Trying to wire an RS-422 cable

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geraldholdsworth
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Trying to wire an RS-422 cable

Post by geraldholdsworth » Tue Jun 11, 2019 3:30 pm

Hi all,

I have a little quandry, which is nothing to do with Acorns. But I'm asking here because this is full of clever chaps and chapesses, and my brain is turning to mush trying to work this out.

I'm trying to wire, for a client, an RS-422 serial cable with DB9 and each end. One end is a Fostex, of which the spec is (from what I can work out from the attached circuit):
Fostex DB9
1 GND
2 !TX
3 RX
4 GND
5 N/C
6 GND
7 TX
8 !RX
9 GND

and the other end is an AMX Central Controller, of which the spec is:
AMX DB9
1 RX-
2 (used by RS232)
3 (used by RS232)
4 TX+
5 GND
6 RX+
7 (used by RS232)
8 (used by RS232)
9 TX-

Right...now for the head messer. There is already a cable in place going to another AMX port (which is an added on card). The spec of this is (and this is not a DB9):
AMX Cardport
1 GND
2 (used by RS232)
3 (used by RS232)
4 (used by RS232)
5 (used by RS232)
6 TX+
7 TX-
8 RX+
9 RX-
(there is actually pins 10 to 20, but 10 is +volts, and 11 to 20 is a repeat of 1-10 but for the second RS-232/422/485 port on the card).

The current wiring is thus:
AMX Cardport -> Fostex DB9
1 (black)
6 (green) -> 3 (green)
7 (black)
8 (white) -> 8 (white)
9 (red) -> 2 (red)
-> 7 (black)
-> 4 (black)
-> 9 (black)
I am assuming that pin 7 on the AMX goes to pin 7 on the Fostex (from process of elimination). I should point out here that the cable in use, and the one I am using, has four black wires, along with a white, red, green and blue. Each black wire is twisted round one of the other coloured wires along the length making it a four pair cable (my colleagues refer to this cable as a Belden cable).

From the current wiring, I have deduced that the new wiring should be:
AMX DB9 -> Fostex DB9
1 (RX-) -> 2 (!TX)
2 N/C
3 N/C
4 (TX+) -> 3 (RX)
5 (GND) -> 1,4,6 and/or 9 (GND)
6 (RX+) -> 8 (!RX)
7 N/C
8 N/C
9 (TX-) -> 7 (TX)

but it doesn't add up. Shouldn't the AMX pin 6 (RX+) go to Fostex pin 7 (TX), and AMX pin 9 (TX-) go to Fostex pin 8 (!RX)? Or is this an oddity with RS-422?

Cheers,

Gerald.
Attachments
Fostex 422 ports.png
Gerald Holdsworth
Repton Resource Page
www.reptonresourcepage.co.uk

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geraldholdsworth
Posts: 493
Joined: Tue Nov 04, 2014 9:42 pm
Location: Inverness, Scotland
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Re: Trying to wire an RS-422 cable

Post by geraldholdsworth » Tue Jun 11, 2019 4:11 pm

So I found this online:
4wire_rs422_fig1.gif
4wire_rs422_fig1.gif (3.43 KiB) Viewed 168 times
Which agrees with what I reckon it should be:
AMX DB9 Fostex DB9
RX- 1 RED -> 2 RED !TX
TX+ 4 GREEN -> 3 GREEN RX
GND 5 BLACK -> 9 BLACK GND
RX+ 6 BLUE -> 7 BLUE TX
TX- 9 WHITE -> 8 WHITE !RX

Should I just take the existing wiring as a red herring and carry on regardless?
Gerald Holdsworth
Repton Resource Page
www.reptonresourcepage.co.uk

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1024MAK
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Joined: Mon Apr 18, 2011 4:46 pm
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Re: Trying to wire an RS-422 cable

Post by 1024MAK » Tue Jun 11, 2019 8:14 pm

Yes, try it and see.

RS422, RS423, RS485 and RS232 all should have protection against short circuits etc...

Mark

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