RISC-PC Not Working

Arc/RPCs, peripherals, RISCOS operating system & ARM kit eg GP2x, BeagleBoard
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BeebMaster
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Re: RISC-PC Not Working

Postby BeebMaster » Sun Oct 15, 2017 10:42 am

Hmmm, interesting. I think that would work. The RPC 600 does exactly what the 700 was doing during its duff clock & crystal escapade, ie. red screen of life on power-on, (floppy drive light probably telling me the problem but it isn't connected up at the minute), then enters supervisor mode with no keyboard functioning. A Delete-power-on or Delete-Reset button gets you back into the RISC OS Desktop every time, but you don't exactly want to be doing that every time.

I probably have all the parts needed to make that clock replacement - I have podule headers, I have the clock chip, I probably have the capacitor, but they don't show the underside of the board so I wouldn't know what connects where, and I don't know what the header pins on the little board are for. Is it as simple as connecting the clock chip pins straight onto the podule header pins and connecting the battery and cap between the appropriate pins?

I wonder if I could combine this with an Econet-Module-on-a-Podule to save a podule slot? Actually the CJE spiel suggests they can sell you a 2 slot backplane with the replacement clock built into the backplane so I reckon combining Econet and real-time-clock on the same podule would be a goer.
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Re: RISC-PC Not Working

Postby danielj » Sun Oct 15, 2017 10:48 am

So, all you have to do is wire up the equivalent to the clock circuit (resistors, crystal, capacitors, battery, CMOS IC) , and attach pins 5 and 6 to pins c20 and c19 respectively on the podule connector?

d.

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Re: RISC-PC Not Working

Postby BeebMaster » Sun Oct 15, 2017 11:01 am

Well, that sounds so simple that even I could get it wrong.

Although I'd need a bit more detail of what pin connects to what other component etc.

I'm tempted to be getting the through-hole version of the clock chip if I'm going to do it.
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Re: RISC-PC Not Working

Postby BeebMaster » Sun Oct 15, 2017 11:28 am

Right, GUESSING after looking at the datasheet, before my impatience completely eats up my whole mind, body & soul:

Pin 1 - Connect to crystal
Pin 2 - Connect to crystal
Pin 3 - dunno
Pin 4 - Connect to battery negative
Pin 5 - Connect to C20 on podule (as above)
Pin 6 - Connect to C19 on podule (as above)
Pin 7 - dunno
Pin 8 - connect to battery positive

Anywhere near?
Capacitor - looks to be 25V 4.7uF oil-drum type, what do I do with that?
Does power to the clock chip need to come through the podule as well as the battery?
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Re: RISC-PC Not Working

Postby danielj » Sun Oct 15, 2017 11:57 am

Have you got a picture of the circuit from Chris's acorns? Riscpc technical drawings? Basically you need to replicate the clock circuit there in its entirety, taking 5v and 0v and the other two signal lines from the podule connector.

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Re: RISC-PC Not Working

Postby BeebMaster » Sun Oct 15, 2017 12:24 pm

Yes, got all that here. Looking now.
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Re: RISC-PC Not Working

Postby BeebMaster » Sun Oct 15, 2017 1:53 pm

It's a bit rough and ready, and of course is only a proof-of-concept, but here is the first ever RISC PC Flying Clock!
RPC600FlyingClock1.jpg
RPC600FlyingClock2.jpg


Works a treat, although the battery hasn't been added yet, and the on-board battery doesn't help, so it forgets itself after a power off but otherwise seems fine.
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Re: RISC-PC Not Working

Postby danielj » Sun Oct 15, 2017 2:43 pm

Great work :)

d.

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Re: RISC-PC Not Working

Postby steve3000 » Sun Oct 15, 2017 2:44 pm

For various reasons, I also had to make a flying CMOS chip recently in my A3010, although I didn't go as far as to replicate the clock part. Works a treat though :)
_20171015_163755.JPG

FYI, if you do ever have a need to rebuild the CMOS RAM circuit off the main pcb, if it's just for testing proposes you don't actually need to include the clock crystal. I did this on my A5000 which had extensive battery damage. In the most basic configuration just powering an 8583T from the podule bus and linking it to the I2C lines was enough to allow the computer to boot (which was otherwise reporting unable to read CMOS RAM).

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Re: RISC-PC Not Working

Postby BeebMaster » Sun Oct 15, 2017 2:59 pm

Very nice, although that wasn't an option for me as the pads on the main board were completely corroded away.

Just working out how to relocate the battery with the minimum of fuss. If I put a Master 128 type battery holder with resistor & diode built-in, across the +5V and +0V I connected the clock chip to on the backplane, will that work? Or lead to overheating? Not in the mood for explosions & fires today.
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Re: RISC-PC Not Working

Postby IanS » Sun Oct 15, 2017 3:25 pm

You need another diode, so the battery isn't trying to power the whole machine when the power is off. Something like this:-
arc_clock.png

Diagram based on the A300 clock/battery circuit.

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Re: RISC-PC Not Working

Postby BeebMaster » Sun Oct 15, 2017 5:37 pm

Diodes, schmiodes, I knew I wouldn't have any, apart from that special Domesday BAV21 type that looks like a chip.

I've taken apart an old shrink-wrap Master battery pack, that has an IN4005, that'll have to do.

Do I need 2 diodes? A300 picture shows two, but the RISC PC technical drawing battery/RTC page only shows one.

Let's make sure I've done things right so far...I've got the 5V and 0V pins of the clock chip soldered onto the solder side of the +5V and 0V pins on the backplane (not the podule bus pins) where there is a big capacitor between them on the component side. Then I've put the battery holder wires onto these same pins.

I'd already played about with battery packs :

With a battery holder with no resistor/diode combo the RISC PC stays part powered on by the battery when it's switched off, so that's no good and I can't check whether the clock settings are being retained when it's off as it's never really off.

With a Master 128 type battery holder the RISC PC goes off and all the CMOS settings are retained whilst the power is off, but the date aways comes back to Wednesday 1st January 2020 at midnight! This even happens with a soft reset.

Looking at the A300 pic it seems I need a diode between 5V on the clock chip and 5V on the backplane, and a second diode between the first diode and the battery 5V, such that 5V chip to 5V backplane goes via 1st diode, 5V chip to 5V battery goes via 2nd diode, and 5V battery goes to a point between the 2 diodes.
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Re: RISC-PC Not Working

Postby philb » Sun Oct 15, 2017 6:44 pm

BeebMaster wrote:Do I need 2 diodes? A300 picture shows two, but the RISC PC technical drawing battery/RTC page only shows one.


You need 2 diodes if you are using a non-rechargeable battery: the second diode blocks the reverse current which would otherwise try to charge the battery when main power is on. If you are using a rechargeable battery, one diode suffices.

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Re: RISC-PC Not Working

Postby IanS » Sun Oct 15, 2017 6:46 pm

philb wrote:If you are using a rechargeable battery, one diode suffices.

Or a master battery pack, which will already have a diode in.

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Re: RISC-PC Not Working

Postby BeebMaster » Sun Oct 15, 2017 7:45 pm

Right. I have fixed it. I even found some 1N1001 diodes, I must have bought them for fixing MDFSes.

With one diode I couldn't get the clock to keep going, it just set itself back to the last time input on every reset. I think that was because the clock chip wasn't being powered directly from the backplane, the diode was in the way.

With 2 diodes in a little triangle affair, it all works.

5V on backplane has 2 diodes connected to it, one in one direction, t'other in th't t'other. One diode goes to the battery 5V so that the diode blocks the battery from keeping the computer powered. The other diode connects to 5V on the clock chip, at that point it is also connected to the battery 5V so that the battery and clock chip are directly connected so it is powered whilst off, and the second diode is blocking the battery sneakily being able to go round the other way and power the computer.

Sounds a bit convoluted and probably is, but it's the only way I could get the clock chip powered by both the computer and the battery and stop the battery sending power to the computer whilst still being able to send power to the clock.

I think.
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Re: RISC-PC Not Working

Postby IanS » Sun Oct 15, 2017 8:05 pm

BeebMaster wrote:5V on backplane has 2 diodes connected to it

Sounds wrong.

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Re: RISC-PC Not Working

Postby BeebMaster » Sun Oct 15, 2017 8:37 pm

Well, it probably is, most of my things are, but it works and hasn't gone on fire yet, so I've reassembled it and I now have two fully working RISC PCs repaired in the space of one weekend. I shouldn't have waited 6 months thinking I couldn't do it before doing it.

I think I'll have to have a go at the MDFSes again soon.
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Re: RISC-PC Not Working

Postby IanS » Sun Oct 15, 2017 9:23 pm

BeebMaster wrote:5V on backplane has 2 diodes connected to it, one in one direction, t'other in th't t'other. One diode goes to the battery 5V so that the diode blocks the battery from keeping the computer powered. The other diode connects to 5V on the clock chip, at that point it is also connected to the battery 5V so that the battery and clock chip are directly connected so it is powered whilst off, and the second diode is blocking the battery sneakily being able to go round the other way and power the computer.

This is what I understand you have, based on your description.
IMG_20171015_221538471.jpg

Is that what you have?
Is the battery pack a standard master pack, so it may have D3 inside?

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Re: RISC-PC Not Working

Postby BeebMaster » Sun Oct 15, 2017 10:13 pm

Yes, that's basically it, except it "looks" like a triangle as the two diodes are both connected to the same 5V point on the backplane. There's no diode in the battery holder, it's just a 3xAA battery holder which has 3 rechargeable batteries in. So what I hope I've done is to allow the RISC PC to charge the batteries, but stop the batteries powering the RISC PC, and allow both the computer and batteries to power the clock chip at the appropriate time.
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Re: RISC-PC Not Working

Postby IanS » Sun Oct 15, 2017 10:37 pm

BeebMaster wrote:Yes, that's basically it, except it "looks" like a triangle as the two diodes are both connected to the same 5V point on the backplane. There's no diode in the battery holder, it's just a 3xAA battery holder which has 3 rechargeable batteries in. So what I hope I've done is to allow the RISC PC to charge the batteries, but stop the batteries powering the RISC PC, and allow both the computer and batteries to power the clock chip at the appropriate time.

So is there a resistor in-line with the battery to limit the charging current?
If it is as my diagram, then the battery will be trying to power the RISC PC via D2, and the batteries will probably be flat by morning.

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Re: RISC-PC Not Working

Postby BeebMaster » Mon Oct 16, 2017 3:34 pm

No resistor, is that vital? What value should I use? They are 2150mAh (mAH? MaH? MAH? Mah?) batteries.

Would it be better to connect:

Chip +5V with a wire direct to backplane +5V
Battery +5V with a wire to a resistor then a diode then to the backplane +5V?

I tried that without the resistor but it didn't appear to be powering the clock chip with the power off and I didn't want to have two wires coming off the same pin of the tiddly surface mount chip newfanglry, that's why I went for the triangle/loop affair with extra diode to prevent the battery powering the RISC PC.
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Re: RISC-PC Not Working

Postby philb » Mon Oct 16, 2017 6:50 pm

BeebMaster wrote:No resistor, is that vital? What value should I use? They are 2150mAh (mAH? MaH? MAH? Mah?) batteries.


It's fairly vital to restrict the charging current into the battery, yes. Otherwise the batteries will overheat and may start venting. The fact that you currently have a diode in series will be helping a bit, and the end-of-charge voltage of the batteries plus the diode drop is probably close enough to +5V that they aren't going to get chronically overcharged at a high rate as long as the batteries stay full, but if the batteries started out from close to empty the machine would be trying to charge them at a very high rate and this will not be good for either them or it. NiMH has a fairly high self discharge rate so it wouldn't take very long with the machine left off for the batteries to empty.

For NiMH batteries you need to restrict the trickle charging current to no more than about C/30, which would be roughly 70mA for your 2150mAh cells. Assuming a start-of-charge voltage of 1V per cell and +5V in, you would need 2/(70/1000) = 28R at a minimum. I would suggest maybe 47R.

NiCd batteries were a bit more tolerant of overcharging at slightly higher rates and this is something to watch out for if making a direct substitution of a modern NiMH battery for an old NiCd one: the resistor value may need increasing. Of course, if replacing (say) the 1.2V NiCd on an A3000 with a 3.6V NiMH, the increase in terminal voltage means that the existing resistor is probably fine.

Would it be better to connect:

Chip +5V with a wire direct to backplane +5V
Battery +5V with a wire to a resistor then a diode then to the backplane +5V?


If you connect chip +5V directly to the backplane +5V then you won't be able to power the chip without also (attempting to) power the whole rest of he machine.

To avoid having to connect two wires to the chip pin what you want to do is:

Get a diode and a resistor
Solder the +ve end of the diode to one end of the resistor
Solder a wire to the point where the diode and resistor meet, and connect this to the chip +5V
Connect the free end of the diode to backplane +5V
Connect the free end of the resistor to the battery pack

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Re: RISC-PC Not Working

Postby BeebMaster » Mon Oct 16, 2017 9:02 pm

Right then, I had better be doing that post-haste before the whole thing goes up in flames. Which is the positive end of the diode - stripe or non-stripe?

I don't know if it's connected to what I've done so far but the clock gained time overnight (5 mins) and also sometimes gains an instant 5 mins on a hard reset. At one point it added on half an hour.
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Re: RISC-PC Not Working

Postby BeebMaster » Tue Oct 17, 2017 10:06 pm

I've done that now, working as expected so far...
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Re: RISC-PC Not Working

Postby z0m8ied0g » Tue Oct 17, 2017 10:10 pm

BeebMaster wrote:I'm never going on holiday again, it's taken me about half an hour to get the MDFS going again, and now I've found the RISC-PC has developed a curious problem.


I just presumed you always took your machines on holiday with you :lol:
Acorn... lots of Acorn... http://blog.retroacorn.net

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Re: RISC-PC Not Working

Postby philb » Tue Oct 17, 2017 10:13 pm

Jolly good. It sounds like you figured it out already, but the answer to your previous question is that the +ve end of the diode is the one away from the stripe.

If your RTC is gaining time then that would suggest you don't have enough load capacitance on the crystal. You could try adding some small capacitors (10pF or thereabouts) between the crystal legs and ground which will slow it down a bit. Some experimentation will be needed to figure out the right values: I think the original circuit had 12pF but your parasitics will be all different. Or if accurate time is not a big deal then you could just live with it!

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Re: RISC-PC Not Working

Postby BeebMaster » Tue Oct 17, 2017 10:15 pm

I switched it on before I did the latest mod when my triangle double diode doings was still in-situ and the time was absolutely bang on even after nearly 24 hours powered off. We'll see what the next 24 hours brings.
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Re: RISC-PC Not Working

Postby philb » Tue Oct 17, 2017 10:19 pm

If you moved the wires around a bit then that would probably have been enough to disturb the parasitic capacitances and change the frequency that the RTC counts at. 10pF isn't much!

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Re: RISC-PC Not Working

Postby crj » Wed Oct 18, 2017 1:28 am

z0m8ied0g wrote:
BeebMaster wrote:I just presumed you always took your machines on holiday with you :lol:

We did take the Beeb on holiday to France in the caravan once when I was a kid. It was great!

As a sign of the times, when reminiscing about this I found myself trying to remember how we got internet connectivity from a campsite. Then I remembered...

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Re: RISC-PC Not Working

Postby BeebMaster » Wed Oct 18, 2017 7:39 am

Usually only teapot, china, tea, kettle etc make it into the holiday suitcase.

It's retained the time dead-on overnight, so I'm going to leave it alone and declare it fixed.

Great news and thanks to everyone for their help!
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