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Old 30th Oct 2019, 8:05 pm   #61
AC/HL
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Default Re: 8085 processor questions

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Old 31st Oct 2019, 9:45 am   #62
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Default Re: 8085 processor questions

I've returned to this machine again - Having purchased another one and got that working.

I have narrowed down the field of concern to the CPU card - by substituting the other cards into my working machine.

Now back to needing the idiots guide to repairing early microprocessor systems.

I have checked by substitution the processor, and the Eprom that is called the CDP - Customer Data Program - again both of these items work in the functional system.

So I think I am now down to a handful of logic chips, and a stack of memory to go through.

Really wish they were all socketed!
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Old 31st Oct 2019, 10:39 am   #63
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Default Re: 8085 processor questions

That's great that you've narrowed it down to one card. In my experience, by far the most unreliable parts on processor boards of this era are RAM chips. A distant second in the unreliability stakes are bus buffers, usually the 74LS245.

Go for the RAM first.

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Old 31st Oct 2019, 4:37 pm   #64
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Default Re: 8085 processor questions

Hi Chris,

Yes, that's where I am heading. Sourcing RAM chips is not a cheap exercise! replacing all 16 will cost more than the machine purchase

But it will be fun.

Any reason I should not put them in sockets?
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Old 31st Oct 2019, 4:50 pm   #65
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Default Re: 8085 processor questions

What are the RAMs? In my experience 2114s (particularly) are 'replace on sight' if you have odd faults.

There is normally no reason not to use DIL sockets. Just about the only thing I could think of is if there's not enough space above the ICs for the case to fit if they're in sockets. The extra stray capacitance of the socket will make no difference if the drive signals are buffered adequately.

But use turned pin (machined pin in the States) sockets. Yes they're more expensive, but so is your time tracing intermittant faults!
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Old 31st Oct 2019, 5:03 pm   #66
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Default Re: 8085 processor questions

I don't know the board your using but...I've got a few BBC Micros and Atoms that when they exhibit memory faults one can get a known good ram chip, bend the legs in slightly and piggy back it on the suspect chip(s), trying in turn until it starts to work.

This isn't guaranteed to work but often its the output drivers of the chips that are goosed and this is a quick way of testing that. Also scoping the Q pins of the chips will often show an inactive or weedy one. You will need to preferably have a logic analyzer or a good scope that can trigger when the chip is selected for read...

D
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Old 31st Oct 2019, 5:09 pm   #67
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Default Re: 8085 processor questions

Tony, Currently installed RAM are AM92L44DPC

Hi Dominic, thanks for the suggestion - no logic analyser here (not sure that I really want one) but have a half decent 100Mhz Tek storage scope - think that ought to do fo something this slow...

Not that I have any idea what I am supposed to be looking for - Im much more at home on analogue stuff
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Old 31st Oct 2019, 5:28 pm   #68
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Default Re: 8085 processor questions

Ah, those look to be static rams rather than the CMOS dynamic ones so the piggy back thing probably won't work.

One thing to look for on rams is whether any of the outputs are stuck at '1' or '0' or not giving a solid high or low voltage. Trigger when chip enable is active. Again it's not an entirely foolproof way of identifying the culprit but gives a good indication.

As cmjones says any '245s should also be treated with suspicion as they are often driven quite hard and even when still more or less working can become tired or slow enough to cause problems.

D
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Old 31st Oct 2019, 5:39 pm   #69
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Default Re: 8085 processor questions

Okay,

So, I will bite the bullet, and pull the RAM out, and install turned pin sokets, this will make plugging RAM in much easier, and should allow for some easier testing I hope

Before I try this, I will see if I can get a 'scope on the chip outputs - guess I am looking for a latched 5ish volt output
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Old 31st Oct 2019, 5:40 pm   #70
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Default Re: 8085 processor questions

Static RAMs, especially the 2114, 2102 and 2101, are just as unreliable as early dynamic RAMs!

Those 9244s look a bit odd, being 4k x 1 bit. I'd be tempted to remove them all and wire in a more modern static RAM, something like a 6264 (8k x 8 bit). Depending on the original circuit, it might need no more logic, or possibly one gate, to generate the /CS, /OE and /WE signals that a modern RAM needs from the /CS and /WE ones on the 9244. If I get chance, I can sketch out the circuit.

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Old 31st Oct 2019, 5:48 pm   #71
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Default Re: 8085 processor questions

Whoosh!

Yup, that was the sound your post made as it whizzed right above my head Chris

Sounds "interesting" (read complicated)

But certainly want to know more - a less ancient device stands more chance of being reliable/available I guess
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Old 31st Oct 2019, 8:40 pm   #72
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Default Re: 8085 processor questions

I've had a go at desoldering one of the RAM chips - I need a better method, double sided through hole plated board.... Just to be extra awkward, the manufacturer has bent the legs of the IC over as well....

Manual solder sucker is not getting to it and solder braid while sort of effective, is still not getting through.

I think dremel through the legs, and pull them out a pin at a time
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Old 31st Oct 2019, 8:57 pm   #73
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Default Re: 8085 processor questions

You don't have an old high end EPROM programmer do you Sean? They often have a 'chip test' utility which can test 7400, 4000 series logic and usually common RAM ICs as well.

Alternatively you could use something like an Arduino (One with enough pins to handle all the address, control and data pins on one RAM device) as a RAM tester for a specific RAM device.

Edit: of course you'd have to get the IC out first....

... Unless... you removed the 8085 and connected a self contained microcontroller (PIC, etc) to the buses and used that to test the RAM 'in circuit'.
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Old 31st Oct 2019, 9:51 pm   #74
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Default Re: 8085 processor questions

>>I think dremel through the legs, and pull them out a pin at a time

less messy to use a sharp pair of side cutters. But, yes; cutting out the chip is the correct approach when the PCB is more valuable than the chips it contains (and especially for multi-layer boards).

I think Chris's suggestion (replace them with a modern RAM) is a good one. If you install turned-pin sockets you can build a daughter-card on vero board that will plug in cleanly and will not stop you from fitting "genuine" memories in the future should you be lucky enough to find some.

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Old 31st Oct 2019, 9:59 pm   #75
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Default Re: 8085 processor questions

Thanks both, No, sorry, no High end eprom programmer - no eprom programmer at all infact.

More I look at this more I think modern RAM is the answer - sharp side cutters are in stock, and yes, much less mess.

I do have an arduino kit here, but have not really looked at it. I think seeing as RAM is at best suspect, testing it may prove to be an exercise in futility - I dont know enough about Arduino to carry out this sort of thing, so any results or lack there of will mean little to me.

Not gonna rush into this - while I would like to get the machine to work, it isnt a race
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Old 31st Oct 2019, 10:25 pm   #76
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Default Re: 8085 processor questions

If you go with Chris' suggestion and make a daughter board with a 6264 you will almost certainly be ok with CS and OE just tied together. The data output is suppressed if WE is asserted so there is no data bus conflict. The 9224 RAMs do have separate data in and data out pins where the 6264 just has data I/O so you could check that in and out are tied together on your board and then the 6264 wiring would be very straightforward.

There are some very similar 4k X 1 bit SRAM chips on eBay in batches of 21 at a decent price but with postage from US. They lack the low power feature of the 9224 so there would be some risk in swapping those in, but it would look more original.

Regards,

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Old 31st Oct 2019, 11:18 pm   #77
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Default Re: 8085 processor questions

I'm not overly worried about originality, reliability (well as much as a 35 year old electronic teleprinter can be) is more of the key here - If the repair is affordable and fairly simple, there could be application for this on other machines in museums - I know of one that has about 20 of these machines, all not functional.

I need to learn something about all these WE CS and OE things.

Power consumption is a minor, but important concern - the power supplies in these machines are a known weak point - I have rebuilt this one with new capacitors, but they still run fairly hot and stressed.
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Old 1st Nov 2019, 12:54 am   #78
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Default Re: 8085 processor questions

Okay, possibly understanding some of the abbreviations now.

WE - Write Enable
CS - Chip Select
OE - Erm, Stumped !

I am also guessing that I would need fewer devices if I used the 6264 - 8K would mean only needing 8 devices? or am I missing something fundamental here?

Is it really as simple as connecting the address lines in the correct order, adding a couple of supplies, and away we go?
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Old 1st Nov 2019, 5:59 am   #79
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Default Re: 8085 processor questions

OE = Output Enable. A signal to turn on the data outputs on the RAM chip for reading the data out.

Note that these signals are often active low at the chip pin. Meaning that when, say, CS is a logic 1 ('high', towards +5V) the chip is NOT selected. To select the chip you have to make CS a logic 0 (low, towards ground). This is indicated on the circuit diagram and data sheet by a line over the signal name, and in typing by putting a '/' after it, so CS/ or OE/

To store 8K bytes you just need one 6264 chip (!). BUT I do feel that you should try to keep the original design and RAM chip type if you can.
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Old 1st Nov 2019, 1:34 pm   #80
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Default Re: 8085 processor questions

I've popped an offer in for some M5T-4044 ICs - the specs seem broadly similar to the original devices.

In the meantime I will order some sockets - this way, I can also look at the potential for making alternative RAM cards as a plug in option
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