Atari STE 4 Bombs - WD1772 replacement?

[matt.lacey]

I've been poking around my STE with a multimeter, and can't find any spurious shorts between VCC and GND, power rails look good, and testing continuity gives pretty consistent values across pins on the DMA controller. It does vary a little on the floppy controller, and I'm wondering if that might be why my machine won't boot with the floppy drive or a floppy emu installed.

To that end I looked around and these chips are anything but cheap, and I got to thinking, what is it TOS is expecting from this chip? It seems like there's only a couple of address lines (obviously being decoded somewhere) and then 8 data access lines. There's a few control lines like read/write and chip select. What I'm wondering is whether it's possible to use a PI PICO or other microcontroller to fake the floppy controller, allowing me to boot the machine without a floppy drive, but make it think there is one.

There doesn't seem to be an init command of any sort in the datasheet, but the status register can indicate things like spin up status, so is the machine just checking for a series of particular statuses? If I'm oversimplifying and being an idiot please feel free to tell me to shut up :)

[rubber_jonnie]

I've been poking around my STE with a multimeter, and can't find any spurious shorts between VCC and GND, power rails look good, and testing continuity gives pretty consistent values across pins on the DMA controller. It does vary a little on the floppy controller, and I'm wondering if that might be why my machine won't boot with the floppy drive or a floppy emu installed.

To that end I looked around and these chips are anything but cheap, and I got to thinking, what is it TOS is expecting from this chip? It seems like there's only a couple of address lines (obviously being decoded somewhere) and then 8 data access lines. There's a few control lines like read/write and chip select. What I'm wondering is whether it's possible to use a PI PICO or other microcontroller to fake the floppy controller, allowing me to boot the machine without a floppy drive, but make it think there is one.

There doesn't seem to be an init command of any sort in the datasheet, but the status register can indicate things like spin up status, so is the machine just checking for a series of particular statuses? If I'm oversimplifying and being an idiot please feel free to tell me to shut up :)

Honestly, the last time I had a floppy problem, it was a dead DMA and these are for sale on ebay for £7. Obviously it will be a used part and it's a risk, but I wouldn't say it's expensive. I do agree that the WD1772 ICs are expensive on ebay though...

What I would add here is that is that that you would need to ensure that DMA is suitable for your particular machine and certainly some are not suitable, such as IMP branded DMA so you have to be very sure to ensure that any parts you buy match what you're replacing.

Assuming your drives work elsewhere, have you checked both the power to the floppy and also checked continuity through the 34 pin cable from the mainboard to the floppy drive? I can't recall if you have but I have run into problems with both in the past, the STE is expecting to detect a floppy at the end of the cable, and at it's simplest, if the cable has a fault, it wont.

[BigSteve]

What version of TOS do you have?

Having just experienced 4 bombs on an STE after swapping to TOS 2.06 and not re-connecting the floppy drive I became aware there is some form of validation test at startup.

Can you test the FFD elsewhere?

[matt.lacey]

The only other Atari machine I have to hand that I could swap the drive into is my Falcon and I'm a bit hesitant to do so, I guess the drive would also work in a PC with the cable the other way up? I've got a DOS machine I could try it with.

I have checked power to the floppy drive, but thinking about it, I've not tested continuity on the cable itself, coudln't see a reason for it to have broken but ruling that out would be the smart thing to do so I'll do that tonight. I get consistent continuity readings across all the pins on the DMA chip, so nothing alarming, but if it's the cheaper part I might start there if I can find a suitable replacement (assuming the cable is good!).

[rubber_jonnie]

The only other Atari machine I have to hand that I could swap the drive into is my Falcon and I'm a bit hesitant to do so, I guess the drive would also work in a PC with the cable the other way up? I've got a DOS machine I could try it with.

I have checked power to the floppy drive, but thinking about it, I've not tested continuity on the cable itself, coudln't see a reason for it to have broken but ruling that out would be the smart thing to do so I'll do that tonight. I get consistent continuity readings across all the pins on the DMA chip, so nothing alarming, but if it's the cheaper part I might start there if I can find a suitable replacement (assuming the cable is good!).

Definitely worth a try, I've had bad cables stump me before. I do tend to replace the soldered in connector with a socket where I can, but the old connector is a sod to get off.

If you want to test in a PC then you'll likely need to change the drive ID to DS1 rather than the DS0 used on the ST. You may have a jumper for this, but otherwise it's moving a soldered on link on the PCB.

[matt.lacey]

Definitely worth a try, I've had bad cables stump me before. I do tend to replace the soldered in connector with a socket where I can, but the old connector is a sod to get off.

Just tested the cable and it's definitely good. Was a little concerned when a couple of pins for the connected seemed to be tied together, then realised that one entire side is just connected to the ground plane.

[rubber_jonnie]

Definitely worth a try, I've had bad cables stump me before. I do tend to replace the soldered in connector with a socket where I can, but the old connector is a sod to get off.

Just tested the cable and it's definitely good. Was a little concerned when a couple of pins for the connected seemed to be tied together, then realised that one entire side is just connected to the ground plane.

Out of curiosity did you check with a floppy drive plugged in, so you're checking from the underside of the PCB to the connection on the disk drive? I only ask because I have run into problems with the floppy plug too and when a drive was plugged in connectivity was iffy, even though it seemed ok when I checked to just the plug.

As for the pins tied together, yes, there's a ton of them but that is normal if you look at the schematic.

Do you have a diag cart? A recent machine I looked at with a bad floppy I was able to boot from the cart and run the floppy tests. I assume that you get an activity light on the FDD (I can't recall if you said you did or not)

[matt.lacey]

Out of curiosity did you check with a floppy drive plugged in, so you're checking from the underside of the PCB to the connection on the disk drive? I only ask because I have run into problems with the floppy plug too and when a drive was plugged in connectivity was iffy, even though it seemed ok when I checked to just the plug.

I did not - I'll give that a go this evening. I whacked a pin header into the end of the cable so that I had easier access to the lines for testing.

Do you have a diag cart? A recent machine I looked at with a bad floppy I was able to boot from the cart and run the floppy tests. I assume that you get an activity light on the FDD (I can't recall if you said you did or not)

I don't yet - I emailed Cream/Ultra to see if the diagnostic images could be used with the UltraDev cart but the ROMs are too big for that. Will probably look to buy one soon!

[rubber_jonnie]

I don't yet - I emailed Cream/Ultra to see if the diagnostic images could be used with the UltraDev cart but the ROMs are too big for that. Will probably look to buy one soon!

There are diag carts in the store: Diag Cart

They are the standard Atari field service diagnostic images and are more than enough for most jobs.

[matt.lacey]

There are diag carts in the store: Diag Cart

They are the standard Atari field service diagnostic images and are more than enough for most jobs.

Just oredered one, along with the cable and some caps for both my STE and Falcon PSUs. Figured it was about time I dealt with those lest I have more problems down the line. Will post back with what I find once the cart arrives.

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