Yes, I do building power distribution systems in my day job. We never allow separate, isolated grounding to exist due to these problems. Wned a process vendor requests an "isolated" ground rod, he gets a ground rod outside the building, and a copper #4/0 stranded conductor extended to the main building ground (loop, counterpoise, building steel, pipe, etc.). In the case you describe, there was easily chance for a serious ground potential and ground currents. The solution would've been to ground the coax shield at the building entrance. For what it's worth... Bill Kichman, P.E. Cornwall, PA USA -----Original Message----- From: Tim To: PICLIST@MITVMA.MIT.EDU Date: Wednesday, August 04, 1999 8:02 AM Subject: Re: LED Moving Message Display >Using common grounds (-ve wire) from to supplies is a common method in micro >based alarm systems and rearly causes problems. I have however beenadvised >by a client that in a CCTV system, 2 cameras in separate buildings cabled >back to 1 monitor with coax had a problem caused by the building earth >stakes being at different potentials causing the camera to blow up. > >Never encounted the problem myself though. > >Tim Norton >----- Original Message ----- >From: Jonathon Doran >To: >Sent: Monday, August 02, 1999 8:28 AM >Subject: Re: LED Moving Message Display > > >> > Thanks, that's a start, but not quite what I had in mind...looking for a >PIC >> > or similar microcontroller-driven set of multiplexed LEDs to allow >roughly a >> > 7 x 80 pixel field to be controlled. Perhaps use a Hitachi LCD display >and >> > a few buttons for operator interface. Any other suggestions will be >greatly >> > appreciated. >> >> How big do you want it? The HCMS 2975 has a 7*80 field :-) I asked about >> this earlier on the list, with no response so far. >> >> But seriously, you might want to build your display the way they did. A >> 320 bit shift register, with some latches. Serially clock your data into >> the shift register, then latch the display. Shifting 5*7=35 bits and then >> latching will cause the display to scroll. >> >> The problem I'm running into with my displays (and I believe all my >problems >> are now related) is that current surges from the LEDs are causing my PIC >to >> brown out. HP recommends splitting the supplies, but when you have >> pins like: >> Vled >> Vlogic >> GND >> >> on the smaller displays, I have no idea how to wire them up without >causing >> ground loops. Surely HP wasn't asking me to connect the grounds of two >> different supplies together. Perhaps they were... >> >> Returning to our original subject, consider using separate supplies. >> >> Jon Doran >