5.25" Floppy drive write-protect fault

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BrianR.
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5.25" Floppy drive write-protect fault

Postby BrianR. » Thu Sep 21, 2017 8:53 pm

Evening all.

I have just acquired a Cumana 5.25" floppy drive. It has a fault in that it is not detecting that the floppy disk write-protect tab has been removed, so any writes to disk fail with a 'read only' error. :(
The drive appears to be identical to the one shown here>>> viewtopic.php?t=8567
Has anyone got any ideas on how to fix this? I wouldn't mind too much if the write protect was disabled permanently, so maybe a link across a couple of IC pins might solve it? (I'm reasonably competent at soldering). A schematic of the drive electronics would be a good start start, if anyone has one.
Alternatively, I might be able to repair the faulty LED/sensor if I could get to it, so teardown instructions would be great.
TIA, (but I know it's a long shot.)

crj
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Re: 5.25" Floppy drive write-protect fault

Postby crj » Thu Sep 21, 2017 10:07 pm

If you want to completely override write protect, you can do that between the drive and the Beeb.

It's pin 28 of the connector; disconnect it from the drive and hardwire it to +5V.

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MartinB
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Re: 5.25" Floppy drive write-protect fault

Postby MartinB » Thu Sep 21, 2017 10:42 pm

Write Protect on disc interface pin 28 is an output from the drive notifying the host that a write-protected disc is present in the drive and that the drive has locally inhibited writes. Disconnecting and tying that line high into the host will indeed make the latter unaware of the write protect but it won't enable writing to the disc.

@BrianR - can you remove the cover and look at the back of the drive chassis for a make and model label such as Teac FD-55... or similar. Cumana didn't make drives, they just cased them up and branded them so without knowing the specific type, it's hard to offer specific technical advice.

crj
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Re: 5.25" Floppy drive write-protect fault

Postby crj » Fri Sep 22, 2017 4:41 am

Are you sure about that? I thought most drives relied on the controller to enforce write protect.

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MartinB
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Re: 5.25" Floppy drive write-protect fault

Postby MartinB » Fri Sep 22, 2017 8:00 am

Yes, I'm sure! :wink:

It is perfectly possible to also implement a soft-protect function in the host fs, in DFS there's a nod to this with the *ENABLE switch for heavily destructive write commands. You could also have say a global disc *WPROT <ON / OFF> but none of this affects or interacts with the write enable functionality within the drive itself which is hard-wired to the open/covered state of the write-protect tab or hole and whose status is reported via the WRITE PROTECT signal on pin 28.

BrianR.
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Re: 5.25" Floppy drive write-protect fault

Postby BrianR. » Fri Sep 22, 2017 8:14 am

Thanks for the responses guys.
However, I have resolved the issue. There is a paper insulating(?) sheet that has a cut-out for the sensor. This sheet had moved, and was covering the sensor. So, an easy low-tech solution, thank goodness.
Martin, FYI, the drive PCB is marked 'TEC' CFE61-05004.

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MartinB
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Re: 5.25" Floppy drive write-protect fault

Postby MartinB » Fri Sep 22, 2017 8:43 am

Well done! Easy! =D>

Brian wrote:Martin, FYI, the drive PCB is marked 'TEC' CFE61-05004.

That means the drive is a TEC 50x where x is likely 1, 3 or 4 and this determines the tracks, sides, densities and capacities etc.. You'd still need to find a model label to know for sure - if it is the basic 501 for example then I think it might just be single-sided and you'll know not to waste your time with double-sided discs. I'd be tempted to find the label and have a Google.... :wink:

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danielj
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Re: 5.25" Floppy drive write-protect fault

Postby danielj » Fri Sep 22, 2017 11:11 am

Great work Brian :) gotta get your hands dirty under the bonnet to keep these things going :D

d.

BrianR.
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Re: 5.25" Floppy drive write-protect fault

Postby BrianR. » Fri Sep 22, 2017 1:44 pm

Thanks again.
Martin, the drive is an FB-504, which is a DS/QD drive.
I have successfully formatted drives 0 and 2 at 80 tracks per side. Result!

This drive has an integrated PSU, but I'm going to bypass that and power it from the Beeb. I'll have to modify the housing somehow.
I've had to bodge the connection from the Beeb's auxiliary power socket though (it aint pretty, but it does the job, and is electrically safe).

It will be used with my Master 128, and the other (Mitsubishi) drive on my 2 Model Bs.

crj
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Re: 5.25" Floppy drive write-protect fault

Postby crj » Sat Sep 23, 2017 12:24 am

MartinB wrote:Yes, I'm sure! :wink:

It is perfectly possible to also implement a soft-protect function in the host fs

I'm not talking about software.

I've just double-checked both the 8271 and 1770/2 datasheets; each clearly states that they inhibit writing to write-protected discs.

Then I dredged out the manual for a random Shugart 5.25" drive. It says the drive contains a link to configure whether it enforces write protect itself. Admittedly a link which was factory-set in the "on" position, but still...

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MartinB
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Re: 5.25" Floppy drive write-protect fault

Postby MartinB » Sat Sep 23, 2017 8:41 am

To be fair, you're now pulling the discussion in other directions. In the context of resolving Brian's reported problem, you originally stated....

crj wrote:If you want to completely override write protect, you can do that between the drive and the Beeb.

It's pin 28 of the connector; disconnect it from the drive and hardwire it to +5V.


...and that was incorrect. Stat.

To repeat my response to your statement, Pin 28 of the 34-pin Shugart interface is an an output from the drive to the host and is used by all drives to report whether the drive has detected a write-protected disc present in the drive. All drives, bar none, will also, by default, internally disable writing to such a disc in response to the write-protected disc detection. Further, if a typical controller such as the 8271 or 177x sees this /WP, they will additionally, as you say, close the loop by inhibiting writes but they do that by withholding Write Gate and actually, they don't need to do this because as we have said, the drive will already have set itself to ignore write data.

Now, you have managed to find a drive that has an option to override this internal write-inhibit and, via the use of a non-standard link, only report the presence of the protected disc but not enforce the inhibit. However, this doesn't change what has been said, it's just an unusual feature of that particular drive. Yes, if Brian had one of these drives then he could have used that that link option in conjunction with breaking the Pin 28 line to disable all protection but this is not where we were.

I really don't understand why you feel the need to wrestle with this but I shall withdraw now because I don't think the discussion is adding any value to the thread.... 8)

crj
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Re: 5.25" Floppy drive write-protect fault

Postby crj » Mon Sep 25, 2017 12:41 pm

Well, what you're saying now is definitely incorrect: that Shugart datasheet also covered a model variant with no write protect mechanism whatever!

Besides, the reason I knew of the technique and mentioned it in the first place is that I actually witnessed it being done with a stock floppy drive and a BBC Micro back in the eighties. You claim this is impossible; it's not. Maybe we were freakishly lucky; maybe not.

I cited that Shugart datasheet because it was basically the only one Google found for me. I didn't sift through datasheets until I found one that said what I wanted. I've just gone looking for more, and did find a high density floppy drive where write protect was hardwired in the drive, but that may well fit a narrative of write protection, over the years going from being an optional extra enforced by the controller, to a standard feature, to a feature optionally enforced by the drive as well as the controller, to eventually being mandatorily enforced by the drive (as well as the controller).

I will note that it would make no sense whatever for controllers to enforce write protection if, as you claim, that was always done by the drive.

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1024MAK
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Re: 5.25" Floppy drive write-protect fault

Postby 1024MAK » Mon Sep 25, 2017 2:30 pm

Claim down everyone. As with most interfaces, the floppy drive Interface has some variation between different manufacturers.
And there are signal and interface differences between 5 1/4in drives and 3 1/2in drives and also 3in drives on various signals.

So yes, the write protect signal is an output from the drive to the controller. Most (but maybe not all) controllers will inhibit the write gate while the drive is indicating that the disk is write protected. And some drives may also have logic that does the same thing internal to the drive's logic system.

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