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30122Re: [midatlanticretro] can a terminal damage a computer?

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  • Mr Ian Primus
    Apr 7, 2013
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      It's going to be really rare for a terminal to damage a computer... beyond possibly burning out the 1488/1489 chips if there is something horribly wrong with the terminal, wired wrong, etc. From what I've read, RS232 spec was designed so that any of the wires could be shorted to any of the others without damaging it. Not that it's foolproof, but...

      Now, there is an exception here if there is bad wiring in the building. If the computer and the terminal are on separate circuits, and one device isn't grounded, AND another fault occurs, then Bad Things can happen, because it's very possible to get a 120v differential between grounds. But, this will be a rather spectacular show of sparks and burning smells. Not a subtle device failure.

      I have many dozens of terminals, and I've never had one damage a computer.

      I had one weird occurrence several years ago while copying some files on an Apple IIe, however. I had the Apple II connected to the PC's RS232 port, and was using Kermit to transfer some stuff back and forth. And, this was one of those rare occasions when all of the covers were actually on everything. Cover on the PC, cover on the Apple II, monitor sitting on top. It was winter, so the air was dry, and in my computer room, I had one of those plastic chair mats, and a wheeled office chair with plastic wheels. This room is wired correctly, and properly grounded.

      I had been typing on the Apple II, moved a foot or two to do something else, went back, and habitually touched the metal bottom casing of the Apple to discharge any static. I get a respectable static zap from it, and then start typing. The serial line isn't responding. I spend a few minutes checking cables, and assume that I must have damaged the RS232 chips in the Apple with the static. Swap out the SSC in the Apple - nothing. Then I happen to try the other RS232 port on the computer - it works. So, I'd burned out the RS232 chip in the computer. The original SSC from the Apple was fine. The only damage was the PC.

      Later on, I happened to have the computer apart and there was a little crater in one of the RS232 line driver chips on the computer's motherboard. Interesting that I was able to blow out the chip like that, with static, on properly wired, correctly grounded machines.

      That has never happened since, and I can't explain it. You'd think the static would have damaged the Apple before the PC - and either way, the metal casing of the Apple is directly grounded. So is the casing of the PC.

      -Ian
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