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Primitive parallel port is driver for i2c bus, which exploits
features of modern bidirectional parallel ports.
Bidirectional ports have particular bits connected in following way:
|
/-----| R
--o| |-----|
read \-----| /------- Out pin
|/
- -|\
write V
|
---
It means when output is set to 1 we can read the port. Therefore
we can use 2 pins of parallel port as SDA and SCL for i2c bus. It
is not necessary to add any external - additional parts, we can
read and write the same port simultaneously.
I only use register base+2 so it is possible to use all
8 data bits of parallel port for other applications (I have
connected EEPROM and LCD display). I do not use bit Enable Bi-directional
Port. The only disadvantage is we can only support 5V chips.
Layout:
Cannon 25 pin
SDA - connect to pin 14 (Auto Linefeed)
SCL - connect to pin 16 (Initialize Printer)
GND - connect to pin 18-25
+5V - use external supply (I use 5V from 3.5" floppy connector)
no pullups requied
Module parameters:
base = 0xXXX
XXX - 278 or 378
That's all.
Daniel Smolik
marvin@sitour.cz