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Old 23rd Oct 2017, 8:27 pm   #1
acollins22
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Default Nascom repair - Looking for clues please

Hello Folks,

I'm trying to breathe some life into my recently acquired Nascom-1 and wonder if anyone can help please.

The previous owner told me that it was working but that after half an hour it stopped and refused to come back to life.

The original symptoms are that it displays a screen full of garbage that won't clear.

What I have so far...
I have the following voltages from the power supply :-
  • 11.93V
  • 4.79V
  • -5.06V
  • -12.06V
All look good on my scope and show very little ripple.
I have seen the clocks and the processor (Z80) is running. I can see the M1 and other signals twinkling.

I have replaced the original 2708 EPROMs with 2716s in adaptors (I can't program 2708s so I'm using the bottom half of 2716s).

I have also blown two ROMs with a tiny piece of software in each. The first fills the screen memory with 0x00. The second fills the screen memory with 0x2A ('*').

Neither seem to run until the reset button is pressed and then I get the two phots below. The results are pretty consistent but not absolutely guaranteed.
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I'm sure there are clues there but I am out of my depth here.

Please can someone come up with some suggestions?

Thanks,

Andy.

P.S. This is a slightly cut down version of the story. You can read about everything I have done here. http://www.randomorbit.co.uk/?cat=129
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Old 23rd Oct 2017, 9:56 pm   #2
SiriusHardware
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Default Re: Nascom repair - Looking for clues please

I see you've also raised this over on the vintage computer forums as well.

Never (regrettably) having owned a Nascom or a Nascom 2, and therefore not knowing how similar they are, I can't help you directly, but there was an interesting thread about repairs to a Nascom 2 on the vintage computer forums here:-

http://www.vcfed.org/forum/showthrea...light=nascom+2

If they used similar methodology in the original Nascom, maybe something in that thread will be useful to you. One of the problems that user had was failure of the fusible-link Bipolar PROMs used as address decoders in the Nascom 2.

Did they use PROMs for that purpose in the Nascom 1 as well?
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Old 23rd Oct 2017, 10:15 pm   #3
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Default Re: Nascom repair - Looking for clues please

How does the display generation work? I'm guessing it's character-mapped as opposed to bitmapped; but is the CPU driving the display directly and then running your program during the vertical flyback, or is there some dedicated logic?

If it's using a character generator ROM, that seems fine, because the characters on screen seem properly formed. Likewise anything downstream of the chargen, and its address generator (if the CPU generates the display addresses itself, it's blameless for now). How about the display RAM? If it is misbehaving, the screen display might well not make sense.

To test whether the CPU is running, write a simple program to toggle an I/O port pin on and off to a PROM, and probe that I/O line. Use a simple DJNZ loop for dwell time in each state, and choose an inititial value to load into B to set the frequency. You could use a single 2716 to replace a pair of 2708s, if you connected up the extra address line A10 to a suitable place on the board.
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Old 23rd Oct 2017, 10:35 pm   #4
SiriusHardware
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Default Re: Nascom repair - Looking for clues please

Quote:
Originally Posted by julie_m View Post
How does the display generation work... is the CPU driving the display directly and then running your program during the vertical flyback, or is there some dedicated logic?
Here you go:

http://www.nascomhomepage.com/pdf/NASCOM1.PDF

Dedicated discrete logic, including a character generator by the look of it.
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Old 24th Oct 2017, 9:02 am   #5
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Default Re: Nascom repair - Looking for clues please

My prime suspect in microprocessor systems of this vintage is almost always the RAM chips. They're by far the most unreliable semiconductor part. Glancing at a NASCOM circuit diagram, it seems to use 2102 chips for both the working and video RAM. They're still reasonably available. If you're stuck, I think I've got a couple of dozen in my arcade spares box (Atari Super Breakout uses them).

The 81LS97 data buffer on the output of the working RAMs is also a suspect. If any of these are in sockets, try swapping them around and see if the symptoms change when running your test program.

The working RAMs would be easy enough to substitute for something newer and more reliable like a 6116, since the circuit doesn't explicitly make use of the 2102's separate data inputs and outputs. However, the video RAM would be more difficult to substitute and is probably better kept original without a significant redesign.

Chris
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Old 24th Oct 2017, 1:48 pm   #6
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Default Re: Nascom repair - Looking for clues please

The circuit is pretty much textbook -- probably not surprising really, though, given that the textbooks were only written after the Nascom was designed!

If the display RAM chips are socketed, try wiring some of the output pins to 0V and leaving others floating. You should get a screen filled with the same character over and over again.
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Old 24th Oct 2017, 2:15 pm   #7
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Default Re: Nascom repair - Looking for clues please

Ahhh the Nascom 1. I cut my teeth on these back in the day, building one from a kit after learning Z80 programming at college.

Enough fond memories, down to your problem.
This looks like an addressing problem. The expected data is not being read from the video memory on 2 lines of 8 repeated twice per frame. Either the data is not being read from the video memory at these locations or , and I think this is more likely, it was not written into it because the computer wont boot from its ROM either, I suspect it can't address these address ranges either.
Follow the address lines out from the Z80 with a scope, look particularly at Addresses A5,A6,A7,A8 as the screen pattern indicates the addressing fault would be in that kind of address spacing.
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Old 24th Oct 2017, 2:57 pm   #8
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Default Re: Nascom repair - Looking for clues please

Looking at the screen shots again the 2 lines of garbage are the same on both pictures so that looks like uninitialized ram content. It looks like the video RAM is being read correctly ( so all the address counters in the video section are working ) but not written to. Also the 2102 memory chips hold 1 bit each so a faulty RAM chip could not produce that pattern.
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Old 24th Oct 2017, 5:28 pm   #9
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Default Re: Nascom repair - Looking for clues please

There are comprehensive circuit descriptions, including coverage of the operation of the video circuit, in the Nascom 1 hardware manual found on the same site as the circuit diagram here:-

http://www.nascomhomepage.com/pdf/N1HM.PDF

Among other things, the parts list in that manual usefully supplies the IC type for each IC circuit number, something the circuit diagram doesn't always bother to do.

For example, the main address decoder is formed by two halves of IC36, whatever sort of IC that is. The circuit doesn't actually say, but the parts list informs us that it is a 74LS139, (a dual 2 to 4 line decoder).
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Old 24th Oct 2017, 7:21 pm   #10
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Default Re: Nascom repair - Looking for clues please

Thank you everyone for your advice. I knew this was the best place to ask.

I figured I should clean the pins of the Z80 if I was going to be scoping about so that was the first job.

I should point out that this evenings tests were carried out with my own screen-filling ROM that fills the screen memory and then loops on its last instruction to effectively halt.

When testing, if there was no activity on a given address line, I pressed reset to try and generate some activity.

I have put a scope on the address pins A5, A6, A7 & A8 as suggested by SiriusHardware.

A5 and A6 are running all the time and look like this...
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and this.
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A7 and A8 need a reset and look like this
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and this.
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I haven't changed the scope settings between these photos so the voltage of A7 and A8 look significantly different to those on the other pins I've looked at.

Also, the screen has changed a bit and now looks like this
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and although the nature of the banding stays pretty much the same, the contents do vary.

Please keep the suggestions coming, I'm out of my depth here and your advice is most useful.


Cheers,

Andy.
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Old 24th Oct 2017, 9:48 pm   #11
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Default Re: Nascom repair - Looking for clues please

Judging by the characters being properly-formed, everything dowmstream of the video memory is temporarily above suapicion.

Write a very simple machine code program to toggle an I/O line continuously from 1 to 0 and back,using only the processor's registers, no RAM; store this in an EPROM, and try running it while probing the driven I/O line with your oscilloscope. If you have all zeros (= NOP instructions) from $0000 to $003f and start your program proper at $0040, it ought to be safe enough from external hardware using RST instructions on interrupts (the easiest thing to do in IM0, which is also how the processor starts up). The Z-80 always starts executing at address $0000 on reset.

If this seems to work, you can try something more ambitious like writing to the display memory. Otherwise, you have processor or memory trouble; or maybe problems with bus logic. Is anything pulling Z-80 pin 16 (!INT) low?
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Old 25th Oct 2017, 7:38 pm   #12
acollins22
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Default Re: Nascom repair - Looking for clues please

Hi folks,

Thanks to julie_m's suggestion I have more information.

I wrote a small piece of code as suggested that tried to write to an I/O port and loop around. When this is running I can see activity on the _IOREQ_ line.

Just for the sake of control I did the same test with the screen filling code and didn't see any activity on the _IOREQ_ line.

This suggests to me that the code must be executing which in turn suggests that the ROM is being read and the CPU is running.

Yea!

Thanks everyone.

Andy.
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Old 25th Oct 2017, 9:03 pm   #13
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Default Re: Nascom repair - Looking for clues please

This is good news The Z-80 processor is mostly functional. Or most probably fully-functional, anyway.

You didn't say whether you had managed to trace the actual I/O line right through to wherever it emerged in the real world; but at least you recorded activity on !IORQ (pin 20) while I/O instructions were being performed. I would not expect any activity on the !IORQ line, unless you were using I/O instructions; you should have seen !MREQ (pin 19) pulsing low when any program was running.

What did your screen-filling program do to the display? Was it still showing random garbage, did your program have some effect on the garbage, or were you able to get the wanted character appearing in some or all positions?
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Old 25th Oct 2017, 9:27 pm   #14
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Default Re: Nascom repair - Looking for clues please

Anyone know if the MK3881 is basically just some other manufacturer's clone of the Z80-PIO, or a distinct device in its own right?

The control and data registers for port A appear to be mapped to I/O addresses 0x07 and 0X05. (Port B Control / Data registers are at I/O addresses 0x06 and 0x04).

As both I/O ports look to be undedicated / available for user use, possibly the simplest observable thing to do would be to output a running binary count to either Port A or Port B and hope to observe activity on all of the port pins. If that works, then you really can be sure that the processor subsystem is essentially working.

Edit: A quick shufty around the 'net does seem to suggest that the MK3881 is a Mostek clone of the Z80-PIO.

Last edited by SiriusHardware; 25th Oct 2017 at 9:33 pm.
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Old 25th Oct 2017, 9:58 pm   #15
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Default Re: Nascom repair - Looking for clues please

Has it been modified for use with CPM?

I built and repaired dozens of these back in the very late 1970's still have a couple.

If so its possible that the boot ROM has been disabled since CPM starts at address 0000 same as the standard Nascom operating system.

When working one of the tasks is to zero the video memory.

I did part time work for a local Nascom supplier and the local colledge students would build them and sometimes (often) they didnt work.

The first thing I did was to check that all the data lines and address lines could change state.

Worth checking all the address lines since the Z80 has a refresh register and is expecting to address dynamic memory so they should be changing state even when the processor is idling.

Incidentaly you can read the refresh register as its asynchronous to other processing functions can be used to create random numbers.

Cheers

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Old 29th Oct 2017, 9:26 pm   #16
acollins22
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Default Re: Nascom repair - Looking for clues please

Hello Folks,

So, a little more progress.

I wanted to know if the character appearing on the screen were coming out of the video memory or were being made up "on the spot" by the fault. I pulled a couple of the chips and saw that the pattern had changed as their corresponding bit had floated high.

I think this means that the problems are related to the addressing logic.

I'm now waiting for a few 74 series chips missing from my spares collection before I work over the addressing logic.


Cheers,

Andy.
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Old 29th Oct 2017, 9:59 pm   #17
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Default Re: Nascom repair - Looking for clues please

While you're waiting for the chips, can you try writing a bit of code which will wiggle an I/O port pin? As before, use no RAM and confine any variables needed to within the registers.

The test which showed _IOREQ activity apparently in response to your code running IN or OUT instructions was indicative, but not absolutely conclusive. The CPU could just have been running wild and executing randomly 'found' I/O instructions.

If you can exert programmed control over one of the I/O ports, then you can be really sure you are able to run working code.
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Old 29th Oct 2017, 10:02 pm   #18
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Default Re: Nascom repair - Looking for clues please

A good point but what I failed to mention was that I also looked at the _IOREQ_ line when running my previous code that just fills the screen with a known character and that showed no I/O activity.

(I'm also trying to hang on to a few EPROMS unprogrammed as I don't have an eraser yet )


Andy.
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Old 29th Oct 2017, 10:26 pm   #19
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Default Re: Nascom repair - Looking for clues please

Re: EPROMs, can your programmer 'do' EEPROMs? I have used a Winbond 27E512 EEPROM for this sort of thing in the past, with an adaptor similar to those you are using already to make your 2716s look like older/ smaller devices.

This type of EEPROM behaves like an EPROM in read mode, but can be erased and reprogrammed in seconds, unlike conventional EPROMs.

I might be able to find you a 27E512 if your programmer can handle them, or maybe you have something suitable already?
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Old 30th Oct 2017, 3:58 pm   #20
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Default Re: Nascom repair - Looking for clues please

If "tanning" is a problem for you, it was certainly possible in the days of the BBC Micro to use a 6264 (I think that was the part number) 8k * 8 SRAM IC with an auxiliary lithium battery back-up supply, as a substitute for a 2764 EPROM. The BBC used only memory ICs designed to run from a simple 5V supply (no +12V, -5V or -12V). Most of the pins are just passed straight through; the rest are carefully bent out of the way and the IC stood in a 28-pin DIL socket.

You would have to handle it very carefully to prevent corruption, but it should be possible to program as a "special voltage" 2764 and then connect it to the NASCOM motherboard.
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