BBC Master weirdness

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pau1ie
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BBC Master weirdness

Post by pau1ie » Wed Jul 11, 2018 8:32 pm

I thought I had messed up the disc controller on my BBC Master somehow. I was using the 5 1/4" floppy and it worked to start with, but suddenly on access it made the click noise it makes when you start accessing it, but continuously. Is that where it shoves the head into the end to make sure it is at track 0? Anyway I decided to try a 3 1/2" drive to check the computer is OK, but the access light is on continuously on that, but the motor doesn't spin.

But after a few cold restarts, it was fine. Anyone else get that sort of behaviour? Or is it related to my next post...
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roganjosh
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Re: BBC Master weirdness

Post by roganjosh » Fri Jul 13, 2018 5:57 pm

Disc drives for the BBC computers have an electronic track 0 sensor and so don't bang the heads against a stopper. That was only done on the Commodore 1541 drives AFAIK. I suppose it's possible it might happen if there was a problem with the track 0 sensor circuitry but it seems very unlikely in your case as it would have to have struck both drives and there are different symptoms with the two drives.

I'm therefore a little puzzled as to what the clicking noise is. If it's a particularly loud click on the 5.25" drive it could be a head-loading servo trying to operate but perhaps not having enough power and ending up cycling. Anyway, whatever the cause it's always worthwhile making sure the +5V line (and 12V on the 5.25 drive) is/are stable when trying to access the drive. If the supplies are stable then the fault sounds more likely to be on the floppy side of the 1770, quite easily the hex inverter buffer (7406) so don't condemn the 1770 hastily. It's easy with a multimeter to test that its outputs are stable and the opposite of the inputs, first without the drive being accessed and then with. The MO line is an obvious one to test given that you say the motor doesn't spin on the 3.5" drive.

Anyway, that's just speculation. There are more experienced heads than me on this list who may help. I just happened to notice your post had gone unanswered and the 1770 is coincidentally currently occupying me (awaiting some JLCPCB pcbs for a 1770 interface I've recently designed for a different machine and writing a DFS for it before they arrive). I'm not familiar with the hardware for your other problem.


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1024MAK
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Re: BBC Master weirdness

Post by 1024MAK » Sat Jul 14, 2018 5:10 pm

AFAIK, all 31/2" drives use an optical sensor or microswitch to sense track zero. And I believe nearly all 51/4" drives do the same.

For most drives, there is however no sensor to detect the last track (40 or 80). So if the system is trying to step beyond track 41 to 42, or 81 to 85 (depends on the drive), there will be a noise as the head assembly hits a stop or frame.

All the signals to/from a floppy disk drive are 5V TTL logic levels.

Mark
Last edited by 1024MAK on Sat Jul 14, 2018 5:11 pm, edited 2 times in total.
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