Mitsubishi 4853 floppy drive problems

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jonb
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Mitsubishi 4853 floppy drive problems

Post by jonb » Fri Sep 14, 2018 3:00 pm

Hi

I have a pair of dual floppy drive units for the BBC Micro with 40/80 track switches.
  • Opus 5802 D
  • Unbranded unit
Both units are using the Mitsubishi 4853 drives. One has a rotating disk lock like a Teac FD55 drive and the other has a sort of "push to close / push to open" locking mechanism. They are all 4853s though (albeit with some differences in the drive electronics. One set has 40/80 track functionality built in, whereas the other set has a single "dual step" board hacked onto the drives that uses a 74LS123 and some discreet components.

Anyway the question is, these drives are supposed to be double sided according to their datasheets, but I have not been able to use them as DS drives. They only work single sided. They appear to have twin heads, so at 40 tracks DS I should get a whole 340k from each drive (wow). I have connected them to a Superbrain and can read the content from disks as long as no part of the file is on side 1, at which point I get a "disk not ready" error.

Is it possible they've been modified to be single sided only, or are they supposed to be double sided?

Thanks
JonB

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danielj
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Re: Mitsubishi 4853 floppy drive problems

Post by danielj » Fri Sep 14, 2018 7:10 pm

If there's a top head with wires coming off it then they should absolutely be double sided...? Single sided drives just have a felt pad on top.

d.

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Re: Mitsubishi 4853 floppy drive problems

Post by Kazzie » Sat Sep 15, 2018 6:29 am

Or to put it another way, if you open up the drive and see nothing but a felt pad on the upper head, there's no way you'll be able to read anything from that side of the disk.
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Re: Mitsubishi 4853 floppy drive problems

Post by jonb » Sun Sep 16, 2018 9:29 am

They are double sided for sure.

But is there a jumper or other hardware setting that makes them appear single sided to the computer? (Not sure why you'd want to do that, mind..)

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1024MAK
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Re: Mitsubishi 4853 floppy drive problems

Post by 1024MAK » Sun Sep 16, 2018 8:03 pm

If the wiring loom for the top head is removed from the connector on the board, that would do it!

Alternatively, there could possibly be a link in the logic circuits that select which head to use. Or the input control from the control and data (the 34 way ribbon) cable by be disconnected somewhere. Any of this would prevent the top head from being selected.

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MartinB
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Re: Mitsubishi 4853 floppy drive problems

Post by MartinB » Sun Sep 16, 2018 10:13 pm

Pin 32 of the interface connector is Side Select so if that were open circuit or shorted to ground, you would only be able to access one side of the drives.


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Last edited by MartinB on Sun Sep 16, 2018 10:29 pm, edited 1 time in total.

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danielj
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Re: Mitsubishi 4853 floppy drive problems

Post by danielj » Mon Sep 17, 2018 2:26 pm

Doesn't look like there's any jumper setting that'd cause that sort of misbehaviour...

http://www.bitsavers.org/pdf/mitsubishi ... ations.pdf

:?

d.

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Re: Mitsubishi 4853 floppy drive problems

Post by MartinB » Mon Sep 17, 2018 2:47 pm

I’m feeling fairly certain as I said earlier that it’s a Side Select issue. Drives aren’t cognisant of their two sides or whether files span both, it’s fully the responsibility of the filing system to manage the two surfaces and thus the FS has to select the active head. If Side Select stays in one state, the drive(s) won’t know or care, they’ll just behave as single-sided drives. DFS for example uses Side Select and the two surfaces to create two logical drives from one physical unit - for drive 0 and 2, the Drive Select lines remain the same thus selecting the one physical unit whilst Side Select is toggled to select the two logical drives.


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Last edited by MartinB on Mon Sep 17, 2018 2:52 pm, edited 2 times in total.

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Re: Mitsubishi 4853 floppy drive problems

Post by danielj » Mon Sep 17, 2018 3:02 pm

You did. I was just confirming that there were no jumpers that disabled side-select :)

d.

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Re: Mitsubishi 4853 floppy drive problems

Post by MartinB » Mon Sep 17, 2018 3:07 pm

Ah.... :lol:

Since the problem manifests on both drives, I’d guess it’s either the cable (including connectors), or one of the two drives is murdering the signal.... :-k
Last edited by MartinB on Mon Sep 17, 2018 3:11 pm, edited 1 time in total.

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Re: Mitsubishi 4853 floppy drive problems

Post by 1024MAK » Mon Sep 17, 2018 3:08 pm

And now breathe out...
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Re: Mitsubishi 4853 floppy drive problems

Post by cmorley » Mon Sep 17, 2018 3:22 pm

Is the chain terminated properly too? That could cause a head select problem I suppose.

My money is on the wiring or a connector though.

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Re: Mitsubishi 4853 floppy drive problems

Post by jonb » Mon Sep 17, 2018 8:01 pm

Thanks, I will have to look into side select. Should have spotted it on the pinout, d'oh!

I wonder if that's the problem with the 'brain..?

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