Tandy 2000 FDC chip question

[Patrick Bureau]

you should not be using a 1.2MB floppy unless you demagnetized it first (you know with those hand-held magnets R/S used to sell),
If the floppy was made in a 1.2 drive the 720 head will have trouble reading the floppy, because of the size of heads on a 1.2 being smaller, and tracks being closer together.
You should be using 5.25" DDDS floppies not 1.2 DSHD floppies anyways with your 2000 FDD as indicated in the Model 2000 Page 4 of the Technical reference manual.

[Chuck(G)]

See post 35. Falter knows this already.

Please review the thread and note the exact nature of the problem.

[Patrick Bureau]

well, Chuck got you sorted out, and is helping you out..
I'll stop blundering, trying to help then...
You are in good hands, he has the knowledge to walk you through your issue.. g'luck

[falter]

Hi Chuck,

I do observe state changes on pins 1 and 2 of the 7416. However on pins 3 and 4, pin 4 on my logic probe shows HI no matter what, and only a brief pulse as pin 3's state changes. I noticed when I accidentally bridged them with my logic probe (shaky hands), it actually caused the drive head to move around erratically. I'm not seeing changes in the direction pin on the actual floppy connector. So I'm thinking you're probably on the money here?

[Chuck(G)]

That's not right. According to the schematic (pdf page 125) of the Tech ref manual, pin 2 of U40 is directly connected to pin 18 of the floppy drives.

So, we can assume one of the following:

1. There's a break on the PCB between U40 and J11.
2. The cable between J11 and the floppy drives is bad
3. You're not measuring what you think you're measuring.

Let me know and we can go from there.

[falter]

Yeah I totally buggered that explanation up. Never work on electronics while under the influence of nerve pain meds.

I'll try to describe as best I can.

For DIR, I can see a low logic signal at pin 17 of U135, that periodically blips up to HIGH, and then down to low again. On pin 3, my logic probe detects nothing until pin 17 switches to high, which is sort of picks up briefly, and then goes back to nothing. At pin 1 of U140 I can see a state change from high to low, and on pin 2 from low briefly to high. Kind of. Sometimes it's kind of a few, like high low high on either side. Anyway, I can see the what's happening on U140 pin 3 at pin 18 on the drive, and I can see the drive head very briefly changing direction to go backwards, although it mostly carries on going forwards towards the hub. Sort of a 1 steps backward, 10 steps forward kind of thing each time it attempts to boot.

On pin 15 of U135 I see low logic signal that pulses periodically, same at pin 5 of same. At pin 3 of U140 I see a low logic signal that pulses briefly, and at pin 4 a high logic signal that does same. This is seen on pin 20 of the drive itself, and the head moves.

On pin 7 of U135 I see a low logic signal that pulses and briefly jumps to high. On pin 13 of U135 I see a high logic signal that stays high, no pulse. At U 139 pin 8 I see high logic signal, no pulse. At pin 9 I see a high logic signal, no pulse. Same at pin 26 of the drive itself.

So if I understand how the 241 works, I feel like we should be seeing something happening on pin 13 of U135, but we aren't. If I'm right, then the question does the 241 have a problem, or is something going on with U 139?

Or I could be totally offbase.

[Chuck(G)]

Okay, let's start with the 241. First thing to notice is that in the 8272A data sheet, some pins are input, some are output and some have dual personalities, depending on what's being done. In the case of pins 38 and 37, the pins are output, but the meaning depends on whether the floppy is seeking or reading/writing. The 8272A has pin 39 to say which is being done.

So, the 241's job is to pass the signal from pins 37 and 38 to the the U140 7416s only if the controller is seeking. Otherwise, the relevant outputs of the 241 are tristated (i.e. they "float"). R52 makes sure that the input to U140 pin 3 is low if the FDC is in read/write mode, which keeps the drive from stepping. If the drive isn't stepping, the state of DIR (direction) doesn't matter. Personally, I would have pulled pin 1 of U140 low in the same way, just to keep garbage from appearing on the direction pin when writing, but maybe the Tandy engineers wanted to save a penny.

So, looking at the outputs of the two sections of the 7416 (U140), we can see that the state of pin 2 matters only when pin 4 goes from high to low. If I had a logic analyzer or 'scope handy, I'd trigger on the high-to-low edge pin pin 4 and capture the state of pin 2.

One more thing...U140 is an open-collector device, which means that the floppy drive must have a terminator installed if it's the only floppy on the cable, or one drive or the other must have a terminator installed if a two-drive system. Without the pullup provided by the termination, the signal from U140 will not reach a real "high" state--and that by itself will create problems.

Does this make sense to you?

[falter]

Yes sort of.. the only part I'm not understanding is how the Track 0 process works. Ie if the 765 generates that to force the drive to find it, or if it sort of waits to hear from the drive that Track 0 has been reached.

It is very possible these drives are not terminated properly.. the previous owner had been checking the system out before selling. I note one of them has an IC installed near the drive edge connector.. the other doesn't? Does that have to do with termination?

[Eudimorphodon]

It is very possible these drives are not terminated properly.. the previous owner had been checking the system out before selling. I note one of them has an IC installed near the drive edge connector.. the other doesn't? Does that have to do with termination?

Check the markings on said IC and see if it's actually a DIP resistor package because, yeah, that's probably it. The drive with that on it should be at the physical end of the cable furthest from the motherboard. (regardless of its ID jumper setting.)

[falter]

These are the drives as they are configured now:

https://drive.google.com/file/d/1t05...w?usp=drivesdk