MDA and CGA video are supported directly by the PC/XT BIOS on the motherboard.
EGA and VGA cards have their own onboard BIOS chip.
This means that you need to disable the video BIOS on the motherboard, else it will try to initialize an MDA or CGA adapter, and EGA/VGA aren't compatible at that level.
On the IBM PC/XT and many clones, this is done via some dipswitches or jumpers on the board.
I believe this documents the machine you have: https://stason.org/TULARC/pc/motherboards/Z/Z … 8088-Z-159.html
It seems to have a 'monitor' selector, labeled SW101, consisting of two dip-switches, and you need to set both to 'on' for the EGA setting.
If that doesn't work, you can try setting it to monochrome (both 'off'), as the PC standard supports a monochrome and a colour card working together.

Hope this helps.

That's amazing, thanks! I'll give it a shot. 8-bit machines are almost foreign to me and I forget how much changed from PC/XT to AT because I've never had hands-on experience with them.

I have a few old XT's and some do not like the switches set to anything but mono as Scali said, if using a VGA card, it depends on the motherboard bios. Good luck !

brassicGamer wrote on 2023-08-13, 21:36:

That's amazing, thanks! I'll give it a shot. 8-bit machines are almost foreign to me and I forget how much changed from PC/XT to AT because I've never had hands-on experience with them.

Technically it works the same on AT machines, except that the settings can be controlled from the BIOS setup, instead of having to set dipswitches.
But there will be some setting for the video type, which can usually be set to mono, CGA or EGA/VGA.
I also think that many AT BIOSes will try to autodetect the video in case of an error, so it generally 'just works'.
Although, I don't think I actually ever used an MDA, Hercules or CGA card in an AT or better, so I've always used a card with its own BIOS.

Damn, it didn't work. On inspection the switches (labelled "monitor type") were already set to EGA, which I now remember doing previously, even though I was successfully running a CGA cards in there.

I will have to investigate further. There exists a manual for the Z-150, but this is architecturally different in that the display circuitry was integral to the main boards. The Z-159 came with EGA as standard so it's weird.

Someone else recently had issues trying to use a 8bit VGA on a XT with no success (vid card was not initialized, no monitor out) ended up being the motherboard bios. Once upgraded it worked fine.
Which EGA cards are you trying to use ?

Horun wrote on 2023-08-16, 00:41:

Which EGA cards are you trying to use ?

It came with your standard PEGA2 Paradise card so that's the first one and the second is an EGA Wonder VIP, which has EGA and VGA. Both cards are known good.

The computer has a built-in 'Monitor' program that runs from ROM and can be loaded at startup and at any time the system is running. If I can find a way to get CGA output (or maybe composite) I could check that for any possible settings.

Update: dug out a composite monitor I forgot I had and the error message I couldn't see before is "incorrect video configuration"! It was set to monochrome but I had a CGA card installed, so I switched it and it works. Now I just need to try the EGA setting again and see what I get. Removing the CPU board in this thing is very tricky.

Update: Fixed! It's quite possible I had the DIP switch set incorrectly, which start at 0 for some reason (arrays, I understand, but humans don't tend to 0-index things) plus needed to set the switches on the EGA card itself. Magiduck is being screwy because it changes resolution to CGA in-game and the refresh rate goes out of range.