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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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18th Feb 2021, 11:01 am | #1 |
Pentode
Join Date: Jan 2021
Location: Northampton, Northamptonshire, UK
Posts: 131
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2708 EPROM Power Sequencing
I'm currently testing my replica SOFTY 1 Programmer. Derek from Bygonebytes has kindly lent me his original board but that has a few blown IC's on it including the 8060 and I've been trying to establish why ?
The Softy 1 needs +12V and -5V in addition to +5V to power the 2708 EPROMs (firmware and the device being programmed). I have my replica board running but on +5V only (with a 2716 for firmware), I havn't yet tested programming. During my testing (of Dereks board) I've been careful to apply the +5V supply first followed by +12 and -5V my assumption being that keeping everything biased with +5V would prevent +12V or -5V from damaging anything ? I've had a look through several 2708 data sheets to see if there is any info on power sequencing. The Intel data sheet states (in a small note) that -5V must be the first and last power supply applied, other data sheets don't actually state that but do reference all maximum supply ratings to VBB (-5V). Dereks Softy is powered by a simple linear power supply, I assume it came with the Softy ? It doesn't look like there is any circuitry to sequence the supplies. I havn't found any info in the Softy 1 manual either that suggest any precautions to be taken with power supplies ? I've found several designs for 2708 programmers on the web but I've seen no specific attempt at power sequencing or mention that it is required ? Presumably the 'apply -5V first and last' is to protect the 2708, my concern is how to protect everything else on the board ! thoughts ? Chris |
18th Feb 2021, 12:56 pm | #2 |
Pentode
Join Date: Nov 2015
Location: Maidstone, Kent, UK.
Posts: 131
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Re: 2708 EPROM Power Sequencing
Just a thought, In my experience it is always worth ensuring that reverse voltage protection diodes are fitted across each power line in multi line systems to protect against different rise and fall times and reverse voltage scenarios. It is particularly important if different supply sources are used when testing. This may not be the cause of your failures but it will certainly eliminate possible future problems.
Orakle42 |
18th Feb 2021, 1:18 pm | #3 |
Octode
Join Date: May 2018
Location: Northampton, Northamptonshire, UK.
Posts: 1,440
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Re: 2708 EPROM Power Sequencing
If the 2708 is the only device that uses the extra +12V / -5V rails, then I would expect that to fail first, before any strange parasitic-powering causes <0V / > +5V to get fed to anything else before the +5V rail is stabilised.
There could be a latch-up issues, where voltages are present on pins and powering IC's, before their main +5V supply is present, but would have to be from +12V rail. I once had a +5V-powered EPROM, causing a parallel CMOS 9V? PLL IC to latch-up and get hot, as it had an RC supply filter causing a lag in this. I resolved this, by putting series resistors between EPROM data outputs and PLL inputs, to limit current below that which could cause latch-up. Putting reverse-polarity shunt diodes (or maybe some moderate-power zeners, just above rail voltage) on the rails, might help prevent incorrect voltages getting onto these. But this maybe a bit over-cautious - like power-supply sequencing - that doesn't seem to be common on equipment from that era. So I presume they got away with not requiring it, otherwise there would have been a lot of failures. I wonder how many Softy-1's are still working / failed after short periods originally, which could have pointed to a design problem. They wouldn't have had this issue with the 'replacement' and sold for much-longer Softy-2 model as it was single-rail only. |
18th Feb 2021, 3:54 pm | #4 |
Octode
Join Date: Mar 2020
Location: Kitchener, Ontario, Canada
Posts: 1,294
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Re: 2708 EPROM Power Sequencing
The older equipment would be almost certain to be using linear power supplies, with simple transformer, rectifier, bucket capacitor and voltage regulator. The -5v supply would be much lower current than +5v, so would use a smaller bucket capacitor, which then allows the -5v faster turn on time than +5v.
Bucket capacitor for 12v could also be chosen to slow the turn on time for 12v, Though this also slows the turn off time. |
18th Feb 2021, 8:09 pm | #5 |
Heptode
Join Date: Jan 2009
Location: West Lothian, UK.
Posts: 761
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Re: 2708 EPROM Power Sequencing
A friend's Nascom 1 board freshly built had its eproms killed by not having the negative supply sequenced properly.
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18th Feb 2021, 8:37 pm | #6 |
Dekatron
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,556
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Re: 2708 EPROM Power Sequencing
This is actually starting to sound like a job for a microcontroller, (to switch the rails on and off in a precisely timed sequence). The microcontroller would need to have a well reservoired supply so that it could switch the output rails off in the correct sequence even in the event of mains input failure (the assumption would be that all the rails would have enough reservoir capacitance to keep them going for a few milliseconds, but on detection of mains failure the microcontroller would switch them off sooner than that, in a precise timed sequence).
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18th Feb 2021, 10:25 pm | #7 |
Pentode
Join Date: Jun 2012
Location: Bristol, UK.
Posts: 115
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Re: 2708 EPROM Power Sequencing
Checking my '2708' data found 3 manufacturers with "Vbb on first/off last" annotation - AMD, Intel, Motorola.
Is the PSU, pictured with the Softy, supplied by Dataman? And is the EPROM one of the damaged ICs? |
19th Feb 2021, 5:04 am | #8 | |
Octode
Join Date: May 2018
Location: Northampton, Northamptonshire, UK.
Posts: 1,440
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Re: 2708 EPROM Power Sequencing
Quote:
The 2708 EPROM had survived OK, despite it appearing t be the most critical on having all 3 supplies present - Luckily, as Chris had originally just been sent that, to readout as it seems no-one had put a copy on the 'net until now. |
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19th Feb 2021, 5:09 am | #9 | |
Octode
Join Date: May 2018
Location: Northampton, Northamptonshire, UK.
Posts: 1,440
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Re: 2708 EPROM Power Sequencing
Quote:
But the DC-DC converter wasn't too reliable and they did mod. it's design a bit over the various PCB-issues, to improve it / prevent incorrect voltages being fed out if it stopped oscillating. And it's possible that a 4116 drawing too much current had killed the DC-DC converter circuit (Although I don't recall finding any dead 4116's with shorted supply inputs) |
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19th Feb 2021, 5:16 am | #10 | |
Octode
Join Date: May 2018
Location: Northampton, Northamptonshire, UK.
Posts: 1,440
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Re: 2708 EPROM Power Sequencing
Quote:
But it does seem a bit overkill for something a bit retro like this, making it rather less original, that presumably was fairly reliable originally (although with so few still around / maybe sold originally, would rely on finding people who used them a lot back then to comment on issues they had, in case there was a major design-flaw) |
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19th Feb 2021, 9:57 am | #11 | ||
Triode
Join Date: Aug 2008
Location: Marlborough, Wiltshire, UK.
Posts: 44
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Re: 2708 EPROM Power Sequencing
Quote:
But it is not so important that the -V holds up until the +V rails disappear during power-down; a properly running chip is not likely to latchup and if it did, it would only be for a few 10's of ms. Quote:
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19th Feb 2021, 9:10 pm | #12 |
Pentode
Join Date: Jan 2021
Location: Northampton, Northamptonshire, UK
Posts: 131
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Re: 2708 EPROM Power Sequencing
My thoughts are to do something along those lines. From a 5V supply use a small switcher to generate -5V, use this to power a small relay that then switches on the +5V which also powers +12V and +26V switchers. I reckon the relay delay will be sufficient to allow the -5V to settle and I can add a diode and capacitor so that it stays up last after power is removed.
I'm just having a go at programming a 2708 on the SOFTY, once thats working I can do some current measurements and work out the details. I bought 5 used 2708, so far two are faulty! |
19th Feb 2021, 10:26 pm | #13 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,998
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Re: 2708 EPROM Power Sequencing
I designed/built/marketed stuff [battery-powered digital pig-weighing scales / ultrasonic back-fat-measuring probes for cattle-markets/abbatoirs] in the early-80s using 2708s along with a 6502 microprocesor and 4116 RAM] and never needed to do anything specific about powering-up the rails in a particular order.
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19th Feb 2021, 10:43 pm | #14 |
Dekatron
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,556
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Re: 2708 EPROM Power Sequencing
I think it's fair to say that at the time all the components were common or garden and easy to find if they ever needed to be replaced. That's probably why you never had trouble with them.
Many of those same components now are expensive, hard to get museum pieces and the need to avoid doing anything which might conceivably blow them up is exponentially greater than it would have been back in the day, so I can understand Chris not wanting to leave it to chance. |
19th Feb 2021, 11:14 pm | #15 |
Pentode
Join Date: Nov 2018
Location: Freckleton, Lancashire, UK.
Posts: 232
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Re: 2708 EPROM Power Sequencing
Just had a rummage in my 'junk draw' and found I have a number of old 27xx series eeproms of varying sizes - didn't see any 2708's though.
They were all good and have been kept in anti static foam. I'm unlikely to use them, pm me with your requirements and I'll see if I can help out. Regards Tim
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Building and tinkering for over 50 years, from 807's to digital amateur TV. |
19th Feb 2021, 11:16 pm | #16 | |
Pentode
Join Date: Nov 2018
Location: Freckleton, Lancashire, UK.
Posts: 232
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Re: 2708 EPROM Power Sequencing
Quote:
Regards Tim
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Building and tinkering for over 50 years, from 807's to digital amateur TV. |
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20th Feb 2021, 1:19 pm | #17 | |
Octode
Join Date: May 2018
Location: Northampton, Northamptonshire, UK.
Posts: 1,440
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Re: 2708 EPROM Power Sequencing
Quote:
So might be worth putting a 5V6 / 6V2 moderate power zener across the +5V supply, to prevent it getting lifted by other supplies, as 7805's etc only source. But may still need to look at possible latch-up at power-on causing some devices to overheat & fail - as someone else said they'd found. Although this might be fairly easy to identify, with a lot of power-cycling testing in case it's intermittent (hopefully finding out before CPU gets damaged!) However, Chris has told me of suspected-issues with the reset circuitry / originally had what appeared to be the Firmware EPROM not programmed correctly, that could have been masking any power-on intermittent latch-up. |
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20th Feb 2021, 1:30 pm | #18 | |
Octode
Join Date: May 2018
Location: Northampton, Northamptonshire, UK.
Posts: 1,440
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Re: 2708 EPROM Power Sequencing
Quote:
What were the fault symptoms with the faulty ones? - Is it possible that the Softy isn't quite working properly yet, so making these appear faulty, unless you've tested these on your other programmers with the DIY 2708-to-2716 readout-adaptor you'd previous used for original readout. I also have the odd 2708 of unknown status, that I acquired amongst other used EPROM's I'd bought cheap from rallies over the years. And another 5off on a rather vintage Fairchild F8 chipset Qume printer-controller? card, that it would be interesting to dump contents of these. |
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21st Feb 2021, 12:53 pm | #19 |
Pentode
Join Date: Jan 2021
Location: Northampton, Northamptonshire, UK
Posts: 131
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Re: 2708 EPROM Power Sequencing
Well out of five 2708s two seem to work - a slightly better yield than I have got from recent purchases of PROMs from China (0 out of 10 that turned up).
One locks up the bus, one doesn't program and one programs and seems to verify but doesn't run reliably. I have now measured the currents so can design a power supply: +5V 330mA / 400mA +12V 27mA / 60mA -5V 22mA / 45mA +26V - / 9mA First figure is with no EPROM in programming socket, second is programming. All measurements made with current clamp meter. |
21st Feb 2021, 4:30 pm | #20 |
Octode
Join Date: Mar 2020
Location: Kitchener, Ontario, Canada
Posts: 1,294
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Re: 2708 EPROM Power Sequencing
Is that peak or average current? You don’t want the peak current to be triggering current limit and cutting out one of the supplies.
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