it will recognize the <C> and will set itself
into CRC mode, and respond by sending the first block as if a <nak> had
been received. If the sending program does not support CRC mode it will
not respond to the <C> at all. After the receiver has sent the <C> it will
wait up to 3 seconds for the <soh> that starts the first block. If it
receives a <soh> within 3 seconds it will assume the sender supports CRC
mode and will proceed with the file exchange in CRC mode. If no <soh> is
Chapter 8 Xmodem Protocol Overview
X/YMODEM Protocol Reference June 18 1988 26
received within 3 seconds the receiver will switch to checksum mode, send
a <nak>, and proceed in checksum mode. If the receiver wishes to use
checksum mode it should send an initial <nak> and the sending program
should respond to the <nak> as defined in the original Modem Protocol.
After the mode has been set by the initial <C> or <nak> the protocol
follows the original Modem Protocol and is identical whether the checksum
or CRC is being used.
8.2.2 Receive_Program_Considerations
There are at least 4 things that can go wrong with the mode setting
handshake.
1. the initial <C> can be garbled or lost.
2. the initial <soh> can be garbled.
3. the initial <C> can be changed to a <nak>.
4. the initial <nak> from a receiver which wants to receive in checksum
can be changed to a <C>.
The first problem can be solved if the receiver sends a second <C> after
it times out the first time. This process can be repeated several times.
It must not be repeated too many times before sending a <nak> and
switching to checksum mode or a sending program without CRC support may
time out and abort. Repeating the <C> will also fix the second problem if
the sending program cooperates by responding as if a <nak> were received
instead of ignoring the extra <C>.
It is possible to fix problems 3 and 4 but probably not worth the trouble
since they will occur very infrequently. They could be fixed by switching
modes in either the sending or the receiving program after a large number
of successive <nak>s. This solution would risk other problems however.
8.2.3 Sending_Program_Considerations
The sending program should start in the checksum mode. This will insure
compatibility with checksum only receiving programs. Anytime a <C> is
received before the first <nak> or <ack> the sending program should set
itself into CRC mode and respond as if a <nak> were received. The sender
should respond to additional <C>s as if they were <nak>s until the first
<ack> is received. This will assist the receiving program in determining
the correct mode when the <soh> is lost or garbled. After the first <ack>
is received the sending program should ignore <C>s.
Chapter 8 Xmodem Protocol Overview
X/YMODEM Protocol Reference June 18 1988 27
8.3 Data Flow Examples with CRC Option
Here is a data flow example for the case where the receiver requests
transmission in the CRC mode but the sender does not support the CRC
option. This example also includes various transmission errors. <xx>
represents the checksum byte.
Figure 13. Data Flow: Receiver has CRC Option, Sender Doesn't
SENDER RECEIVER
<--- <C>
times out after 3 seconds,
<--- <C>
times out after 3 seconds,
<--- <C>
times out after 3 seconds,
<--- <C>
times out after 3 seconds,
<--- <nak>
<soh> 01 FE -data- <xx> --->
<--- <ack>
<soh> 02 FD -data- <xx> ---> (data gets line hit)
<--- <nak>
<soh> 02 FD -data- <xx> --->
<--- <ack>
<soh> 03 FC -data- <xx> --->
(ack gets garbaged) <--- <ack>
times out after 10 seconds,
<--- <nak>
<soh> 03 FC -data- <xx> --->
<--- <ack>
<eot> --->
<--- <ack>
Here is a data flow example for the case where the receiver requests
transmission in the CRC mode and the sender supports the CRC option. This
example also includes various transmission errors. <xxxx> represents the
2 CRC bytes.
Chapter 8 Xmodem Protocol Overview
X/YMODEM Protocol Reference June 18 1988 28
Figure 14. Receiver and Sender Both have CRC Option
SENDER RECEIVER
<--- <C>
<soh> 01 FE -data- <xxxx> --->
<--- <ack>
<soh> 02 FD -data- <xxxx> ---> (data gets line hit)
<--- <nak>
<soh> 02 FD -data- <xxxx> --->
<--- <ack>
<soh> 03 FC -data- <xxxx> --->
(ack gets garbaged) <--- <ack>
times out after 10 seconds,
<--- <nak>
<soh> 03 FC -data- <xxxx> --->
<--- <ack>
<eot> --->
<--- <ack>
Chapter 8 Xmodem Protocol Overview
X/YMODEM Protocol Reference June 18 1988 29
9. MORE INFORMATION
Please contact Omen Technology for troff source files and typeset copies
of this document.
9.1 TeleGodzilla Bulletin Board
More information may be obtained by calling TeleGodzilla at 503-621-3746.
Speed detection is automatic for 1200, 2400 and 19200(Telebit PEP) bps.
TrailBlazer modem users may issue the TeleGodzilla trailblazer command to
swith to 19200 bps once they have logged in.
Interesting files include RZSZ.ZOO (C source code), YZMODEM.ZOO (Official
XMODEM, YMODEM, and ZMODEM protocol descriptions), ZCOMMEXE.ARC,
ZCOMMDOC.ARC, and ZCOMMHLP.ARC (PC-DOS shareware comm program with XMODEM,
True YMODEM(TM), ZMODEM, Kermit Sliding Windows, Telink, MODEM7 Batch,
script language, etc.).
9.2 Unix UUCP Access
UUCP sites can obtain the current version of this file with
uucp omen!/u/caf/public/ymodem.doc /tmp
A continually updated list of available files is stored in
/usr/spool/uucppublic/FILES. When retrieving these files with uucp,
remember that the destination directory on your system must be writeable
by anyone, or the UUCP transfer will fail.
The following L.sys line calls TeleGodzilla (Pro-YAM in host operation).
TeleGodzilla determines the incoming speed automatically.
In response to "Name Please:" uucico gives the Pro-YAM "link" command as a
user name. The password (Giznoid) controls access to the Xenix system
connected to the IBM PC's other serial port. Communications between
Pro-YAM and Xenix use 9600 bps; YAM converts this to the caller's speed.
Finally, the calling uucico logs in as uucp.
omen Any ACU 2400 1-503-621-3746 se:--se: link ord: Giznoid in:--in: uucp
10. REVISIONS
6-18-88 Further revised for clarity. Corrected block numbering in two
examples.
10-27-87 Optional fields added for number of files remaining to be sent
and total number of bytes remaining to be sent.
10-18-87 Flow control discussion added to 1024 byte block descritpion,
minor revisions for clarity per user comments.
Chapter 10 Xmodem Protocol Overview
X/YMODEM Protocol Reference June 18 1988 30
8-03-87 Revised for clarity.
5-31-1987 emphasizes minimum requirements for YMODEM, and updates
information on accessing files.
9-11-1986 clarifies nomenclature and some minor points.
The April 15 1986 edition clarifies some points concerning CRC
calculations and spaces in the header.
11. YMODEM Programs
ZCOMM, A shareware little brother to Professional-YAM, is available as
ZCOMMEXE.ARC on TeleGodzilla and other bulletin board systems. ZCOMM may
be used to test YMODEM amd ZMODEM implementations.
Unix programs supporting YMODEM are available on TeleGodzilla in RZSZ.ZOO.
This ZOO archive includes a ZCOMM/Pro-YAM/PowerCom script ZUPL.T to upload
a bootstrap program MINIRB.C, compile it, and then upload the rest of the
files using the compiled MINIRB. Most Unix like systems are supported,
including V7, Xenix, Sys III, 4.2 BSD, SYS V, Idris, Coherent, and
Regulus.
A version for VAX-VMS is available in VRBSB.SHQ.
Irv Hoff has added 1k blocks and basic YMODEM batch transfers to the KMD
and IMP series programs, which replace the XMODEM and MODEM7/MDM7xx series
respectively. Overlays are available for a wide variety of CP/M systems.
Questions about Professional-YAM communications software may be directed
to:
Chuck Forsberg
Omen Technology Inc
17505-V Sauvie Island Road
Portland Oregon 97231
VOICE: 503-621-3406 :VOICE
Modem: 503-621-3746 Speed: 19200(Telebit PEP),2400,1200,300
Usenet: ...!tektronix!reed!omen!caf
CompuServe: 70007,2304
GEnie: CAF
Unlike ZMODEM and Kermit, XMODEM and YMODEM place obstacles in the path of
a reliable high performance implementation, evidenced by poor reliability
under stress of the industry leaders' XMODEM and YMODEM programs. Omen
Technology provides consulting and other services to those wishing to
implement XMODEM, YMODEM, and ZMODEM with state of the art features and
reliability.
Chapter 11 Xmodem Protocol Overview
CONTENTS
1. TOWER OF BABEL................................................... 2
1.1 Definitions................................................. 2
2. YMODEM MINIMUM REQUIREMENTS...................................... 4
3. WHY YMODEM?...................................................... 6
3.1 Some Messages from the Pioneer.............................. 7
4. XMODEM PROTOCOL ENHANCEMENTS..................................... 10
4.1 Graceful Abort.............................................. 10
4.2 CRC-16 Option............................................... 10
4.3 XMODEM-1k 1024 Byte Block................................... 11
5. YMODEM Batch File Transmission................................... 13
5.1 KMD/IMP Exceptions to YMODEM................................ 16
6. YMODEM-g File Transmission....................................... 18
7. XMODEM PROTOCOL OVERVIEW......................................... 20
7.1 Definitions................................................. 20
7.2 Transmission Medium Level Protocol.......................... 20
7.3 File Level Protocol......................................... 21
7.4 Programming Tips............................................ 22
8. XMODEM/CRC Overview.............................................. 24
8.1 CRC Calculation............................................. 24
8.2 CRC File Level Protocol Changes............................. 25
8.3 Data Flow Examples with CRC Option.......................... 27
9. MORE INFORMATION................................................. 29
9.1 TeleGodzilla Bulletin Board................................. 29
9.2 Unix UUCP Access............................................ 29
10. REVISIONS........................................................ 29
11. YMODEM Programs.................................................. 30
- i -
LIST OF FIGURES
Figure 1. XMODEM-1k Blocks.......................................... 12
Figure 2. Mixed 1024 and 128 byte Blocks............................ 12
Figure 3. YMODEM Batch Transmission Session (1 file)................ 16
Figure 4. YMODEM Batch Transmission Session (2 files)............... 16
Figure 5. YMODEM Batch Transmission Session-1k Blocks............... 16
Figure 6. YMODEM Filename block transmitted by sz................... 16
Figure 7. YMODEM Header Information and Features.................... 16
Figure 8. YMODEM-g Transmission Session............................. 19
Figure 9. XMODEM Message Block Level Protocol....................... 21
Figure 10. Data flow including Error Recovery........................ 22
Figure 11. Message Block Level Protocol, CRC mode.................... 24
Figure 12. Example of CRC Calculation written in C................... 25
Figure 13. Data Flow: Receiver has CRC Option, Sender Doesn't........ 27
Figure 14. Receiver and Sender Both have CRC Option.................. 28
- ii -