That would work if I could enter the BIOS and set the drives, but I can't.
Anyway, I probed the ISA slots, scope shows all is good. Wrote a program to read/write all the 8 bits through RAM in the RTC, all OK. No stuck data lines, at least not the low byte ones.
Then I removed the ROM on Realtek and probed the socket pins, and what do you know, the /OE is toggling when the /MEMR signal arrives, so the chip is detecting correct address 0xc0000. In fact the address bits on the socket follow ISA slot and I can see the increments. If I pull the ISA slot data pin with resistor to ground I get a bit change on the card. The program works, the read works, so why is there 0xff on the bus.
Well, under the ROM there is a '245 and it's controlled by the Realtek chip to allow reads and writes. And it's permanently disabled. Nothing gets through in any direction. So as a last resort I put the Trident in the slot - and test card shows the ROM data now. So Trident ROM read works but apparently the card itself does not. It could be the ALE signal though. But why doesn't Realtek work? I suspect the card needs some register write to enable it first. So a modern BIOS will do it while probing hardware but these old ones don't do that. One more reason to hate this piece of sh... The card still works in a 386 system, I checked.
You said it's unlikely that I have 2 cards that don't work, but I'm pretty sure now I'm just unlucky with this combo and that is exactly my problem. I see a Trident 8900C for sale, nice clean card and I can live with the price but it's missing RAM. I have two more working DRAM chips so I can at least use the VGA modes, so I think I'll buy it.