| ??? 03/02/04 00:04 Read: times |
#65796 - RE: vga signals isolation problem ( I th Responding to: ???'s previous message |
I think that it's more likely that the chip is getting fried due to electrostatic discharge or from a difference in "ground" potentials when it's hooked up. I think that is precisley what is happening, the signals are terminated by 75 Ohms and 22 Ohms on the computer board. The building is not properly earthed and the monitors might be powered from a different phase than the computers, would this cause any problems? The cable is hopefully matched to the video impedance, and although it's been noted that's "a lot of capacitance", this is not how a matched impedance cable works! The correct cable should present a constant resistive load regardless of how long it is. This is not like RS232; RS232 has an impedance of about 2000 ohms. Video is usually standard at 75 ohms. I guess the cable is matched properly because the video signal on the monitors is extremely good. To protect from ESD and voltage differences, from every signal line to ground, put suppression devices. I would use something like the AVX VC060305A1500 TransGuard. If you can get to the traces on the video card, that's great. However if this can't be done, you might be able to build a module from a M/F pair of VGA connectors. I would try anything to correct this problem, thanx for the advice. If you're located in North America, I can send a few of the suppressors. They are surface mount 0603. Email me off line. Thanks, that is very generous of you, I live in Jordan, have to order them through RS or Farnel I guess. Do I protect both sides of the cable or just the computer side? every now and then a small monitor screen gets blown up also, so I replace it with a newer better quality one, non of the big screens has blown up so far, so it must be protected well. Thank you so much Michael, great comments which could save me and the company a fortune. Mahmood |
