RussellMc writes: >=20 > Another nice "trap" is to have two "Y" capacitors on the input side from > each AC line to chassis ground, and have output ground also connected to > chassis ground, BUT no external ground connected to chassis ground. The > chassis is now at (about) VAC/2 and various other couplings may then allo= w > various magic to work. >=20 > With typical Y cap values you can typically feel the 1/2 AC chassis=20 > voltage > as a not-nice sharp but not too too painful bite on the backs of your > fingers. Touching the chassis with a firm finger contact MAY not cauise=20 > any > shock sensation. >=20 > Connecting the equipment to another item via a signal lead that is chassi= s > grounded at both ends (eg PC to printer with only one of the two items > having a mains ground) can result in destruction of the port at one and/o= r > other end and possibly more complete destruction of either item. Long lon= g > ago I had a new Centronics-cable-connected printer destroyed on first=20 > being > plugged into a PC's printer-port because only one of the two had an earth > wire in its cord :-(. (That's one way to learn such lessons). My last full-time job lasted from 1990 until retirement in 2015. I worked as a systems engineer in the Network Operations group for Oklahoma State University. Times were certainly different in 1990 and OSU had a new Ericsson phone and data system plus a campus-wide serial data network. One could rent a telephone which had what was called a TAUT or Telephone Access Unit Telephone which had an Ericsson voice telephone unit and a module on the rear with a RS-232 25-pin connector that was female, just like a dial-up modem. It even answered to Hayse commands for dialing, etc sent from a connected computer. I got a call, one day, from our help desk in which someone told me that on some of those TAUT's customers were reporting that the light signalling data transmission and reception were flickering even when there were no data being passed, not even any connection being made. I asked the lady who was telling me all of this whether or not people were getting shocked or had any damage to the TAUT or their computer as this was concerning to those of us who know why this might be happening. As far as I know, nothing ever burned up or nobody got shocked that way but I think we just had a big ground/Earth loop between the telephone and whatever Earth was where the subscriber was. These telephones were not ISDN but an Ericsson proprietary 128-KB/S data format in which 64 KB was the voice channel and another 64 KB could be a second voice or data channel. You could send and receive data at 9600 baud or, if you didn't need voice, the TAUT would work at 19,200 baud. Back then, we thought that was fast. I never found out how much ground loop voltage was involved but had that been an analog phone line, there probably would have been quite a hum. Martin McCormick WB5AGZ --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .