Date: Fri, 16 Feb 1996 20:43:26 +1100 (EST) From: michael butler <imb@scgt.oz.au> To: msmith@atrad.adelaide.edu.au (Michael Smith) Cc: jkh@time.cdrom.com, hackers@freebsd.org Subject: Re: Multidrop serial (422/485) driver Message-ID: <199602160943.UAA29040@asstdc.scgt.oz.au> In-Reply-To: <199602160756.SAA27443@genesis.atrad.adelaide.edu.au> from "Michael Smith" at Feb 16, 96 06:26:02 pm
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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. > The 2000 olympics stadium at Homebush in Sydney is riddled with the stuff > (cue Michael Butler 8), .. Not quite yet .. first phase is the Cricket Ground and Football Stadium site (where the soccer will be played) with integrated fire and intrusion alarm monitoring (being implemented now). Building and plant monitoring management (including irrigation) will follow. Because it's IP-encapsulated, I can route it the 4km to here (at home) or across the (~14km) ISDN link to Homebush as easily as any other data stream. If any "disaster" occurs to that link (which will be B-ISDN by 2000) it's no pain at all to dynamically re-route over a standby microwave channel. At no time, however, do I expect the comms link to be real-time .. IP just isn't .. what I do expect is to be able to download event scheduling information and since each micro has a clock with 5mS resolution .. it just looks like it does. Having all but ditched the commercial unix implementations, FreeBSD now forms the core of this network. When it's all up and running as I want it, I will be making more "noise" about this :-) > .. we use it for communications between our data acquisition systems and > the associated radar transmitters; with an optically-isolated interface it > doesn't suffer for being run in the same cable trays as the main antenna > feeds (100KW+ @54MHz). The SCG also have about 30km of fibre installed .. not so much because of RF EMI (we only run weather radar, Bendix hand-helds and the local mobile-phone cell) but because of lightning strikes .. averaging the demise of one colour CCTV camera every two weeks at the moment .. ~75 metre high lighting towers don't help :-( michael
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