Bring XT floppy controller from Compaq Portable back to life

[modem7]

Well I'll get back to this once I'm able to write something on the keyboard. It still needs some restauration.

Per the last few posts, issuing commands to the controller chip on the floppy controller can be done in a number of ways (even via DEBUG),
What works and what doesn't work narrows down the circuitry to look at on the controller.

... but it lets you talk directly to the uPD765-based FDC (I assume that's what the portable uses)

@Dallas_Green
The P8272A on your controller card will be functionally equivalent.

 

[Chuck(G)]

After dealing with floppies since the mid-70s, they're a bit boring.

 

[mogwaay]

So do almost all of the Sydex utilities, as well as ImageDisk use direct hardware interfacing. No BIOS there.

I don't know about the Sydex utils will add to my list next time I'm fiddling with my floppies. ImageDisk is awesome but I could only get it to work in DMA mode, so I could not use it in my machine and i just wondered if being able to use FDC in non-DMA mode (PIO mode) might help rule DMA issues out for the OP.

 

[Chuck(G)]

DMA is the standard for PC compatibles. IMD is no exception and neither are the Sydex utilities--and a whole bunch of other ones. To wit, from the PC98 specification (Microsoft), chapter 18, page 288:

Not saying that it's not possible, but non-DMA transfer on a PC is a real pain and almost always involves disabling interrupts of one kind or another; the standard FDC chip based on the NEC µPD765 does not have any internal buffering for data, so a "bubble" can mean errors.

 

[mogwaay]

Not saying that it's not possible, but non-DMA transfer on a PC is a real pain and almost always involves disabling interrupts of one kind or another; the standard FDC chip based on the NEC µPD765 does not have any internal buffering for data, so a "bubble" can mean errors.

Tell me about it! It took me ages to get PCjr-esque read-only DMA-less floppy disc support into Sergeys 8088 BIOS code to work in my 'XTjr' homebrew machine.

It's just neat that OmniDisk lets you read floppies in a DMA-less mode and doesn't need BIOS support (I think it can use either polling or interrupts instead) which might help to see if it's DMA issues that might be causing a Floppy controller on a PC to fail, especially if it's a PC/XT class which isn't using DMA for much else - (hmm apart from memory refresh which would probably make itself very obvious very quickly).

 

[Dallas_Green]

Hello community
After some weeks without touching this hobby I found someone who sold to me a spare floppy controller of the Portable 1 and it worked immediately. So it's definitely the controller itself which doesn't pass the data.

Since on the motherboard I had to replace single ICs as well I guess there is also a broken one on the floppy controller here. Actually it has the same ICs SN74LS245N soldered which failed that time on the motherboard. So I go for them first and then lets see whether I can bring this controller back to life. =)

Regards Dallas