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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 1st Jan 2018, 2:15 pm   #1
Sean Williams
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Default 8085 processor questions

This post relates to some problems I am having with a BT Telex Machine (schematics requested elsewhere, but no luck as yet)

The beating (or not) heart of the machine is the 8085.

I am trying to get to the bottom of the problems that this system has, so thought attacking it from the basic computer that it should be will be a start.

I am well out of my depth here, so any assistance will be welcomed.

In a nutshell, when powered up, the screen is a mass of mixed characters, this has been the case since day 1.

I have checked all the supply rails, and the power supply is fully functional.

I have noticed that the processor is getting warm, and there is a green status led that should be illuminated, but this is not happening. I reason that the actual processor chip is dead, so am awaiting a replacement. (at least substituting this for a new processor should rule it out)

Attempting to scope the clock crystal results in no activity at all.

I have popped the processor out, and powered the machine up without it, the screen still fills with random characters.

So, I wonder what is normal for the 8085 to drive a CRT monitor? is the monitor driven straight from the processor, or is there some other trickery at play here?

Cheers
Sean
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Old 1st Jan 2018, 2:26 pm   #2
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Default Re: 8085 processor questions

I would be tempted to check why the crystal is not oscillating, check the crystal, driver chip, load capacitors .etc (I have known a crystal to fail before in my sinclair ql). if the cpu is not getting a clock signal it won't do anything and the junk on screen could just be indeterminate values from ram powerup.
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Old 1st Jan 2018, 2:41 pm   #3
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Default Re: 8085 processor questions

In this application, the crystal appears to be driven by the 8085 - there appears to be no capacitors connected to the crystal either.

Perhaps I should pull the crystal and test it in a simple oscillator circuit?
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Old 1st Jan 2018, 2:49 pm   #4
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Default Re: 8085 processor questions

If the crystal is just connected between two pins of the processor (X1 and X2) with no other driver chippery then it will be at a fairly high impedance and the act of scoping it may be enough to stop oscillation. You may be better looking at the "CLK (OUT)" pin on the processor.
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Old 1st Jan 2018, 2:52 pm   #5
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Default Re: 8085 processor questions

Thanks Dickie, interesting point that I had not considered - will try that.
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Old 1st Jan 2018, 3:02 pm   #6
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Default Re: 8085 processor questions

I had problems with an 8085 in a Marconi Signal generator.

I found a cheap logic probe from Maplin very useful for checking for stuck address and data lines.
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Old 1st Jan 2018, 3:22 pm   #7
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Default Re: 8085 processor questions

If you find a clock signal, then have a look on the data and address buses for activity. If there is some then the processor is running and you can look for stuck lines. If there is none then check that the hold, reset, and various interrupt lines (rstxx) are at sensible levels and not stopping things running.
Unless it is a very simple application, the 8085 will not be driving the CRT directly.
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Old 1st Jan 2018, 3:42 pm   #8
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Default Re: 8085 processor questions

It is a teleprinter, so I would say not simple, well other than decoding and displaying text.
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Old 1st Jan 2018, 3:45 pm   #9
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Default Re: 8085 processor questions

The other chip types should give you an idea as to how the display is driven.
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Old 1st Jan 2018, 4:01 pm   #10
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Default Re: 8085 processor questions

Not sure about your specific beastie but usually these have a character-generator chip that holds a matrix of 1s and 0s to represent each displayable character on a suitably-sized matrix 9x6 dots or similar] - if you're getting a locked picture with recognisable characters [even if gobbledegook] displayed on the screen then it at least shows this character-generator is OK [I've known them go bad].

Easy way to test that the CPU clock is running without imposing any possible disruptive load on the clock circuitry is to listen for it with a shortwave receiver! The clock freq will be marked on the xtal can - set your SW radio to SSB or CW then attach a short length of wire to the antenna terminal and dangle it near the clock xtal. Tune around a few KHz of the marked frequency and you should easily detect the carrier as a raspy buzz [it's near-enough a square wave so don't expect a 'T9' note!]
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Old 1st Jan 2018, 4:06 pm   #11
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Default Re: 8085 processor questions

Certainly getting a locked picture - and a good selection of identifiable characters.

Will go have a listen for the oscillator - 5.136MHz is certainly a strange frequency

Logic probe ordered as well - can't grumble at a fiver!
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Old 1st Jan 2018, 4:22 pm   #12
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Default Re: 8085 processor questions

There are a couple of display controllers from this period that i am familiar with. More common was the Motorola 6845 which generated all the clock signals for character generation and sync pulses and had a separate character generator memory which could be ROM or RAM, RAM offered you user definable characters. The 6845 will only generate proper sync pulses after its registers have been initialised so if you have a 6845 and a stable picture then the CPU is doing at least some of its initialisation.

Less common and very down market was the 6847 which would generate NTSC signals including the colour burst. It was low resolution and I doubt you would find it in a serious instrument (it was used in some of the home computers like the Dragon 32). The 6847 generates all the timing and colour signals without any external initialisation.

I am sure there must have been other packaged display controllers and it was not uncommon to do all the timing signals in 74LS logic, probably a dozen chips.

If you could post a list of all the large chips (more than 20 pin) it might help to guess what is what.

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Old 1st Jan 2018, 4:45 pm   #13
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Default Re: 8085 processor questions

Ok, well, i am now certain that the clock is not working.

Scoping clock out, just reads noise, coupling an antenna to a nearby hf radio picks up no noise at all.

Coupled to that, the processor does not get warm, it gets HOT!

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Old 1st Jan 2018, 4:47 pm   #14
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Default Re: 8085 processor questions

Roger, there are lots of 74LS type chips inside this thing.

I have checked all of the 20 pin IC's and none of them come up listed as a CRT driver, so perhaps your latter suggestion applies.
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Old 1st Jan 2018, 7:04 pm   #15
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Default Re: 8085 processor questions

I have to agree that in the past I have had real difficulty using a scope on the crystal as the capacitance of the probe stops it working. Scoping on other pins like clock out to the other ICs is a better indication. It has been many years since I worked with that micro (a bad experience during my degree project work) so can't help any further I'm sorry.
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Old 2nd Jan 2018, 1:27 am   #16
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Default Re: 8085 processor questions

It's a shame I didn't know you were going to pose this question as I recently intercepted three or four new-old-stock 8085A CPU ICs on the way to the WEEE waste. One is an original Intel part, the others are other-manufacturer clones (Toshiba, NEC).

Warm running does not necessarily indicate a fault in CPUs of this era, although excessive heat does. By excessive, I mean, you can't even keep your finger on it after a minute or so. Anything less warm than that could be quite normal.

The weird crystal frequency will (like nearly all weird CPU crystal frequencies) be something to do with either tone frequency generation/detection or baud rate generation - in this case most likely the latter.

The trick of using a receiver to 'sniff' an oscillator which might stop dead if you connect a scope directly to it is an old favourite of mine as well, but it's also worth switching your scope probe to X10 and increasing the sensitivity of the scope channel by ten and trying again.

That EPROM with the unprotected window is making me quite nervous, if it has always been like that then you could be suffering from data fade in the EPROM, assuming that's where the firmware is.
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Old 2nd Jan 2018, 7:48 am   #17
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Default Re: 8085 processor questions

Thanks Sirius,

Yes, this is a problem i have also considered - I am looking to see if I can find someone with another machine that is functional - if I can, I will take a copy of a functional eprom, just in case.

If the datasheet of the 8085 is indicative of this application, then the firmware will almost certainly be on that eprom.

The processor is too hot to touch after 5 minutes of operation, so I think this is a fairly reliable indication of an issue.

Yes, it is a shame that I have already ordered a selection of replacement 8085s, I should have enquired here first I guess.

Back to work today, so playing time is limited again

Thanks
Sean
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Old 2nd Jan 2018, 8:57 am   #18
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Default Re: 8085 processor questions

The CPU clock not running could certainly cause these symptoms. Check with your scope on pin 37 which is the clock output: that doesn't run the risk of the probe capacitance affecting the crystal oscillator. Another likely possibility is the reset or interrupt lines being stuck. Check those with a multimeter or logic probe.

RAM faults are also common. There's a bank of 16 chips on the left hand side of the board which looks like RAM to me, though I can't read their part numbers. They have 18 pins which makes me think they might be 2114 static RAMs, which are amongst the most unreliable chips ever made! Check what part they actually are.

Chris
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Old 2nd Jan 2018, 3:33 pm   #19
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Default Re: 8085 processor questions

Have a look at the oscillator xtal and see if there are a couple of small capacitors [22 33 or 47pF being typical] one from each xtal pin to earth.

I recall a case in the past where one of these failed and it was enough to unbalance the circuit and stop the oscillator from doing its thing.

There's also a phenomenon known as "Sleeping Sickness" in crystals - generally believed to be due to contamination during manufacture. Basically if the equipment isn't used for some time the crystal becomes _very_ reluctant to oscillate. Confusingly, if you remove a crystal suffering from this and put it on a test-rig that drives it a bit harder than the equipment it was fitted to, it 'wakes-up' and works just fine, as it then will when refitted to the equipment. But if the equipment's not used for a few months the problem recurs.

[I had a batch of VHF "Stornophone 500" walkie-talkies that were afflicted by this. Annoying, since they were specified for emergency-use so typically spent six months languishing in their holsters unloved and unused, just long enough that the crystals went sleepy and the radios then didn't work when needed]
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Old 2nd Jan 2018, 7:29 pm   #20
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Default Re: 8085 processor questions

A little progress update.

A replacement 8085 appeared in the post today, so I slotted it in - result..... Well, the green CPU led is now working, and the clock is running, and producing a T9 note as well

There is another oscillator on the card behind that wasnt wiggling at all, this was traced to a dead 74S04 - I only has 74LS04 in stock, so tried one of those instead - the oscillator is now also running.

Still nowt but rubbish on the screen though, so I guess next stop will be logic probe and see what the address lines are doing.

Thanks for all the helpful comments so far though.

Sean
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