Lazy 8" floppy disk question

[Dwight Elvey]

The time delays and step time are set by the specify command.
This is the value 03h sent to the 765 controller. It would be followed by writes of 2 bytes :
1st byte:
bits D7 to D4 Step Rate in 1 ms increments 1 to 16
bits D3 to D0 Head Unload Time 16ms steps of 0 to 240 ms
2nd byte
bits D7 to D1 Head Load Time in 2 ms steps, 1 to 256 ms
bit D0 No-DMA

These are usually controlled by software in the boot BIOS EPROM.
There is a note that the specify command must be issued for each drive selected.
You need to look at the data in the EPROM and see where the specify command is written. It is likely that there is a table in the EPROM with all the time delays for the different drives selected. I would suggest that if you can find where in the code it writes the specify command that you modify both the set time and head load time to the maximum. You can then try to format again. If it works, you can try reducing it a little at a time. When it fails, add a couple counts and be happy.
It is possible that it has a table for each drive.
Dwight

You'd be looking for the out instruction to port 8Fh or CFh depending on which board you have.
Dwight

 

[alan8086]

An update for you both - thanks for continuing the discussion.

I found a couple of hours to pick up where I left off before my chip tester died. I was just starting a 3 rd pass at testing all the RAM ICs. My chip tester is fixed now so I carried on testing - including the glue logic ICs on all 4 16k static RAM boards.

I came across two faulty ICs - a 74LS240 (pin14) on one board and a 74LS85 (pin6) on another of the boards. I've ordered replacements and am hoping to see some improvement in performance. A week or so ago I did manage to format one disk once. I wonder if it was at that point one or both if these ICs died?

I do intend to scope the index pulse - worth a look to check what the waveform looks like if replacing these chips makes no difference - and as a learning excercise

Disassembling the BIOS should be on my to do list - im trying to get this machine up and running so im in a position to learn and understand all you have discussed here!

 

[Chuck(G)]

Yusta be that you'd look for bad 74XXX chips; now the failures seem to have crept to the later 74LSXXX. Wonder when it'll hit the 74ALSXXX or 74HCTxxx ones?

"This fals world is but transitory,
The flesh is bruckle, the Feynd is slee"

--William Dunbar (1460-1520)

 

[alan8086]

So when these machines were new, were the 74 and later 74LS ICs considered rock solid reliable? Is it just age that is getting to them now? This is probably the most use I've had out of this IMS 8000 these last few weeks - when I first took delivery of it, I worked out enough to get hyperterminal connected and get to a CP/M prompt before it died not long after. Now I have the chip tester, its like permanent CPR!

I'm hoping power cycling and just 'use' is weeding out the weaker ICs - I found no defective 2114's this time. Next job is maybe replace all the voltage regulators, then the tantalums and maybe even those huge expensive smoothing caps in the linear power supply?

Yusta be that you'd look for bad 74XXX chips; now the failures seem to have crept to the later 74LSXXX. Wonder when it'll hit the 74ALSXXX or 74HCTxxx ones?

"This fals world is but transitory,
The flesh is bruckle, the Feynd is slee"

--William Dunbar (1460-1520)

 

[glitch]

If the supply checks out (no excessive ripple, sufficient holdup under load) then there's no need to replace the main filter caps. They often reform just fine -- my IMSAI, and many other systems I either have or have worked on, still have their original main bulk filter caps. The regulators are worth testing, they'll sometimes get electrically noisy or drift out of regulation, but again the majority of S-100 boards I've got or have worked on had no regulator issues. Tantalum caps will usually blow up or not, though replacing them isn't the worst thing you can do

7400 and 74LS is still reliable stuff (it's still made), but environmental conditions (e.g. too high +5V rail, too much heat) can shorten their lives. Everything fails eventually, though, like Chuck said.

In general, I've found vintage S-100 stuff to be pretty reliable, once I get it gone through and working. My IMSAI is probably the vintage system I hack on the most, and I rarely have to do anything to it.

 

[Chuck(G)]

Well, it used to be that bipolar logic was considered to be more stable than MOS or CMOS. Early 4000-series CMOS was really pretty awful.

I think it's probably the packaging slowly failing. It would be interesting to compare failure rates between CerDIP and plastic packaged ICs of the same vintage.