--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/DUART-cable	Sat Dec 14 06:24:24 2019 +0000
@@ -0,0 +1,95 @@
+The Calypso chip has two UARTs, called MODEM and IrDA in the chip docs.  The
+primary UART (MODEM) has 4 signals (TxD, RxD, RTS and CTS), whereas the other
+UART (IrDA) has data leads only (TxD and RxD).  TI's C-Sample and D-Sample
+boards have also established a convention whereby Calypso GPIOs 2 and 3 have
+been assigned as supplementary modem control signals for the primary MODEM UART,
+with GPIO 2 being an output from Calypso signaling DCD to the host and with
+GPIO 3 being an input to Calypso receiving DTR from the host.  Note that DSR
+and RI are still missing in this arrangement.
+
+In FreeCalypso we have further established our own convention: all FC hardware
+products that bring out both Calypso UARTs do so by way of a single 10-pin (2x5)
+2.54 mm header in a fixed pinout given below.  This convention was started with
+FCDEV3B, our first FC hw product, and is now being continued with MMTB1 and
+other boards.  Our standardized DUART header pinout is as follows:
+
+Header pin	Calypso signal
+1		GND
+2		GND
+3		TX_IRDA
+4		TX_MODEM
+5		RX_IRDA
+6		RX_MODEM
+7		GPIO2_DCD
+8		RTS_MODEM
+9		GPIO3_DTR
+10		CTS_MODEM
+
+Pins 7 and 9 were originally left unused (they are unconnected on FCDEV3B), but
+they have been assigned as DCD and DTR (from the host's perspective) starting
+with MMTB1.  Note that while DCD and DTR in the table above are named from the
+host's perspective, all Calypso signals ending with _MODEM or _IRDA are from
+the chip's perspective, i.e., the opposite.
+
+Our standard recommended practice for connecting to our two UARTs from a regular
+PC or laptop is to use FT2232D breakout boards from the following vendor:
+
+http://pldkit.com/other/ft2232d-module
+
+FT2232D is a versatile USB-to-various-things adapter, but in its default
+configuration which we use here it acts as a bridge from USB to two UARTs,
+presenting both UARTs behind a single USB device.  The breakout board made and
+sold by PLDkit has two rows of header pins, featuring 15 pins on each side, and
+the two sides of the board neatly correspond to the A and B sides of FT2232D.
+
+Our standard recommended method for connecting our two UARTs to a PLDkit FT2232D
+board is to use a 10-wire ribbon cable that is custom-assembled as follows:
+
+* One end is terminated with a 10-pin female connector that mates with the DUART
+  header on our modem boards;
+
+* The other end is split into two parts as described below, and each split end
+  is terminated with a 15-pin female connector that mates with one side of the
+  FT2232D breakout board.
+
+The split between A and B sides on the FT2232D end is not symmetric, instead 7
+wires go to side A for the MODEM UART and 3 wires go to side B for the IrDA
+UART.  Assuming the standard color code, the 10-wire ribbon cable gets split
+between violet and grey wires: brown to violet go to side A, grey to black go
+to side B.  After each wire has been stripped and fitted with a crimp-on female
+terminal, FT2232D side A wires need to be inserted into the connector housing
+as follows:
+
+Wire	Pin position	FT2232D signal	UART signal (host perspective)
+brown	2		ADBUS0		TxD
+red	3		ADBUS1		RxD
+orange	4		ADBUS2		RTS
+yellow	5		ADBUS3		CTS
+green	6		ADBUS4		DTR
+blue	8		ADBUS6		DCD
+violet	15		GND		GND
+
+FT2232D side B wires need to be inserted into the other connector housing as
+follows:
+
+Wire	Pin position	FT2232D signal	UART signal (host perspective)
+grey	2		BDBUS0		TxD
+white	3		BDBUS1		RxD
+black	15		GND		GND
+
+The wires on the other end of the cable (also stripped and fitted with crimp-on
+female terminals) need to be inserted into another connector housing that will
+mate with the DUART connector on FreeCalypso modem boards; the pin assignment
+is as follows:
+
+Pin position	Calypso signal	Wire
+1		GND		black
+2		GND		violet
+3		TX_IRDA		white
+4		TX_MODEM	red
+5		RX_IRDA		grey
+6		RX_MODEM	brown
+7		GPIO2_DCD	blue
+8		RTS_MODEM	yellow
+9		GPIO3_DTR	green
+10		CTS_MODEM	orange