Tristan Mumford wrote:
Michael J. Mahon wrote:Tristan Mumford wrote:G'day! Today I had a feeling, which I acted upon. I wondered if the little short the adaptor board had didn't cause more damage than I thought. It seems it may well have. Just to find out, the Atlas 8 was plugged in to do a few tests. It can read perfectly, as expected. However trying to format a disk results in the same failure to write I have been seeing. Based on the assumption* that the short was from -12v to WREQ I traced the lines to what seems to be an open-collector 74LS06 (M53206P) on the controller.Usually, it's an 'LS05--but still an OC inverter.So, should I replace it with an '06 or an '05?
Either should be fine--it was designed to be a '05--and it's just driving a 1K pullup to +5v in the drive. (BTW, since you're intercepting it, you need to pull it up to +5v, too.)
Holding my amazing LED and resistor logic probe on the WREQ line and attempting to perform a write (ProDOS format) results in no difference in the logic high state.-snip-One odd thing to note is the IC in question doesn't seem to have any continuity between +5 and the signal line (when not powered) as I would have thought for this problem.* Visually it looked like a solderbridge was between WREQ and -12v. Also that was where I had just done an alteration to add a line to monitor WREQ to know when to use the disk writing code in the uC. So it seems logical that this was the issue.If it's an open collector output, then I wouldn't expect any connection with +5. The only +5 it sees is from an external pullup. More likely, it's lost its ability to pull the line *down*, because its output transistor/metal is blown.I see. Oh well hopefully the presumed cause is correct and it's only one commodity IC to replace.
That's probably all it is--TTL ICs do a good job of clamping voltages to within +5v and 0v. -michael NadaNet networking for Apple II computers! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it's seriously underused."