What's the worst that could happen?

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crj
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What's the worst that could happen?

Postby crj » Wed Jan 03, 2018 11:27 pm

For once, this is not a rhetorical question.

I want to develop some hardware connecting to a Beeb's ROM socket. I used to have a lot of spare machines, but then I donated them all to the National Museum of Computing in Bletchley. The one 8-bit Acorn machine I have left is my trusty old Master Turbo that I've had since I was a kid.

I would very much rather not break it.

I'd intended to pick up a cheap model B on eBay, but after three or four weeks I've still not managed because LOLPRICE. )-8

Either I have to pay more than I feel is reasonable for a model B, or I have to plug prototypes into my beloved childhood computer. So I've begun contemplating the latter and wondering... really, what's the worst that can happen?

Suppose I screwed up completely and put a random assortment of 3.3V and 5V signals at anything up to 300MHz on the various pins of a ROM socket. Suppose I was a complete idiot and shorted power to ground. Suppose I ended up double-driving buses everywhere, or driving the data bus to an indeterminate logic level. Obviously, any data the machine was working on would be toast and it would be folly to have an important floppy in the drive, but how much danger is there of damaging the hardware?

One small mercy is that I wouldn't be playing with any voltages outside the 0V-5V range relative to the Beeb's ground. That might be enough to limit the damage I could do.

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1024MAK
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Re: What's the worst that could happen?

Postby 1024MAK » Thu Jan 04, 2018 1:16 am

Err, first off, yes, it's better to use a Model B rather than a Master. Simply because most of the chips in a B are still available. Whereas, the Master uses a lot of custom chips. These are in a limited supply, and we don't have much data on them :-(

74LSxxx and 74ALSxxx chips are relatively hardy and can survive brief short circuits on their outputs caused by other chips outputs being connected to the same point. As can most NMOS chips like the CPU and the DRAM.
So more than one chip driving the same bus/line is not normally terminal. As long as the duration is limited to short pulses.

Intermediate levels should not cause any permanent problems. As it's unlikely enough current will flow to cause any damage.

Should however, an output be connected to either +5V or 0V (GND), it's less certain that no damage will be done. It all depends on the chip, the duration of the short and the impedance of the short circuit.

However, nearly all of the chips in a Beeb can be replaced. So if you do kill a chip, or multiple chips, it will most likely still be repairable.

There are two things that are certain to kill chips, either a voltage significantly above their rating (ether on it's supply pin(s) or it's inputs), or incorrect polarity/negative voltages (except where permitted, like on 4116 DRAM negative supply pins for example).

Shorting the computers +5V and 0V (GND) lines, however may cause a PCB track to fuse open circuit. As although the PSU will limit it's output power, it's possible than enough current will flow to overheat a thin PCB track.

Mark
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Re: What's the worst that could happen?

Postby Boydie » Thu Jan 04, 2018 4:35 pm

I have 3 faulty Model B motherboards, which you’re welcome to borrow and have a crack at fixing. I’ve also got a spare keyboard (don’t know if it works) which you could also borrow.

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Re: What's the worst that could happen?

Postby crj » Thu Jan 04, 2018 6:11 pm

Thanks for the advice. It would be great if you could elaborate on a few points...

1024MAK wrote:74LSxxx and 74ALSxxx chips are relatively hardy and can survive brief short circuits on their outputs


What does "brief" mean? Are you talking about a 10ns transient glitch, or ten seconds while I notice something's wrong? (-8

There are two things that are certain to kill chips, either a voltage significantly above their rating (ether on it's supply pin(s) or it's inputs), or incorrect polarity/negative voltages (except where permitted, like on 4116 DRAM negative supply pins for example).


To prevent that, all I have to do is make sure I wire up ground and Vcc on every component correctly and never work with anything above 5V, being certain to maintain a common ground. Given the only sources of power will be the ROM socket itself and USB, I'm confident on that point, at least.

Shorting the computers +5V and 0V (GND) lines, however may cause a PCB track to fuse open circuit. As although the PSU will limit it's output power, it's possible than enough current will flow to overheat a thin PCB track.


Hmm. Though it's hard to be sure without knowing the copper thickness, it looks as though the Beeb's power and ground rails are easily hefty enough to sustain the 3.5A the PSU is rated for. I'm assuming they are rated to minimise voltage drop rather than just for the current they'll carry. Then again, I don't know what current the PSU drives into a dead short; the limit current is presumably higher than the maximum rated load, but by how much?

(A friend who built himself a pulser for intentionally blowing out internal tracks in multi-layer boards had it deliver 100A momentarily, ISTR.)

It would be great to hear anecdotal evidence from anybody who's actually shorted a PSU and/or run silly power through the PCB traces.

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1024MAK
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Re: What's the worst that could happen?

Postby 1024MAK » Thu Jan 04, 2018 9:49 pm

crj wrote:Thanks for the advice. It would be great if you could elaborate on a few points...

1024MAK wrote:74LSxxx and 74ALSxxx chips are relatively hardy and can survive brief short circuits on their outputs


What does "brief" mean? Are you talking about a 10ns transient glitch, or ten seconds while I notice something's wrong? (-8

Generalising here.. I would say in the order of 10ms or less. Less of course being better. But some may survive for longer. One problem is that the circuitry that is connected to the pin may overheat within the chip die.

Ten seconds would definitely be pushing it though...

crj wrote:
Shorting the computers +5V and 0V (GND) lines, however may cause a PCB track to fuse open circuit. As although the PSU will limit it's output power, it's possible than enough current will flow to overheat a thin PCB track.


Hmm. Though it's hard to be sure without knowing the copper thickness, it looks as though the Beeb's power and ground rails are easily hefty enough to sustain the 3.5A the PSU is rated for. I'm assuming they are rated to minimise voltage drop rather than just for the current they'll carry. Then again, I don't know what current the PSU drives into a dead short; the limit current is presumably higher than the maximum rated load, but by how much?

(A friend who built himself a pulser for intentionally blowing out internal tracks in multi-layer boards had it deliver 100A momentarily, ISTR.)

It would be great to hear anecdotal evidence from anybody who's actually shorted a PSU and/or run silly power through the PCB traces.

Somewhere on this forum, someone had a board that they were repairing where a short (on the RGB output I think) had taken a track out.

You would be unlucky to do the same, but it may be possible. If you are concerned, you can always use a polyfuse or a low value series resistor to limit the fault current.

Mark
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Re: What's the worst that could happen?

Postby crj » Fri Jan 05, 2018 3:19 am

OK.

On the one hand, I'm not especially error-prone. On the other, I'm scared enough that I'm heading back to eBay and making soothing noises to calm my wallet. )-8

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Re: What's the worst that could happen?

Postby helpful » Fri Jan 05, 2018 5:00 am

Might be worth contacting the Centre for Computing History to see if they have any spares for sale in return for a donation to the museum. If you can go there (always a good plan!) to collect it you get a nice day out and avoid postage and ebay fees too :-)

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Re: What's the worst that could happen?

Postby hicks » Fri Jan 05, 2018 4:03 pm

I'd assume the worst will happen and you destroy the device you're working on. How much will you have wished you'd bought a spare to work on in that case?

Based on the above, revise your ebay price range accordingly, or keep holding out for a reasonable price. I had to wait a few months to finally win an Electron for a reasonable price.

Might sting in the wallet to have a spare, but your stress levels will thank you when you're experimenting, plus anything happens to your main system, you've got spares! :)

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Re: What's the worst that could happen?

Postby flaxcottage » Fri Jan 05, 2018 9:47 pm

crj wrote:Suppose I screwed up completely and put a random assortment of 3.3V and 5V signals at anything up to 300MHz on the various pins of a ROM socket. Suppose I was a complete idiot and shorted power to ground. Suppose I ended up double-driving buses everywhere, or driving the data bus to an indeterminate logic level. Obviously, any data the machine was working on would be toast and it would be folly to have an important floppy in the drive, but how much danger is there of damaging the hardware?


Another way to break your computer would be to repeatedly hit it with a hammer. :wink: Now I presume you will not do that because you can foresee problems. So, to answer your question, don't do the silly things you propose. Read the circuit diagrams, work out what signal levels are acceptable. 300MHz will not be recognised by the Beeb which works at 2MHz so that will be OK but it cannot be applied to just any pin of a ROM; you have to know why you are applying what to where.

Check your constructions for shorts BEFORE connecting to the beeb. The Read/Not Write line, the clock and the address bus can be used to ensure you do not drive a data bus when something else is putting data onto it. Intermediate levels should not be a problem if you use 5v TTL.

Adding bits to the beeb is fun and can be very rewarding. Try the 1MHz bus for starters. There is a lot of information about that in the archives. Mike Cook's articles about Beeb body building in the Micro User will be a good place to start.
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Re: What's the worst that could happen?

Postby myelin » Fri Jan 05, 2018 10:28 pm

Definitely avoid doing initial testing of any hardware on a machine you *really* care about :) Maybe drop Mark at Retroclinic a line and ask if he has any beat up Model B machines or motherboards to sell. For your particular application you would probably be quite happy with one with a lot of damage (no disk, tape, serial, missing VIA etc) as long as the CPU, RAM, video, keyboard, and ROMs are working!
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Re: What's the worst that could happen?

Postby crj » Sat Jan 06, 2018 2:41 am

flaxcottage wrote:So, to answer your question, don't do the silly things you propose.

Well, obviously I intend not to...

(I'm currently winning an auction on a Model B, with only 12 hours to go. Fingers crossed.)

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Re: What's the worst that could happen?

Postby daveejhitchins » Sat Jan 06, 2018 5:50 am

crj wrote:(I'm currently winning an auction on a Model B, with only 12 hours to go. Fingers crossed.)
Good luck with your bidding, however, it's seems, these days, it's the last 12ns that count :lol:

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Re: What's the worst that could happen?

Postby crj » Sat Jan 06, 2018 9:02 pm

Lost that one too. Bidding on another.

I'm now up to twice what I originally intended to pay. Grr!

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Re: What's the worst that could happen?

Postby myelin » Sat Jan 06, 2018 9:11 pm

crj wrote:Lost that one too. Bidding on another.

I'm now up to twice what I originally intended to pay. Grr!

What’s your budget? I’m sure someone on here might be able to help. I feel like random Beebs usually sell on eBay for £65-80, with Retroclinic-refurbished ones about double that.
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Re: What's the worst that could happen?

Postby crj » Sun Jan 07, 2018 1:57 am

Well, I was originally hoping to pick one up for £50 including P&P. I'm now up closer to £100 including P&P, so more than double the P&P-exclusive price.

If that's the going rate, that's the going rate, but hey.

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Re: What's the worst that could happen?

Postby cmorley » Sun Jan 07, 2018 9:00 am

I've got 3 B's I'm going to sell. Repaired motherboards with new PSU (X & C9) caps. Cases in typical goodish condition with ashtray popped... it's not a dev machine if the lid is on though! I need sensible money for one but not ebay lolprice (esp if no fees). I cleaned and repaired them last year just need to sort out the keyboards.

Mod the PCB for a 64K EPROM or I've got solderless kits which take <5mins to fit for the same (also EEPROM) to get ROMs (BASIC, filesystem, EXMON II etc) in without filling all ROM sockets or having a massive ROM board in the way of dev toys.

PM me if you're interested.

Or just watch all the sales over the next six weeks. The prices usually go down in Jan/Feb then back in March (people are skint after Christmas so demand drops) if you wait long enough you should get a fixer upper untested (means I tried it and it was broken) for £25-35 + P&P. Especially collection only. Good luck!

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Re: What's the worst that could happen?

Postby crj » Sun Jan 07, 2018 8:39 pm

cmorley wrote:I've got 3 B's I'm going to sell. [...]PM me if you're interested.

Thanks, but I just finally won an auction: £77.00 + P&P

Not LOLPRICE, but definitely OUCHPRICE. Still, I've bought it now. Assuming it arrives in one piece and works, I can get moving!

(I already have a magical mystery box which, amongst other things, contains some Retroclinic capacitors in preparation for this day. I'll do a this-month-I-received posting in due course, to save people the tedium of loads of little today-I-received postings.)