Monitor with Dual Video Inputs -> Ground Pins

Monitor: Dell™ UltraScan P1110
Video Input: 2

This monitor can be connected to TWO computers (say A and B). There is a switch to select which input signal to be displayed. Do you think both computers can be powered on (at the same time) if both of them are connected to the monitor?

I'm wondering about the ground pins. Do you think the ground (in figure below) is referred to earth_ground or 'ground' of the motherboard/display_card? Would there be any potential difference between the ground pins connected to computers A and B?

The ground pins on the video connector are signal return path grounds. The shell of the conncetor is grounded to the case of the computer, which is generally connected through the computer case to the grounding pin of the AC power cord (earth ground).

The signal ground is isolated from the case ground by virtue of the fact that power for a PC is supplied by a switching power supply that provides isolation from the AC line.

Only a fault in the power supply or motherboard would have the ability to create a possible hazard; however, the power supplies and motherboard are protected with fuses and/or polyswitches to guard against just such a hazard.

Both computers can be powered on while connected to the monitor without causing damage. The monitor is designed for two computer operation.

Can I say that the video signals (R, G, B), together with each (return) ground, are differential signals?

No, not differential. R, G, B are single-ended 75-ohm lines.
Differential would have R+, R-, G+, G-, B+, B-, and ground.

The PC's mainboard (and case) happens to be connected to AC mains ground.If everything is fine,that is.
So the PC is grounded even when switched off.This can easily be measured by an ommeter.
The two PC's cases are short-circuited.

House_Cat said:

Only a fault in the power supply or motherboard would have the ability to create a possible hazard; however, the power supplies and motherboard are protected with fuses and/or polyswitches to guard against just such a hazard.

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I have two computers (or CPUs), say A and B. Both computers were connected to LAN, and they shared the same monitor (ie. the Dell P1110). I accessed/controlled computer B from computer A through Remote Desktop (available in Windows XP).

Incident
Both computers were ON.
Computer B was linked to computer A via Remote Desktop.
When I connected a USB peripheral device (ie. digital camera) to computer A, the Intel ICH5 chipset burnt (can see a hole on top of the IC). During the incident, computer A shut down immediately, and after that, it could not reboot anymore.
Then, I powered off computer A, while computer B was still ON (not logged off).
I switched the display of the monitor (using the Select button on front panel) to computer B, but the screen was 'empty' or 'black'. It looked to me that computer B was also affected (e.g. shut down or reboot or whatever reason) during the incident.
Then, I removed the VGA cable of computer A from the monitor.
So, the monitor was only connected to computer B now.
However, I have difficulty to login to Windows in computer B.
After trying few times (reboots), I finally got into Windows, but after using for 5 ~ 10 minutes, 'blue screen' (dump memory screen) appeared and the system reboot itself.
After troubleshooting few days, I finally found out it was caused by a faulty RAM. After I remove the RAM, the system became stable again.

Question
Do you think the faulty RAM in computer B was damaged because of the incident happened in computer A?

Do you think the faulty RAM in computer B was damaged because of the incident happened in computer A?

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If I understand the situation, the only physical connection between the two computers was the connection to the monitor. That physical connection would have been cabinet ground only. The LAN connection, and the video signals arrive at each computer via other chips and circuitry.

There is no direct connection between system RAM and the video output - there is a video card with video chips and an system bridge chip between the video output connector and system RAM. Likewise, there is no direct connection between system RAM and the LAN connector - there are LAN chips and a system bridge chip.

No, there is virtually no way that a USB short on one computer could cause a RAM fault on the other computer.

I suspect your logon problem after the hardware failure on the other computer was related to the fact that you were doing a remote software operation when the hardware failure occured. It probably left the operating system in the B computer in a mess regarding cache clearing, final directory updates - etc. The RAM problem was most likely there all the time, and you found it while the system was trying to restore the OS and clean up the software mess.