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Old 5th Apr 2021, 10:10 am   #1
Buzby123
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Default MK14 SN74S571 PROM Power reduction test

I got some tiny P-channel MOSFETs for a different project, so I thought I might have a go at the power reduction idea mentioned in the Micky thread.

I just wired the FET to apply 5v to the PROM when /OE ( pin 13 ) was low. The FET gate was wired directly to the /OE pin.

Results were quite promising. The current consumption of the MK14 dropped from 470mA to 380mA. The current into a single PROM normally was 85mA, so this idea has reduced that by half.

There is plenty room for improvement.

The way the FET, ( an AO3401 ) is wired is far from optimum. The FET has a huge input capacitance, nearly 700pF. The gate in IC17 can easily drive the FET 'on', but takes ages to turn it 'off'. I tried a few pullup resistor ideas to improve switching times, but in reality I don't think a FET is the way to go.

Definately an idea with legs.

Cheers,

Buzby
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Old 5th Apr 2021, 11:13 am   #2
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Default Re: MK14 SN74S571 PROM Power reduction test

Thanks for posting your findings on this.
I presume putting x10 etc. 'scope probes on the input and output, would show how long the FET is taking to turn off relative to input signal, to gauge how much more power-saving should be possible with a more suitable FET / better drive to it.

MOSFET input capacitance is generally proportional to output current (so inversely proportional to RDSon). So rather than using a 4A one, if a much lower rated one is used then input capacitance wll be much lower.
e.g. Just checking the spec. of a popular P-Ch FET, the BSS84 (Id(Max)=130mA), this has an input capacitance of 25pF typ.

So this should do for a single PROM, as you've used one for each, but would need something a bit better higher Id(Max) rating to power both PROM's from a single device (but maybe not as easy to implement with single switch on the MK14's layout).
And a parametric search on Digikey etc. could help find some more optimum ones.
Driving the MOSFET with a fairly low Rds(on) N+P Ch. CMOS style output stage, would also help to turn it off very quickly, compared to open-drain resistor pull-up RTL style logic.

So don't necessarily have to resort to a bipolar PNP device (But maybe could try something like a BC212 (Ic=200mA Max.) or BC327 (Ic=500mA Max) instead.
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Old 5th Apr 2021, 2:59 pm   #3
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Default Re: MK14 SN74S571 PROM Power reduction test

Bipolar proms have an access time of approximately 30-50ns, but the read cycle of the 8060 is an order of magnitude higher. If you could delay the turn on time from the output enable you might get further power reduction.
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Old 5th Apr 2021, 3:28 pm   #4
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Default Re: MK14 SN74S571 PROM Power reduction test

Yes, you could insert an RC delay circuit from /OE line to delay it a bit.

But you would then need a schmitt-trigger buffer, to ensure a fast switch-on of the power-switch, as having it slowly ramping wouldn't be too good for power reduction and reliable operation.
And would also need a diode bypass across the the resistor of the delay circuit, to provide a fast switch off when /OE goes high again.
- Although a set-reset flip-flop might work better for doing this, working faster and avoiding needing peak currents in the delay capacitor.

However this does add quite a bit more complexity to it allfor maybe only slight power savings. And could save even more, if you could get the CPU in a CMOS version - maybe an implementation of it in a modern FPGA (but then rather moving away from originality of it)
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Old 5th Apr 2021, 3:58 pm   #5
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Default Re: MK14 SN74S571 PROM Power reduction test

I think this is technically interesting but ultimately will make the machine look a bit untidy, just the way the add-on single-step circuit did. Even if it cut the current consumption by half I don't think I would do it to either of my MK14s but it is a shame SOC didn't design it in. Certainly the small heatsinks typically fitted on MK14 regulators are barely adequate so anything which would have reduced the current consumption at the original design stage would have been a good idea.

I've asked Kan_Turk, when he gets a chance, to fish me out another couple of those AMD 27S13As which I'm going to programme with the 'new' OS and fit in my issue VI to see if it draws less / runs cooler.

One of my other nerdy interests is amateur radio and I can think of at least two late seventies - mid eighties era synthesised handheld radios which used current-sucking Bipolar PROMs for their channel programming. That would have been a much more obvious application in which to try this trick. I'll have to have a look at the diagrams and see if they actually did.
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Old 5th Apr 2021, 4:58 pm   #6
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Default Re: MK14 SN74S571 PROM Power reduction test

Yes, unless you're going to be using the MK14 a lot, it probably isn't generally worth the effort. Although for a replica one, that you don't mind experimenting a bit with add-ons, then it's something you can always try. And can probably do it in a way that's easily reversible with plug-in boards etc. rather than butchering the PCB a bit.


In Radio-transceivers, the PROM's would usually have been permanently selected if used with parallel-input PLL's that were commonplace then. And would involve a bit more complexity, having output latches etc. so that the PROM's could be shutdown.

It would probably be easier to just use a usually much lower-power EPROM instead (as many CB conversion boards did), and could probably find some room inside to add a converter board - especially as there's less concern about keeping these original and don't yet command high retro-collectable prices that computers etc. can.
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Old 5th Apr 2021, 8:12 pm   #7
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Default Re: MK14 SN74S571 PROM Power reduction test

The problem was size - something like a PYE PFX or a Storno 4000 was already tightly packed with parts, so there wasn't really any room for a wide-bodied 24 pin or 28 pin EPROM in there. You only really had that luxury in mobile radios.

Quote:
And can probably do it in a way that's easily reversible with plug-in boards etc. rather than butchering the PCB a bit
I forgot to say that Buzby couldn't have done that mod any more neatly, I was imagining bits of vero stuck on the underside of the PCB.
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Old 5th Apr 2021, 8:33 pm   #8
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Default Re: MK14 SN74S571 PROM Power reduction test

I'd only encountered PROM's in mobile PMR's (Think there's one in a Storno 5000 I'd been given), where there was generally plenty of room to slip in an EPROM board, and not encountered them in a handheld.
But these days, you could probably fit a small board with a smaller SMD parallel EEPROM or maybe a small uC - both allowing them to be reprogrammed.

Yes, Buzby did quite well managing to insert those SOT-23 devices in these.
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Old 5th Apr 2021, 10:11 pm   #9
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Default Re: MK14 SN74S571 PROM Power reduction test

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Originally Posted by ortek_service View Post
Yes, Buzby did quite well managing to insert those SOT-23 devices in these.
I must admit this was the first time I'd used SOT-23 devices, and they are smaller than I expected. ( Yes, I know the specs give the dimensions, but the numbers just don't seem that small when written in text. )

The technique was to solder all 16 legs of the wire-wrap socket to the header, then snip a tiny piece out of one leg. Judicious use of a pair of tweezers, some BluTac, and my inherent myopia allowed me to solder the device into the gap. A tiny piece of solderable enamel wire completed the circuit.

The module could have been made much more low-profile, but I wanted enough room to experiment, such as adding resistors, or another SMD type device, if needed.

I will continue with the tests, and try a PNP transistor as shown in the original NatSemi databook. They must have had a reason to put that app note in the book !
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Old 5th Apr 2021, 11:53 pm   #10
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Default Re: MK14 SN74S571 PROM Power reduction test

Sirius asked for a comparison of temperature between the original DM74S571N and the Phillips 82S131N in a different thread, but I thought it might make sense to add the results to this thread on PROM power reduction.

Measurements were taken using a cheap, non-contact thermometer. The laser pointer is not useful at the range I’ve been using it at, approx half inch above the surface of each chip, so I’m just scanning the surface and noting the max temperature measured. I left the default emmissivity at 0.95 which should be correct for black epoxy ICs.

Temperatures are reasonably stable after 30 minutes. I’ve noted the temperature at 10 minute intervals.

DM74S571N fitted to original MK14 issue IV. 82S131N fitted to issue VI replica. Both without on board regulators and powered from the same power supply.

DM74S571N measures 46.5C to 47.0C.
82S131N measures 43.5C to 44.5C

I’m not sure I could tell the difference with a touch test.
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Old 6th Apr 2021, 12:00 am   #11
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Default Re: MK14 SN74S571 PROM Power reduction test

I wonder if the same power switching mod would work for other manufacturers bipolar proms or if only the nat semi parts have floating outputs when the supply is switched off.
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Old 6th Apr 2021, 12:21 am   #12
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Default Re: MK14 SN74S571 PROM Power reduction test

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Originally Posted by Mark1960 View Post
I wonder if the same power switching mod would work for other manufacturers bipolar proms or if only the nat semi parts have floating outputs when the supply is switched off.
Probably a good chance with most equivalents to the NS ones, as they are likely to be quite similar in design (as power-consumption figures might confirm)
Although maybe the Tesla ones might be a bit different if they use different programming arrangements.
And although TI don't do an equivalent in this size, the 'half-size' SN74S287 may be rather different, with its inverted programming / bits seems to default to 1 rather than 0 when blank.
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Old 6th Apr 2021, 8:20 am   #13
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Default Re: MK14 SN74S571 PROM Power reduction test

Unfortunately I don't have a spot-thermometer like Mark's - interesting that the 82S131s for him run barely any cooler than the furnace-like 74S571s. When I get the 27S13As I'll compare the overall current consumption from 5V with those fitted and with 74S571s.
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Old 6th Apr 2021, 10:09 pm   #14
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Default Re: MK14 SN74S571 PROM Power reduction test

Slightly OT, but interesting anyway.

The current consumption drops from 380mA to 290mA when the bubble display is removed, showing that the display is using 90mA.

Doesn't that seem like quite a lot for such a little display ?.
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Old 6th Apr 2021, 10:26 pm   #15
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Default Re: MK14 SN74S571 PROM Power reduction test

When you consider that those displays were widely used in calculators running from a PP3 (9V) battery, that does seem quite high although I suppose in that application digits were only lit when you were activiely using the unit. At most, there may have been a single '0' lit when the calculator was waiting for input.

It'll be interesting to see what the restored 'large' display adds to (or even subtracts from) that.

Did you manage to fix the bubble display, then?
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Old 6th Apr 2021, 10:26 pm   #16
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Default Re: MK14 SN74S571 PROM Power reduction test

Quote:
Originally Posted by Buzby123 View Post
Slightly OT, but interesting anyway.

The current consumption drops from 380mA to 290mA when the bubble display is removed, showing that the display is using 90mA.

Doesn't that seem like quite a lot for such a little display ?.
Not really - If display is showing six 0's then that's only an average current of 90 / 36 = approx 3mA per LED segment
- Although peak current is likely to be 8x that, as the multiplexed display has to be scanned one digit at a time, but the peak-current is only actually limited by the 7408 source current as there's no current-limiting resistors.

And it seems the LED's in those days weren't too efficient, compared to later ultrabright etc. ones. Plus they are rather small and maybe bubble lens over all the segments doesn't focus as well as larger-LED ones..
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Old 6th Apr 2021, 10:28 pm   #17
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Default Re: MK14 SN74S571 PROM Power reduction test

Quote:
Originally Posted by SiriusHardware View Post
Unfortunately I don't have a spot-thermometer like Mark's - interesting that the 82S131s for him run barely any cooler than the furnace-like 74S571s. When I get the 27S13As I'll compare the overall current consumption from 5V with those fitted and with 74S571s.
Ambient temperature in the basement was only 18C, not sure if thatís making a difference, but was the same for both types.

Iíll take some current measurements when I get time. That should help us get a better comparison with your measurements. Let me know what configuration you use for current measurements, my original only has 256 bytes ram, I could add the 8154 back in if that makes it closer to yours. I donít want to add more ram unless I find some retro sockets.
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Old 6th Apr 2021, 10:58 pm   #18
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Default Re: MK14 SN74S571 PROM Power reduction test

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Originally Posted by SiriusHardware View Post
Did you manage to fix the bubble display, then?
No, I'm now using the 'new' bubble display I bought years ago, when I was tempted to re-build an Acorn System-1 on some original boards I still had.

That never happened, so I sold the pair of boards on eBay. Got £100 for them !.
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Old 7th Apr 2021, 1:06 am   #19
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Default Re: MK14 SN74S571 PROM Power reduction test

If they had been assembled originals, it seems they've recently gone for £500 for each board of the System-1 pair!
So after discovering I got some original Acorn system 1 / 2 boards (unfortunately not the top keyboard / display / cassette one) in surplus 50p Radio Rally bags a few years ago, which had all the IC's etc. removed, I've been re-assembling them with suitably vintage IC's I mostly had to get a working mostly-original rack system.

I did recently buy a Russian ALS318A version of the display (for < £15 with the VAT that's now being charged on orders from their) - as the surplus ones I'd got from Greenweld back in the 80's were slightly smaller / different pinout ones commonly used in 70's pocket calculators - to complete a replica Mk14.
But have only tested it on DMM's diode-test, so it was fairly dim at those currents.
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Old 20th Nov 2022, 10:17 am   #20
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Default Re: MK14 SN74S571 PROM Power reduction test

Thread reopened at OP's request.
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