Can't access disc drive

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Sniffer
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Can't access disc drive

Post by Sniffer » Sun May 05, 2019 4:13 pm

Having got a troublesome beeb booting (viewtopic.php?f=3&t=17060&start=30), thanks to the guys here, I now want to access a memory test prog I have saved on disc. The trouble is, I can't access the drive.

I have initially installed the 1770 disc upgrade but can't access the attached drive. I tried different ICs in 79 & 80 incase they were bad and a different 1770 module. The setup (drive, floppy, cable & power) is good, it works well with other beebs. I tried next the 8271 disc upgrade, just for completeness but that results in the same situation, I cant access the drive.

Code: Select all

*cat
just hangs, no drive activity, no timeout, nada.

I've been going round checking continuity, using the board schematics, which all seems good.

Has anybody experienced this or a similar issue?

I intend to start scoping all the various pins (having learned alot in the thread linked to above) in the hope that something stands out but voltages all appear good.

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Re: Can't access disc drive

Post by Kazzie » Sun May 05, 2019 4:50 pm

Good to see you're getting more confident about scoping and testing things.

Have you tried seeing what's happening on the data cable to the floppy drive itself? When looking for a pinout you'll need to bear in mind that the Beeb uses the original Shugart pinout, not the PC variant. You could stick your probe on the Beeb's port, on the floppy drive board, or even the end of a floppy cable.

Good places to poke around would include the MOTOR ON and DRIVE SELECT 0 pins, as these are how a computer would typically tell a floppy drive to start spinning up and doing things.

These signals are active low, so should fall toward 0V when activated by the Beeb. If you don't find any sign of them, that implies that no signal's getting out of the box in the first place.
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Re: Can't access disc drive

Post by AndyF » Sun May 05, 2019 4:56 pm

Unlikely but one problem I had with a certain type of kit was the 'links' provided would not seat or stay/place nice in the IC sockets. In the end I got that annoyed I soldered links over the IC pins instead (component side so it was visible to the unwary!)

As I say I don't think that's your issue here however it may be prudent perhaps with a multi meter (power off!) to check for continuity where it should exist maybe.
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Re: Can't access disc drive

Post by Sniffer » Sun May 05, 2019 5:38 pm

These signals are active low
Meaning when, for example, a *cat is issued, the pin would go low, otherwise stay high? So, on power up, pin 2 of the 8271 will be high?

If so, this beeb conforms to that, in so far as on power up, pin2 is high, but then a *cat changes nothing, still high. Pressing escape causes pin 2 to go low and another *cat makes it go high again. I guess I didn't learn so much afterall :D
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Re: Can't access disc drive

Post by Sniffer » Sun May 05, 2019 5:41 pm

Unlikely but one problem I had with a certain type of kit was the 'links' provided would not seat or stay/place nice in the IC sockets
You mean the metal links between pins 1&4 and 9&12 when installing the 1770 upgrade? Yeah, they can be loose in the sockets but this problem exists with the 8271 as well and there are no links with that upgrade, all ICs.

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Re: Can't access disc drive

Post by 1024MAK » Sun May 05, 2019 6:24 pm

Are you using the correct disk filing system ROM (EPROM) chip for the disk controller that you have in use?

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Re: Can't access disc drive

Post by Kazzie » Sun May 05, 2019 6:51 pm

Mark makes a good point about keeping the correct FS ROM with the correct FDC chip!

I assume, from our discussions on the previous thread, that S9 is open now that you've fitted a FDC. It may also be worth going through the DFS installation instructions to make sure everything's in place correctly, if you haven't already. Beebmaster's instructions for fitting their erstwhile 8271 upgrade kit lists everything that ought to be done.

You've understood the "active low" signals correctly. As you didn't see any sign of them coming out of the Beeb, it would be worth checking whether they're coming out of the 8271 itself: The MOTOR ON (or HEAD LOAD) signal comes from pin 38, and DS0 from pin 2. Note, however, that they're active high on the 8271 itself, and then they're inverted on their way to the connector. So they should be low until you *CAT a disc, when they should go high.
Last edited by Kazzie on Sun May 05, 2019 6:52 pm, edited 1 time in total.
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Re: Can't access disc drive

Post by Sniffer » Sun May 05, 2019 7:19 pm

Are you using the correct disk filing system ROM (EPROM) chip for the disk controller that you have in use?
Yes, I used the relevant roms.
assume, from our discussions on the previous thread, that S9 is open now that you've fitted a FDC.
Yes, I removed the jumper I had fitted and I did actually run through both relevant fitting instructions, though the Acorn 1770 instructions make no reference at all to link settings, apart from S9.
it would be worth checking whether they're coming out of the 8271 itself

That is where I was checking :) So, the fact that pin2 of the 8271 is high at power on is wrong?

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Re: Can't access disc drive

Post by Sniffer » Sun May 05, 2019 8:06 pm

Note, however, that they're active high on the 8271 itself
So, to me, it looks like the beeb thinks I'm connecting to drive 1. On the 8271, pin 32 & pin 5 are both high on power on. Pins 2&6 are both high on power on. On doing a *cat, pin 2 remains high but pin 5 goes low.

I only have one drive.

Editted to say, IC77 (74ls00) does not seem right, only A1, B1 and Y1 seem normal, as in high or low. The other input/outputs seem to be sqawking.
Last edited by Sniffer on Sun May 05, 2019 8:19 pm, edited 1 time in total.

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Re: Can't access disc drive

Post by Kazzie » Sun May 05, 2019 8:57 pm

For IC77, the gates at pins 4-6 and 8-10 should be constantly switching, as they're fed by the 2MHzE clock and the CPU's Read/Write signal. On the last gate, pin 12 is connected to A2 on the address bus, so will change values when the Beeb's actually doing other stuff (but not as often as the other two gates). Pin 13 should be low unless the Beeb is talking to the 8271 (see the /FDC line nearby on the schematic), which means that pin 11 should stay high unless the Beeb is talking to the 8271.

They should all be spending most of their time at valid high/low TTL levels, of course.

If any of that doesn't match with what you've seen, feel free to post a few scope traces for us to take a glance at.
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Re: Can't access disc drive

Post by BeebMaster » Sun May 05, 2019 10:12 pm

Is it a known good DFS ROM (working in another machine) and a known good floppy drive (working in another machine) and correctly connected to the disc drive port (not 1MHz bus) and powered?

Does the DFS ROM show up in *HELP and *HELP DFS ? And is DFS being selected on BREAK, or D-BREAK, or with *DISC ?

For 8271 FDC, DFS should be version 0.90, 0.98 or 1.20. For 1770 FDC, 2.0j or later but make sure it isn't one of the versions intended for Master series machines.

Make sure S9 is definitely completely cut and if it's earlier than an issue 4 board, make sure the two additional board mods have been done correctly.

If the 1770 upgrade was done first (ie. straight from tape-only) then I would completely undo the 1770 upgrade and restore the machine to tape only before starting again with the 8271 upgrade, as it's easy when going from 1770 to 8271 to accidentally leave in place a step for 1770 only which isn't intended for 8271.
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Re: Can't access disc drive

Post by BeebMaster » Sun May 05, 2019 10:14 pm

Also try accessing all drive numbers (with *DRIVE 0-3) then *CAT in case the drive or the cable isn't set up for drive 0.
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Re: Can't access disc drive

Post by Sniffer » Sun May 05, 2019 10:34 pm

Is it a known good DFS ROM (working in another machine) and a known good floppy drive (working in another machine) and correctly connected to the disc drive port (not 1MHz bus) and powered?
Yep, the setup works well with other machines. It's an external PACE badged 5.25in external drive with good cable connected correctly.
Does the DFS ROM show up in *HELP and *HELP DFS ?
Yep. DFS 1.20.
And is DFS being selected on BREAK, or D-BREAK, or with *DISC ?
Yes on all 3. S9 has a link fitted with no jumper and is open.
If the 1770 upgrade was done first
Don't know the history or what steps would need undoing if the 1770 was fitted first? As far as I'm aware, the steps to installing a 1770 are cut S9, fit two ICs at 79 & 80, fit two wire links at 86 & 87, fit the 1770 module and fit the rom?
Also try accessing all drive numbers (with *DRIVE 0-3) then *CAT in case the drive or the cable isn't set up for drive 0.
No change.
Last edited by Sniffer on Sun May 05, 2019 10:35 pm, edited 2 times in total.

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Re: Can't access disc drive

Post by Sniffer » Sun May 05, 2019 10:46 pm

If any of that doesn't match with what you've seen
Well I am a little confused about what I've seen to be honest. Take the gate pins 4-6. Pin 4 has a mean of 4.24v, so high? Pin 5 has a mean of 11.2mv, so low? Meaning pin 6 should be high, yes? Pin 6 actually has a mean of 2.24v, which is below the output minimum of 2.7v, so is therefore low?

The last sentence is clearly my interpretation of the measuremnts and could be total poop :)

Edited to add image of pins 4,5&6.

IC77-4-5-6.png
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Re: Can't access disc drive

Post by Kazzie » Mon May 06, 2019 5:59 am

Pin 6 has a mean value of 2.24V, but from looking at the signal, it's a square wave that spends roughly half its time high and half it's time low (50% duty cycle) so that mean of 2.24V will be about half-way inbetween those high and low values. It has a peak-to-peak measurement of 5.28V; the low is ~5V lower than the high, which is good! And because the duty cycle is around 50%, we can halve the 5.28V peak-to-peak reading, and add/subtract it from the mean to estimate the high and low values.

Doing that gives high and low values of roughly 0V and 5V. So I'm not concerned about the values on pin 6.

The mean and peak values for pin 5 are off, though. That nice square wave is labelled on the diagram as 2MHZE, and should be a 2MHz clock signal that goes to good clean high and low voltages like pin 6. I suspect that you may have knocked the 10x switch again when measuring that pin. :wink:

If pin 5 actually measures a clock signal oscillating between 0V and 5V, I'd be happy with the behaviour of that gate. Pin 4 is high most of the time, and while that's the case the NAND gate will effectively invert the value of pin 5 to get the output value for pin 6. Pin 5 is high ~half the time, and so is pin 6.

Note that because you've measured each pin independently (with a single probe) that the horizontal timings aren't synchronised. So it's not unusual that pin 5's high periods coincide with pin 6's.

Your scope may have other dials or knobs to let you measure the voltage at any particular point on the waveform, or describe the scale of the background grid. Have an explore and see what you can find. :)
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Re: Can't access disc drive

Post by 1024MAK » Mon May 06, 2019 7:16 am

How many channels does you ‘scope have?

Also does it have moveable on screen markers?

When taking photos of digital signal waveforms, one thing I do to help others understand what they are looking at, is provide a reference for the 0V/GND.

So if I have a spare unused channel, I turn the trace for that channel on, but select it to show 0V/GND (most ‘scopes have a selection for input coupling with AC, GND and DC selections).

I then position that trace near the bottom of the screen (if there is a on-screen grid, aligned with one of the horizontal grid lines).

Next, on the channel(s) that I am using to display signals, I switch them to GND. Then align them to the trace that is displaying GND. Then I switch them to the DC input coupling setting.

Of course, depending on the gain / volts/division setting and the signal amplitude, you may have to adjust the settings and do this more than once to get a nice display.

If your ‘scope has moveable on screen markers, instead of using a another channel, you can use these to show the GND level.

Mark

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Re: Can't access disc drive

Post by Sniffer » Mon May 06, 2019 7:29 am

provide a reference

Ok Mark, thanks. I'll try with pin10 on IC37 as a reference and I'll try as you suggest above with 0v/gnd and see what I can get.

Thanks for the info.

Edited to add image of pins 4,5&6 of IC77, using pin10 of IC37 as a reference (I hope). The first sub image is of pin10 IC37 only (as the wave display changes when channel 2 is hooked up). Channel 1 is on top and channel two is IC77.

IC77-4-5-6_2.png

Gotta take the dogs out but I'll try with 0v as a reference to see if I can improve my efforts.

Sorry, but I clearly dont't know what I'm doing :) Here's an image of pins 4,5&6 with gnd as a reference (at least my attempt);

IC77-4-5-6_ref_gnd.png
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Re: Can't access disc drive

Post by BeebMaster » Mon May 06, 2019 11:43 am

Just one more thing which might be worth checking: test for continuity on all the socketed ICs in the disc interface - between the actual IC leg in the socket and the socket pin on the underside of the board. The service manual recommends this for empty sockets at production time in case bent socket pins etc scraped past the quality control.
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Re: Can't access disc drive

Post by Sniffer » Mon May 06, 2019 1:18 pm

test for continuity on all the socketed ICs in the disc interface
Thanks. I've done continuity checks, the first thing I always do, as even I can do that :)

I'm on hold at the moment for a few days as I seem to have fried IC28 during a 'socketing' session. No damage to the board but the 74LS51 didn't survive the operation :( Anyway, not the end of the world.
Last edited by Sniffer on Mon May 06, 2019 1:18 pm, edited 1 time in total.

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