Date: Fri, 16 Feb 1996 02:43:14 -0800 From: Andrew McRae <amcrae@cisco.com> To: hackers@@freebsd.org Subject: Re: Multidrop serial (422/485) driver Message-ID: <199602161043.CAA19529@doberman.cisco.com>
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michael butler writes: >Michael Smith writes: > >> It's called "industrial control". RS-485 is very heavily used in >> machine interfaces; properly wired it's nearly immune to interference, >> you can run it hundreds of metres over telephone cable, and it takes a >> $0.75 part to interface it to almost any UART or microcontroller. > >It can be considered to be a similar balanced line technology to the HSSI >port at the back of your average Cisco but because we don't want (or need) >E1 or T1 speeds we can be far more "liberal" with both cabling and >termination requirements. > >My test bed runs with 38k4 async between "photo-size" (5"x7") micro >controllers (of my own design) in (almost raw) IP framing .. a little like >an ethernet actually. With sufficient care, it can be made to work extremely >well. This may seem to be a little off the freebsd topic, but maybe not... Why does it seem that only Australian companies are using RS-485! At my previous job (MITS, used to be Megadata) we were using RS-485 to run Token Bus through electrical substations. Because of the differential nature, the length of bus is inversely proportional to the speed. We were running at 2 Mbits, which would run hundreds of metres. The Motorola 68824 TBC interfaced to a RS-485 twisted pair modem; the TBC did all that nasty media access to give you a nice looking LAN which we ran IP over. None of this dinky low speed async stuff :-) The same setup also had freebsd running on an industrial PC running a graphical interface for the substation, gatewayed via ethernet to the rest of the internet (which often included WAN links to other substations etc.). A serious FreeBSD application. Lots of people are using RS-485 in industrial applications, and I applaud the effort to get a working FreeBSD interface to it. Now if only those pesky PLC people would get their act together and start to develop some *real* protocols instead of the crap that they come out with... BTW I wouldn't say that there is a HSSI at the back of your average cisco box, nor is it a good comparison to RS-485. RS-485 is more closely akin to RS-449, just like RS-423 is the differential version of RS-232. HSSI (at 52 Mbits/sec) uses ECL drivers and is point-to-point (which my *new* employer designed :-) Cheers, Andrew McRae (amcrae@cisco.com)
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