rpocc wrote on 2024-02-05, 23:16:

The main question here is if there any well-known 16-bit Multi-IO card (such as Prime2, or something generic like that), having the similar 8-bit compatibility mode as TVGA cards, or a known method of connecting a standard drive to this 20-pin interface? I know about specific modern cards but I’m effectively unable to acquire one of these due to logistic issues.

Of course, I could install an XTIDE ROM but anyway I’d have to insert an 8-bit IDE card, which I don’t own, and the main goal is support for floppies.

Many generic multi-IO cards work fine on XT machines, for floppy drive, serial and parallel ports. IDE will only work with 16-bit ISA. XT BIOS can only access up to 720kb floppies, but with the help of a program called 2M-XBIOS, XT machines can read and write 1.44 floppies. With a GOTEK floppy emulator, up to 6MB "floppies" can be used.

Some Trident cards have a different BIOS version that won't work in 8-bit mode.

rpocc wrote on 2024-02-05, 23:16:

1. My MB works with a number of 8-bit MDA/EGA cards, outputting text to my amber dispkay when SW1 switches 5-6 are both at “ON” position, otherwise it just performs triple long-short beep code and doesn’t show anything, including Trident in 8-bit mode with any of 4 possible combination of switches. Is there any way to make it work with this VGA card, including switching to alternative firmware ow something?

With Trident 8900D cards, you need to set a variety of jumpers on the card itself to set it to 8-bit mode - it's not required for 8800CS, 8900B or 9000i-3 but I can't speak for other chipsets.

The main question here is if there any well-known 16-bit Multi-IO card (such as Prime2, or something generic like that), having the similar 8-bit compatibility mode as TVGA cards, or a known method of connecting a standard drive to this 20-pin interface? I know about specific modern cards but I’m effectively unable to acquire one of these due to logistic issues.

Contrary to what the previous poster said, you can use a 16-bit multi I/O card for IDE on a XT - I'm using a Pine PT-606G with my NuXT (as the built-in IDE is broken for my requirements) but the important thing is to make sure that the XTIDE ROM is explicitly set to 16-bit controller in 8-bit mode. That said, there are reports of it being dependent on the multi I/O chipset.

DEAT wrote on 2024-02-06, 05:38:

With Trident 8900D cards, you need to set a variety of jumpers on the card itself to set it to 8-bit mode - it's not required for 8800CS, 8900B or 9000i-3 but I can't speak for other chipsets.

Thanks, I've made a series of tests with couple of dozens various 9000B/C/I/I-1/I-3 cards and found out that only 9000B chipset works with my XT. Both cards have VGA BIOS of version D3.0.

Will try my IO cards later. Mostly it's:
NoName/LCS/LG/China (Goldstar Prime 2/2C, UMC UM82C863F/865F, HMC HM83740(Q), Winbond W83757(A)F)
PROTECH LE-451
Protech PM-510A
JPN Corp. CA8302E-1 (CA8391, CA8392, CA8392F)
WDXT-GEN F300A
WD42C22A-JU

Gustavo wrote on 2024-02-06, 00:15:

Some Trident cards have a different BIOS version that won't work in 8-bit mode.

I have a hard suspicion that later BIOS does work in 8-bit slots, but only with 16-bit processors, in 286/386 motherboards with 8-bit slots. Maybe, with 8086 as well.
My "XT" processor is 8088 and probably it requires only a 100%-compatible 8-bit code, which doesn't rely on MPU, UMB or something like that.
I've tried to put a 3.0 BIOS from 9000B card to a random 9000C card and got turned-on display with no output, and no signs of life with vice-versa, so apparently they're not interchangeable.

Hi. It's possible to use some IDE ports found on multi i/o cards in an XT or in an 8-Bit slot.

I think that depends on the card design and the software.
CF cards and some HDDs can work with an 8-Bit interface, too.

Edit: I heard some HDDs may lose storage capacity if being used with 8-Bit bus, but still work.
What seems to happen is that the lower or upper 8-Bit are being lost/ignored due to lack of data lines on PC bus.
Not sure if that's true, though. My memory is a bit sketchy here.

And by using multiplexing, an 8-Bit bus can handle 16-Bit data, too.
I think later revisions of XTIDE hardware do use that technique.

That way, from a HDD's point of view, the interface looks like an ordinary IDE port.
No special support is needed that way, which increases compatibility.

But strictly speaking, from a hardware perspective, IDE as such was based on 16-Bit wide ISA bus, rather than the original PC or PC/XT bus (8-Bit "ISA").
So the previous poster wasn't wrong, either. IDE is a variation of 16-Bit ISA.

I mean, that's why early IDE HDDs were described as being AT-Bus HDDs, or why IDE also was known as ATA (AT Attached).

AT refers to the AT bus here, the bus originally used in the IBM Model 5170 (IBM AT).
- Which in turn was being built around the 80286 processor.

AT bus was essentially the 80286 front side bus (early PCs didn't use chipsets yet. Everything was sitting on the bus in parallel).

When the 32-Bit wide EISA bus was being specified by the Gang of Nine in the late 80s, the underlying AT bus was being specified as ISA, as well.
(The new name also circumvented a trademark issue with IBM.)

That's why it settled for ~8 Mhz, also.
The IBM Model 5170 never was released in a faster version (there was an 6 and 8 MHz version).

Edited.

rpocc wrote on 2024-02-05, 23:16:

The only 8-bit disk IO card I have is a WD MFM 34-pin HDD interface having two 20-pin presumably FDD-interfaces, non-familiar to me, since all FDD I own are a standard 34-pin drives.

Those aren't FDD interfaces. An MFM HDD has two cables, the large one is shared when two drives are connected to that controller.

DEAT wrote on 2024-02-06, 05:38:

Contrary to what the previous poster said, you can use a 16-bit multi I/O card for IDE on a XT - I'm using a Pine PT-606G with my NuXT (as the built-in IDE is broken for my requirements) but the important thing is to make sure that the XTIDE ROM is explicitly set to 16-bit controller in 8-bit mode. That said, there are reports of it being dependent on the multi I/O chipset.

Previous poster was correct. If we look into it in more detail a real ISA IDE controller is on the HDD itself. What we commonly call an "IDE controller card" is just an address decoder circuit (0x1F0-0x1F7 and 0x3F6) - can be as simple as one PAL chip - and a physical adapter routing ISA pins into 40 PIN ribbon cable connector. Thats all there is to an ISA "IDE controller".

This is a legit fully featured ISA "IDE controller":

In fact its vastly overbuilt. That chip on the right, als245, is fully optional 😀 DIY 16-bit ISA IDE interface?

Your Pine PT-606G is weird/more advanced, because instead of just routing IO it seems um82c863f buffers and invisibly translates between 8-16 accesses.

^Hi, yes. Another lesser used term was "IDE Host Adapter".
It's essentially just an ISA bus interface, with a limited address range.

Real IDE "controllers" may exist for EISA/MCA/PCI and can do fancy stuff like bus mastering.
Provided that the HDD can do it, too.