Kahenraz wrote on 2024-02-18, 21:49:

I thought that I would experiment with a Dell Optiplex 760. It seemed like a good fit for a retro machine with Windows 98 and possibly even DOS support. However, it does not have any native PS/2 ports and the legacy USB support is completely broken.

Actually, I believe these machines (socket 775 Optiplex-es) should all have native PS/2 support. For whatever reason, Dell just didn't fit a PS/2 port on the motherboards for this generation of Optiplex-es. However, most of them do have a header on the motherboard that allows you to add PS/2 ports (and a Serial port) via an add-on bracket, like this:
download/file.php?mode=view&id=186297
That being said, before you go on eBay or the like to purchase one, make sure your motherboard does actually have the header for it. It should be labeled "LPC" on the motherboard and can be found above the floppy header (also close to the PCI-E slot.) It's a 14-pin header in a 2x7 pin configuration.

If your motherboard does not have one installed but does have the space / holes for it, then you can solder one in, provided you're up to this task. That's what I did on a Dimension E5150 motherboard I have, which is from the same socket 775 era (model name "Connolly", which I believe it's the same model or a very similar one to what's in an Optiplex 620.) In my case, I didn't even have the proper Dell PS/2-Serial header, so I made my own with Ethernet wire and a spare PS/2 bracket from an industrial PC. The pinout of the industrial PS/2 bracket was different, so I had to do some tracing work to wire it properly, but it ended up working fine and on the first try. If interested and in case you cannot find a Dell PS/2 bracket for sale, let me know and I'll try to post the info here of what I did. I still have the E5150 and luckily it's not buried under a pile of stuff.

I also have an Optiplex 760, but it's currently in storage and I probably won't get to it for another week or two.

Kahenraz wrote on 2024-02-18, 21:49:

Has anyone ever tried using one of these old Dell machines for a retro computer and found a workaround for the broken USB keyboard support in DOS and Windows setup?

Now that, I have not tried. On both my Dimension E520 and E5150, I only set these up with Windows 7 and XP. I'm way behind on my Win9x builds (have a lot of machines/hardware, but haven't dedicated the time for it.)

These optiplex does have PS/2 ports, need to purchase this one.

Cheers.