diff --git a/README b/README
index 14ec93d..11bf50c 100644
--- a/README
+++ b/README
@@ -143,8 +143,8 @@
 ** Use it
 
 If you have correctly built and flashed the firmware, you will have
-the Pico led blinking at the rate of twice a second, and a couple of
-/dev/ttyACM* being advertised by your host:
+the Pico led blinking at the rate of twice a second, and one or two
+/dev/ttyACM* devices being advertised by your host:
 
   [708023.097129] usb 1-4: new full-speed USB device number 72 using xhci_hcd
   [708023.265195] usb 1-4: New USB device found, idVendor=2e8a, idProduct=000a, bcdDevice= 1.00
@@ -169,25 +169,22 @@
   This is the Central Scrutinizer
   Control character is ^_
   Press ^_ + ? for help
+  P0: Probing port 0
   P0: VBUS OFF
-  P0: Device ID: 0x91
+  P0: Device ID: B_revB (0x91)
   P0: Init
   P0: STATUS0: 0x80
   P0: VBUS OFF
   P0: Disconnected
   P0: S: DISCONNECTED
-  P0: Empty debug message
-  P0: IRQ=0 10 0
-  P0: Connected: cc1=2 cc2=0
+  P0: Connected: cc1=2 cc2=1
   P0: Polarity: CC1 (normal)
+  P0: VCONN on CC2
   P0: VBUS ON
   P0: S: DFP_VBUS_ON
-  P0: Empty debug message
-  P0: IRQ=71 4 0
   P0: S: DFP_CONNECTED
   P0: >VDM serial -> SBU1/2
-  P0: IRQ=71 4 1
-  P0: <VDM RX SOP"DEBUG (5) [504f] 5ac8052 91340000 306 0 0
+  P0: <VDM RX SOP"DEBUG (5) [508f] 5ac8052 44740000 306 0 0
 
 If you see the ">VDM serial -> SBU1/2" line, the serial line should
 now be connected and you can interact with the M1. Note that you can
diff --git a/m1-pd-bmc.h b/m1-pd-bmc.h
index 83abc9c..9e62316 100644
--- a/m1-pd-bmc.h
+++ b/m1-pd-bmc.h
@@ -59,6 +59,8 @@
 void set_upstream_ops(bool serial);
 bool upstream_is_serial(void);
 
+int get_valid_ports(void);
+
 void upstream_tx_str(int32_t port, const char *ptr);
 
 #define PRINTF_SIZE	512
diff --git a/start.c b/start.c
index 8bcc75e..383a5b5 100644
--- a/start.c
+++ b/start.c
@@ -7,6 +7,13 @@
 #include "m1-pd-bmc.h"
 #include "FUSB302.h"
 
+static uint8_t valid_ports = 0;
+
+static inline int __get_valid_ports(void)
+{
+	return (valid_ports == 3) + 1;
+}
+
 static const struct gpio_pin_config m1_pd_bmc_pin_config0[] = {
 	[M1_BMC_PIN_START ... M1_BMC_PIN_END] = {
 		.skip	= true,
@@ -229,12 +236,18 @@
 		gpio_pull_up(pin->pin);
 }
 
-static void m1_pd_bmc_system_init(const struct hw_context *hw)
+static bool m1_pd_bmc_system_init(const struct hw_context *hw)
 {
 	init_system(hw);
 
 	for (unsigned int i = 0; i < hw->nr_pins; i++)
 		m1_pd_bmc_gpio_setup_one(&hw->pins[i]);
+
+	/*
+	 * If we can't infer pull-ups on the I2C, it is likely that
+	 * nothing is connected and we'd better skip this port.
+	 */
+	return gpio_get(hw->pins[I2C_SCL].pin) && gpio_get(hw->pins[I2C_SDA].pin);
 }
 
 static bool apply_waveshare_2ch_rs232_overrides(void)
@@ -259,6 +272,9 @@
 
 static void usb_tx_bytes(int32_t port, const char *ptr, int len)
 {
+	if (__get_valid_ports() != 2)
+		port = 0;
+
         if (!tud_cdc_n_connected(port))
 		return;
 
@@ -283,6 +299,9 @@
 {
 	uint8_t c;
 
+	if (__get_valid_ports() != 2)
+		port = 0;
+
 	if (!tud_cdc_n_connected(port) || !tud_cdc_n_available(port))
 		return -1;
 
@@ -372,6 +391,11 @@
 
 static int wait_for_usb_connection(void)
 {
+	bool connected;
+	int ports;
+
+	ports = __get_valid_ports();
+
 	do {
 		static bool state = false;
 		static int cnt = 0;
@@ -383,9 +407,18 @@
 		cnt %= 256;
 		if (!cnt)
 			state = !state;
-	} while (!tud_cdc_n_connected(0) && !tud_cdc_n_connected(1));
 
-	return !tud_cdc_n_connected(0);
+		connected = false;
+		connected |= tud_cdc_n_connected(0);
+		connected |= ports == 2 && tud_cdc_n_connected(1);
+	} while (!connected);
+
+	return ports == 2 && tud_cdc_n_connected(1);
+}
+
+int get_valid_ports(void)
+{
+	return __get_valid_ports();
 }
 
 int main(void)
@@ -398,10 +431,15 @@
 	success = set_sys_clock_khz(133000, false);
 
 	board_init();
+
+	if (m1_pd_bmc_system_init(&hw0))
+		valid_ports |= 1;
+	if (m1_pd_bmc_system_init(&hw1))
+		valid_ports |= 2;
+
 	tusb_init();
 
-	m1_pd_bmc_system_init(&hw0);
-	m1_pd_bmc_system_init(&hw1);
+	sleep_ms(200);
 
 	if (apply_waveshare_2ch_rs232_overrides()) {
 		set_upstream_ops(true);
@@ -411,6 +449,8 @@
 		port = wait_for_usb_connection();
 	}
 
+	upstream_ops->flush();
+
 	__printf(port, "This is the Central Scrutinizer\n");
 	__printf(port, "Control character is ^_\n");
 	__printf(port, "Press ^_ + ? for help\n");
@@ -418,8 +458,9 @@
 	if (!success)
 		__printf(port, "WARNING: Nominal frequency NOT reached\n");
 
-	m1_pd_bmc_fusb_setup(0, &hw0);
-	if (!upstream_is_serial())
+	if (valid_ports & 1)
+		m1_pd_bmc_fusb_setup(0, &hw0);
+	if ((valid_ports & 2) && !upstream_is_serial())
 		m1_pd_bmc_fusb_setup(1, &hw1);
 
 	m1_pd_bmc_run();
diff --git a/usb_descriptors.c b/usb_descriptors.c
index a084ae6..e63ffa6 100644
--- a/usb_descriptors.c
+++ b/usb_descriptors.c
@@ -44,8 +44,7 @@
 
 #define USBD_MAX_POWER_MA       (250)
 
-#define USBD_DESC_LEN           (TUD_CONFIG_DESC_LEN + \
-				 TUD_CDC_DESC_LEN * CFG_TUD_CDC)
+#define USBD_DESC_LEN(n)        (TUD_CONFIG_DESC_LEN + TUD_CDC_DESC_LEN * (n))
 
 enum {
 	ITF_NUM_CDC_0,
@@ -55,11 +54,10 @@
 	ITF_NUM_TOTAL,
 };
 
-static const uint8_t usbd_desc_cfg[USBD_DESC_LEN] = {
-
+static const uint8_t usbd_desc_cfg_dual[USBD_DESC_LEN(2)] = {
 	TUD_CONFIG_DESCRIPTOR(1, ITF_NUM_TOTAL,
 			      USBD_STR_LANGUAGE,
-			      USBD_DESC_LEN,
+			      USBD_DESC_LEN(2),
 			      TUSB_DESC_CONFIG_ATT_REMOTE_WAKEUP,
 			      USBD_MAX_POWER_MA),
 
@@ -80,6 +78,22 @@
 			   USBD_CDC_DATA_SIZE),
 };
 
+static const uint8_t usbd_desc_cfg_single[USBD_DESC_LEN(1)] = {
+	TUD_CONFIG_DESCRIPTOR(1, ITF_NUM_TOTAL - 2,
+			      USBD_STR_LANGUAGE,
+			      USBD_DESC_LEN(1),
+			      TUSB_DESC_CONFIG_ATT_REMOTE_WAKEUP,
+			      USBD_MAX_POWER_MA),
+
+	TUD_CDC_DESCRIPTOR(ITF_NUM_CDC_0,
+			   USBD_STR_CDC_0_NAME,
+			   EPNUM_CDC_0_CMD,
+			   USBD_CDC_CMD_SIZE,
+			   EPNUM_CDC_0_DATA & 0x7F,
+			   EPNUM_CDC_0_DATA,
+			   USBD_CDC_DATA_SIZE),
+};
+
 const uint8_t *tud_descriptor_device_cb(void)
 {
 	return (const uint8_t *)&usbd_desc_device;
@@ -88,7 +102,10 @@
 const uint8_t *tud_descriptor_configuration_cb(uint8_t index)
 {
 	(void)index;
-	return usbd_desc_cfg;
+	if (get_valid_ports() == 2)
+		return usbd_desc_cfg_dual;
+
+	return usbd_desc_cfg_single;
 }
 
 #define DESC_STR_MAX_LENGTH 20
diff --git a/vdmtool.c b/vdmtool.c
index 29228ea..ae3abec 100644
--- a/vdmtool.c
+++ b/vdmtool.c
@@ -666,15 +666,7 @@
 		.serial_pin_set		= 2, /* SBU1/2 */
 	};
 
-	/*
-	 * If we can't infer pull-ups on the I2C, it is likely that
-	 * nothing is connected and we'd better skip this port.
-	 */
-	if (!gpio_get(PIN(cxt, I2C_SCL)) || !gpio_get(PIN(cxt, I2C_SDA))) {
-		cprintf(cxt, "I2C pins low while idling, skipping port\n");
-		cxt->hw = NULL;
-		return;
-	}
+	cprintf(cxt, "Probing port %d\n", port);
 
 	gpio_put(PIN(cxt, LED_G), HIGH);
 	/* No swapping */