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The Answer Guy

By James T. Dennis, tag@lists.linuxgazette.net
Starshine Technical Services, http://www.starshine.org/

(?)High Speed Serial (RS422) under Linux

From Sujeetharan Sivasubramaniyam on 18 Aug 1998

Hi I want to use RS422 card under linux. I am not sure whether this is suppored by linux already. Also I would like to see some documentation on this. Any pointers will be greatly appreciated!


Since you don't mention which specific card you are hoping to use, I'll assume that you are willing to buy a new/supported card if necessary.
Naturally, with any question about supported hardware, the first place to check is the Hardware-HOWTO (http://sunsite.unc.edu/LDP/HOWTO/Hardware-HOWTO.html).
At first we might be led astray (searching on serial gets us to section 26.6: "Modems and Serial Cards" --- but that's in reference to PCMCIA cards). However, we don't find any references to 422 under "Controllers, I/O" and "Controllers, Multiport" nor anywhere else in the HOWTO.
Running the Excite search on the LDP site using the string "422" doesn't net any useful results either.
So, we have to dig a bit deeper. A Yahoo search using the string "+rs422 +linux" nets about 53 hits. The first of these is from NTLUG (North Texas Linux User's Group) which lists a company called Vikom, in Irving, TX with the note:
"manufactures RS232 and RS422 multiport cards for Linux"
... No URL or contact information is provided.
Another result of this Yahoo! search is for GTek Inc. (a long-time manufacturer of serial hardware for PC's).
They list a Cyclone 6 and note that there are "RS422 and RS485 interfaces available" and separately they say:
"Drivers are available for all popular operating systems: WindowsNT, Windows95, Linux, OS/2 and DOS."
(Granted, this second lead is buried under about a dozen unlikely results that I didn't follow).
You might also search the Equinox, Digi, Specialix, and other web pages for manufacturers of specialized and multi-port serial hardware to see if they have something that suits your needs.
Presumably there is some RS422 support built into the Kernel --- at least in the PPC (PowerPC) port. As far as I know most Mac's and Mac clones shipped with built-in RS-422 (a.k.a. EIA-422) ports for their modems (and printers?).
I also found a note that seems to confirm my own experience --- that is is possible to interface RS-422 connectors to most RS-232 devices using the proper cabling (the classic "Mac to PC Modem" cable). There is even one message about using a special cable to connect an Apple printer to a PC's serial port. This suggests that some RS-422 devices can be interfaced to RS-232, as well. (I mention this since your requirements aren't clear from your message --- potentially you don't need an RS-422 interface at all).
Obviously if you already have a specific device and you are interested in details about that RS-422 card you'll have to provide details about it. Since I don't have any first-hand experience with any RS-422 hardware on PC's and I've shown exactly how I did my searches, it would make sense to do your own searches for support on your card. If you can't find a manufacturers label or any docs on the card (i.e. you inherited it in "used" condition) then you're probably out of luck.

Copyright © 1998, James T. Dennis
Published in Linux Gazette Issue 32 September 1998

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