Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.3 4.3bsd-beta 6/6/85; site ucbvax.ARPA Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxn!ihnp4!houxm!mtuxo!mtunh!mtung!mtunf!ariel!vax135!petsd!pesnta!greipa!decwrl!decvax!ucbvax!info-vax From: info-vax@ucbvax.ARPA Newsgroups: fa.info-vax Subject: Re: VT241 (really cabling woes) Message-ID: <8966@ucbvax.ARPA> Date: Thu, 11-Jul-85 06:08:16 EDT Article-I.D.: ucbvax.8966 Posted: Thu Jul 11 06:08:16 1985 Date-Received: Sat, 13-Jul-85 16:23:40 EDT Sender: daemon@ucbvax.ARPA Organization: University of California at Berkeley Lines: 45 From: Bob SutterfieldAs Richard Garland @CU20B has pointed out, pins 1 (protective gnd) and 7 (signal gnd) are wired together on most DEC VT devices. I had this confirmed for me in a most distressing situation by a local Terminals Field Service rep: My standard station cabling connects pins 2, 3, 7, and 20, with 2 and 3 crossed (standard `null modem'). When we wired an office complex with many users on Rainbows (acting like VT102's) to Xyplex cluster controllers, most came out allright. When one secretary turned hers on one morning, with new station cable connected to COMM port, she noticed funny lights, sounds, and smells coming from her machine. Upon further investigation (most of the day), it turned out that the office had been wired for power about 23 years ago, before it was common practice to wire the ground plug of the 120V outlet to a real ground. All the `ground plugs' in that complex are tied to the neutral leg of the AC!!!! This makes them float wherever they happen to go, with no reason to agree on a common reference. In short: ground plug goes to chassis of Rainbow, into RS232 asynch controller area, where pin 1 (`protective' ground, remember?) is wired to pin 7 (signal ground), thence over station cable into cluster controller (on pin 7), where pins 1 and 7 aren't wired together, and pin 1 is really wired to power ground. Well, it turns out that there happened to be somewhere near *82 volts* between the two `grounds' that day. That voltage pushed current through the ICs on boards in both the cluster controllers and the Rainbow. Chips really exploded, and signal traces on the printed circuit boards acted like fuses (several layers deep). And, I had no idea what the problem with the cluster was (thought it was software at first), so I used some (clear throat) ineffecient diagnosis techniques, roasting yet more equipment. The carnage: Two Rainbow motherboards Two Xyplex cluster controllers Probably around $7000 in all. All was replaced under maintenance, although neither Xyplex nor DEC really should have - it was a personal-favor-for-a-big-customer situation on both counts. The moral: Check the validity of your grounds before installing any DEC terminal products in an area with older wiring. Mothers, don't let your babies do what I done...