I think I can save you some more confusion. Here’s the key to all of this that you’re missing:
RS-422 is a variant of RS-232. Boy, that’s helpful, right?
Actually, it is. Rs 232 and 422 are communications protocols. There are two kinds: one-way and two-way.
Let me list them…clarity is coming, just hang in there.
First, the simple switch lines. These go either high or low and indicate stuff to the devices on each end:
This is oversimplification, but let’s just say that because these are in a steady state i.e. either “on” or “off”, they are simply connected to each other on both ends and the devices sort out what to do after detecting the voltage states.
DSR Data Set Ready
DCD Data Carrier Detect
DTR Data Terminal Ready
Then, there are:
HANDSHAKING
RTS Request To Send
CTS Clear To Send
DATA
TXD Transmitted Data
RXD Received Data
These are ever-changing digital data carriers. They require that the transmit on one end be connected to the receive on the other end… in both directions.
SO… this requires what we call a crossover cable. The transmit pin on one end has to cross over to the receive pin on the other end.
This would be great BUT we’re talking about Mr. “Can’t leave well enough alone” Jobs here remember… he had to do it differently, so he used RS-422 with two pairs of lines for the data and handshaking. Why? they’re balanced pairs. Just like a mic cable. They have far better noise / interference rejection than single wires, so the cables can be longer, and more importantly, unique to the Mac and therefore incompatible with PCs. Instead of the first three indicator lines, it has just one:
GPI General Purpose Input
This gets nuts very quickly when trying to use these ports to connect to the rest of the non-Mac world. Just as you can force a servo-balanced audio output to operate unbalanced by grounding one side, you can do the same to Mac serial outputs as long as you don’t need long lines.
The bottom line is: You need to get MOTU to give you the pinout on their unit. I suspect that you’ll find a standard Mac crossover cable will work, but if not, you can match the pinout once you know what it is.
In the meantime, wire up a standard Mac crossover and try it. I’ll bet it works.
Wire it like this:
1 — 2
2 — 1
3 — 5
4 — 4
5 — 3
6 — 8
7 — 7
8 — 6
Shield — shield
Final notes:
1) Ground goes to ground, naturally.
2) the extra wires in the cable are just that - extra wires.
3) What the hell is a straight-through cable for then? Simple… one-way communications, where one transmitting device is connected to a receiver. This gets confusing in Mac-land because they were known to work sometimes for printers (one-way communications, remember?) even though they were out-of-spec.
4) The GPI is where you input a MIDI clock signal from your serial MIDI interface.
I’m out of time, gotta get back to work. I hope this helps.
I see that while i was writing this, you found a cable that should work… I agree - it should.
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