C64/1571 Oddness

[developstopfix]

I picked up a 1571 drive over the weekend that was sold as untested. Got it home and it appeared to work okay with one quirk: when powered on before the C64C I had it plugged into the spindle motor would run continuously but once the C64C was turned on and the drive reset the motor would stop and the drive seemed like it was working as it should. I messed with it again last night, this time using an older breadbin, and this time the drive functioned as it should when powered on before the C64; the motor spun briefly then stopped. I tested it again with the C64C and it was back to spinning continuously when powered on if plugged into that specific computer. It doesn't matter if the C64C is plugged into the power adapter or anything else, if the drive is plugged into its serial port and the computer is not running the motor keeps running when powered on. I poked at the serial port and nothing seems out of the ordinary - no shorted pins or anything like that. The only difference I could see between this C64C and the breadbin was that on the breadbin I was measuring about 3k Ohms resistance between ground and the other pins of the serial port, on the C64C it was just above 1k for most of them and then 600-something Ohms between ground on pins 4 and 5.

I don't have any other drives or C64s to test with, and I've never used a 1571 (or C128) before. Has anyone run into something similar?

 

[brain]

The lower resistance would indeed hold RESET low. I've noticed it on some machines here, but never worried about it. DO you have a concern?

For reference, here's a page describing what you are seeing. Any resistance or 2K or less will register as a logical 0 on the 74LS14 logic in the 1571 for the RESET line.

LED power from 54/75 HC/HCT

groups.google.com

 

[developstopfix]

Thanks for the quick reply. No concerns really since the drive does seem to work OK once the C64C is powered on and there aren't any other noticeable issues with that computer, I was mostly curious if anyone had an idea of what the root cause was and if it was something I should worry about

 

[Istarian]

For what it's worth, the Commodore floppy drives for the VIC-20, C64, C128, etc are essentially another 6502 computer dedicated to managing the actual disk drive.

So it's rather more complex than the floppy drives for the Apple II which are software-driven by the system's microprocessor or the PC floppy drive which has a dedicated floppy disk controller either on the motherboard or an expansion card.

 

[Jonas1ne]

That sounds like the C64C may be pulling one of the IEC serial lines low when unpowered, causing the 1571 to behave differently during startup. Since the drive appears to work normally once the computer is powered on and resets the bus, I’d be inclined to suspect something on the C64C’s serial interface rather than the drive itself.The resistance differences you measured are interesting, but without comparing against another C64C it’s hard to say whether they’re abnormal. I’d check the CIA chips, IEC protection components, and serial port circuitry on the C64C. If possible, try another IEC device or another serial cable to rule those out as well.





The fact that the breadbin behaves normally with the same drive suggests the 1571 is probably not the primary culprit.