I am making a custom connector that will carry several signals to a test board including RS-422 RX+/RX- and TX+/TX-. The strange thing is that I didn't find a standard pinout for the DB9 connector used for the RS-422!

So I checked the manufacturer of the RS-422 PCI card in my PC and found their own pinout which means that my test board can only be interfaced to this RS-422 from this manufacturer and won't work on solutions from others?! Is that normal for this "standard" protocol?!

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    \$\begingroup\$ There isn't a standard connector or pinout for RS-422, AFAIK. \$\endgroup\$ – Leon Heller Mar 31 '13 at 14:38

From Wikipedia:

RS-422 only specifies the electrical signaling characteristics of a single balanced signal. Protocols and pin assignments are defined in other specifications. The mechanical connections for this interface are specified by EIA-530 (DB-25 connector) or EIA-449 (DC-37 connector), however devices exist which have 4 screw-posts to implement the transmit and receive pair only. The maximum cable length is 1500 m. Maximum data rates are 10 Mbit/s at 12 m or 100 kbit/s at 1200 m. RS-422 cannot implement a truly multi-point communications network such as with EIA-485, however one driver can be connected to up to ten receivers.

So, according to the Wiki, there indeed is no standard for the pin assignments of RS-422 on a DB-9 connector.

In addition to that, different websites say different things:

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So there probably indeed is no standard.

  • \$\begingroup\$ Whenever RS422 and RS485 are mentioned we always ask for the pin assignments because the only standard was for equipment from the same manufacturer..... and even then it was never assumed. We got good at making adaptor leads to cope with it. \$\endgroup\$ – Spoon Mar 31 '13 at 14:54
  • \$\begingroup\$ Thank you. This means that I must check the pinout of every RS422 card I'm going to interface my board to and modify my pinout... Grr! \$\endgroup\$ – Abdella Mar 31 '13 at 16:23
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    \$\begingroup\$ @Abdella I'm sorry, but that's what it looks like. But don't accept the answer too fast! Unaccept it and wait for one, two days. Someone else might come up with a better answer. \$\endgroup\$ – user17592 Mar 31 '13 at 16:39

Most Professional broadcast equipment will use a D-Sub9 with the standard set by Sony:

7 - TX+
2 - TX-
6 - GND

3 - RX+
8 - RX-
4 - GND

The above pinout is for a "Controlled" device such as a VTR. (for a "Controlling" device, TX becomes RX and vice versa).

But as the other contributors have said, you always need to check with the manufacturer. And however things are pinned out, be sure to keep the +/- pairs on the same twisted pair cables. If you, for example, accidentally pair TX+ with RX- on the twisted pair you will get intermittent errors which are nasty to figure out.


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