Standard pinout for RS-422?

Question

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?!

Answer 1

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:

So there probably indeed is no standard.

Answer 2

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.

Answer 3

There isn't an accepted standard like with RS-232 since there isn't a common PC accessory, and the TIA/EIA-422 spec doesn't specify the connectors. However, there are some common devices that people use, such as MOXA UPort Serial Converters or FTDI, both of which have this pinout for a DB-9:

1 - TxD-
2 - TxD+
3 - RxD+
4 - RxD-
5 - GND

So yes, if you have a specific accessory that has its own pinout, you have to conform to that and not worry about general portability.

Answer 4

As previous answers have stated, there does not appear to be a standard for 422/485 pinout on a DB9 connector.

However, it is interesting to note that if you are designing a board using a transceiver that can switch between different protocols (LTC2870IUFD), or if your equipment is using such a device internally, then your hands are effectively tied.

With this particular LTC device, which contains 2 drivers and 2 receivers and supports 232, 485 and 422, the low voltage TX signal will be connected to Driver Input 1 such that RS232 TX will emerge on the Driver Output 1 (pin 19 on the TSSOP package) in 232 mode. In 485/422 mode, this signal becomes TXD+ and the corresponding TXD- emerges on Driver Output 2 (pin 17 on the TSSOP package).

Without further analog multiplexing on the output side, if these signals are routed to your DB9, then the 422 pinout is effectively defined for you:

For RS232 compatibility the TX/TXD+ signal must get routed to pin 3 (DTE) The second driver is used for RTS in 232 mode, thus the RTS/TXD- signal must get routed to pin 7.

As can be seen, the pinout of the right hand table (above) is emerging. If you continue the exercise, CTS/RX+ end up on pin 8 and RX/RX- on pin 2.