Tried using several computers with a KVM switch that uses the 25th port on the VGA connection for USB (for the keyboard and mouse - StarTech.com SV1631DUSBUK). Manufacture says the cable they use to connect the PC is proprietary and it allows them to eliminate having an external USB cable. For PC's that only have HDMI I am using a HDMI to VGA converter. That works fine so long as the USB part of the cable is plugged in. Once thats plugged in the screen keeps refreshing. This happens both in BIOS and once Linux boots. On systems where there is HDMI and VGA ports it only happens when connecting via the HDMI port. When the USB cable gets plugged in the kernel throws EDID errors. It's strange since I would expect that from the HDMI and not when the USB cable is plugged in. Any ideas what I could possibly disable in BIOS or put in between the USB cable and PC to eliminate the issue or just get a new dumb KVM switch?

  • 1
    I think I would consider getting a less dumb KVM switch. – Michael Hampton Feb 18 '18 at 21:37

If they are saying the cable is proprietary its a 15 pin cable and USB is the 4 pin plus shield so what I am guessing they have done like manufactures before them is hooked the USB onto cables note used by the KVM. For Examples the Vesa monitor ID bits are not needed. One of few things could be happening.

  1. You converter has linked or grounded one of the unused pins that is still connected in the cable causing the refresh.

  2. Pins are sharing a common ground which the converter does not like causing the refresh.

  3. The manufacturer has been really slap dash and the unused pins are still connected in VGA end as well as the USB, although i would hope this is not the case.

  4. The converter is picking up interference from the closely packed cables.

First thing I would do is get the multi meter and check all the pins, draw yourself a little pin out diagram.

Pin 9 used causes a few issues with high end AV gear it used to be a +5V to power the ID Chip for Display Data Channel information but is missing. It used to cause a spliter some issues so we ended up removing the pin everything that connected to it.

VGA Pinout

  • I was thinking the same but the common denominator is HDMI + USB. If I just have the HDMI connected it works fine. If #1 was the case I would have issues with VGA as well. #2 seems plausible. #4 thats possible again but why only when the USB is connected. – Dovid Bender Feb 19 '18 at 15:10
  • Ok so for #1. your Kvm doesn't care about monitor identification so say its using those pins, you HDMI converter might be using them or have them connected and thus causing your problem. #4 because the interference is coming from the USB using the unused cables in the 15pin cable, in my experience HDMI to VGA converters can be very flaky. – RickWeb Feb 19 '18 at 18:22

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