What to do with a Model A?

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steve3000
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What to do with a Model A?

Post by steve3000 » Fri May 08, 2020 10:54 am

With help from stardot (viewtopic.php?f=3&t=14503) I have finally got my unexpanded Issue 2 model A running with it's original PSU. While testing the computer I realised just how much better condition this computer is than my main beeb...especially the case and keyboard.

I know I could just swap the "A"s keyboard/case over to my beeb - to improve my beeb, but I'd like to keep my "A" altogether, and besides that, my beeb has other issues that need addressing (such as a graphics glitch that still persists after many attempts to fix: viewtopic.php?f=3&t=16063)

So I started thinking what can I do to my "A" to make it usable for most of my 8-bit needs (programming and some games/demos), but keeping the ability to easily restore it back to original "A" (ie. no soldering to the main PCB) in future?

I've already added RAM to get up to 32kb, fitted the missing 6522 and ADC, and can easily add the other missing ICs, but are there any storage solutions that don't need ports fitted in place underneath (plug in SPI SD interface?)... And what about RGB output - can this be tapped form somewhere internally without need to solder in a socket? Anything else to consider?

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Re: What to do with a Model A?

Post by KenLowe » Fri May 08, 2020 11:00 am

steve3000 wrote:
Fri May 08, 2020 10:54 am
are there any storage solutions that don't need ports fitted in place underneath (plug in SPI SD interface?)...
GoSDC.

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Re: What to do with a Model A?

Post by steve3000 » Fri May 08, 2020 11:08 am

KenLowe wrote:
Fri May 08, 2020 11:00 am
steve3000 wrote:
Fri May 08, 2020 10:54 am
are there any storage solutions that don't need ports fitted in place underneath (plug in SPI SD interface?)...
GoSDC.
Wow, I'd never seen that before! Yes, that looks like it should work...

But, it's a bank holiday weekend, and I was thinking more along the lines of something to DIY build...? For example could I mount the 6522 on a plug in piece of strip board, and build an internal user port maybe?

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Re: What to do with a Model A?

Post by SteveBagley » Fri May 08, 2020 11:18 am

steve3000 wrote:
Fri May 08, 2020 10:54 am
I've already added RAM to get up to 32kb, fitted the missing 6522 and ADC, and can easily add the other missing ICs, but are there any storage solutions that don't need ports fitted in place underneath (plug in SPI SD interface?)... And what about RGB output - can this be tapped form somewhere internally without need to solder in a socket? Anything else to consider?
Some of the raspberry pi retailers were selling pressure fit pin headers for the pi zero at one stage, I wonder if they could be used to pick up signals from the user port, RGB etc. non destructively.

Example: https://shop.pimoroni.com/products/gpio ... 5643318026

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Re: What to do with a Model A?

Post by DutchAcorn » Fri May 08, 2020 12:14 pm

The model A seems to have edge connectors? Would it work to make a plug for the user port using something like this? Or would you need to solder additional components to the mb?
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Re: What to do with a Model A?

Post by steve3000 » Fri May 08, 2020 1:03 pm

DutchAcorn wrote:
Fri May 08, 2020 12:14 pm
The model A seems to have edge connectors? Would it work to make a plug for the user port using something like this? Or would you need to solder additional components to the mb?
Oh, that’s a thought, has anyone tried this?

I imagine the clearance with the case could be an issue, but I’ll take some measurements tonight to see.

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Re: What to do with a Model A?

Post by 1024MAK » Fri May 08, 2020 1:28 pm

Yes you could make a socket adapter to plug into the ‘user’ 6522 socket.

However if you only need a small number of connections, and you don’t mind soldering to the chip pins, that would be less work.

IC pin hook connectors could also be used.

If you do made a socket adapter, the biggest problem is that unless you extract and modify pin contacts out of a stamped pin socket, most pin headers are bigger in thickness/diameter than IC pins. Hence you risk damaging the socket contacts in the socket on the main board.

It may be possible to use edge-connectors. However you will need the type with a 0.1” / 2.54mm pitch. Like ISA bus motherboard connectors. But you will have to cut them to size yourself. Also edge-connector card female connectors will leave minor marks on the board contacts.

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Re: What to do with a Model A?

Post by system11 » Sat May 09, 2020 5:28 pm

I think I'd be tempted to leave it vanilla and either sell or store it if it's not good enough for regular use. It's only going to rise in value if it's unmolested and cosmetically excellent. This isn't from a 'pound signs in the eyes' point of view, simply pragmatic and in line with preservation.

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Re: What to do with a Model A?

Post by BeebMaster » Sat May 09, 2020 5:49 pm

I think if it was 1984 we'd all be saying upgrade to full Model B spec, but in 2020 I'd leave it alone. On the one hand it's relatively "useless" compared to what can normally be done with a Beeb, but I'd say that is outweighed by its rarity and curiosity value.
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Re: What to do with a Model A?

Post by steve3000 » Sat May 09, 2020 8:14 pm

BeebMaster wrote:
Sat May 09, 2020 5:49 pm
I think if it was 1984 we'd all be saying upgrade to full Model B spec, but in 2020 I'd leave it alone.
I helped upgrade several of my school's "A"s back in the mid-80s. Probably my first real use of a soldering iron was fitting sockets to those :shock: I still have an original unused RGB socket from a "A" to "B" upgrade kit in one of my spares boxes... but I have no intention of soldering it onto my current "A"!

I won't be making any irreversible changes to my "A" because I appreciate it's quite unusual, but I do like to make use of my computers - if anything, to keep them functioning and fully tested.

At least sockets are fitted for all the ICs on the Issue 2 PCB except econet interface, so upgrading "A" to "B" ICs is easy and reversible. I've currently upgraded the memory to 32kb, added the ADC and the missing 6522 to IC69... So this is my Issue 2 "A" at the moment:
20200509_191603.jpg
Issue 2 "A" with ICs fitted to upgrade to "B" spec
After some investigation, I don't think I'll go down the route of edge connectors for my MMC/SD adaptor - although likely to work, as Mark says they'll leave some evidence of use on the PCB contacts and will need to be careful not to scrape the case too... Fitting the new 6522 on a daughter board, and plugging this into the main PCB IC69 socket was my original plan, but it's a good point that this may damage the socket, which I'd really want to avoid.

So soldering onto the newly fitted 6522 is a good alternative as it doesn't affect the PCB, but I also like the idea of pressure fitting connectors, so am investigating fitting a 40pin socket over the 6522, which may work once secured (although may still need to solder this on to the 6522, but socket still gives more expansion flexibility).
20200509_191636.jpg
40pin socket fitted onto 6522

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Re: What to do with a Model A?

Post by BeebMaster » Sat May 09, 2020 10:00 pm

Looking very nice. I have an issue 2 B, and the 1982 service manual suggests there were only 5,000 issue 2 boards.

What does it have in the way of OS & BASIC? Looks to be BASIC 1 from the picture. Is it OS 1.20?
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Re: What to do with a Model A?

Post by KenLowe » Sat May 09, 2020 10:20 pm

steve3000 wrote:
Sat May 09, 2020 8:14 pm
Fitting the new 6522 on a daughter board, and plugging this into the main PCB IC69 socket was my original plan, but it's a good point that this may damage the socket, which I'd really want to avoid.
I built a 40 pin break out board to help diagnose a faulty 6502 Co Pro. I've got a couple of spare PCBs if you're interested. To avoid damaging the soldered in socket, you might want to consider plugging the adaptor board into an intermediate 40 pin dual wipe socket and then plugging that into the motherboard socket. The pins on the dual wipe sockets are unlikely to damage the socket that's soldered into your motherboard.

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Re: What to do with a Model A?

Post by steve3000 » Sun May 10, 2020 10:42 am

BeebMaster wrote:
Sat May 09, 2020 10:00 pm
Looking very nice. I have an issue 2 B, and the 1982 service manual suggests there were only 5,000 issue 2 boards.

What does it have in the way of OS & BASIC? Looks to be BASIC 1 from the picture. Is it OS 1.20?
Oh, that's not many issue 2s then! There's actually an issue 2 B, with Serial #3452 in part of a bundle on eBay at the moment: https://www.ebay.co.uk/itm/264719763574

And quite right B04 and B01 ROMs. So I should upgrade to BASIC 2, which will help more things run, and easily reversible.

Of interest, I have OS 1.00 ROM somewhere which I collected back in the 80's when upgrading an A to a B... but as you can't do much with 1.00, I'm not planning on installing it here.

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Re: What to do with a Model A?

Post by steve3000 » Sun May 10, 2020 10:45 am

KenLowe wrote:
Sat May 09, 2020 10:20 pm
I built a 40 pin break out board to help diagnose a faulty 6502 Co Pro. I've got a couple of spare PCBs if you're interested. To avoid damaging the soldered in socket, you might want to consider plugging the adaptor board into an intermediate 40 pin dual wipe socket and then plugging that into the motherboard socket. The pins on the dual wipe sockets are unlikely to damage the socket that's soldered into your motherboard.
Thanks Ken - that could be really useful! I'll send a PM.

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Re: What to do with a Model A?

Post by KenLowe » Sun May 10, 2020 12:46 pm

With the extra socket, it's a bit of a tight squeeze under the keyboard, but I think you could just about get it to work:
20200510_123717.jpg
.
Instead of the straight header break out pins that I'm using, I would recommend right angle pins, and I'd also solder them on the underside to give you a bit more room. You can rotate the whole board through 180 degrees if you want the header break out pins on the opposite side. You'd obviously need to rotate the 6522 as well, to keep pin 1 in the correct location.

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Re: What to do with a Model A?

Post by gfoot » Mon May 11, 2020 12:26 pm

steve3000 wrote:
Sun May 10, 2020 10:42 am
Oh, that's not many issue 2s then! There's actually an issue 2 B, with Serial #3452 in part of a bundle on eBay at the moment: https://www.ebay.co.uk/itm/264719763574

And quite right B04 and B01 ROMs. So I should upgrade to BASIC 2, which will help more things run, and easily reversible.

Of interest, I have OS 1.00 ROM somewhere which I collected back in the 80's when upgrading an A to a B... but as you can't do much with 1.00, I'm not planning on installing it here.
That's interesting - I have an Issue 4 Model B with OS 0.10. So it's a later machine, but with an older OS ROM. I'm not sure whether it was made that way, or whether it was downgraded at a later date for some reason. I ought to check the date codes on the other chips in the system.

Both yours and the one in the eBay auction seem to have OS 1.20. I'm guessing the eBay one had an OS upgrade to go along with the disk upgrade, as I don't think that worked with OS 0.10.

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Re: What to do with a Model A?

Post by Kazzie » Mon May 11, 2020 12:51 pm

gfoot wrote:
Mon May 11, 2020 12:26 pm
Both yours and the one in the eBay auction seem to have OS 1.20. I'm guessing the eBay one had an OS upgrade to go along with the disk upgrade, as I don't think that worked with OS 0.10.
Correct: an upgrade away from OS 0.10 was one of the steps listed in the Service Manual for installing a disk upgrade.
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
Acorn System 1 home-made replica

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Re: What to do with a Model A?

Post by steve3000 » Sun May 17, 2020 1:31 pm

Thanks to Ken's breakout PCB, I've assembled a low-profile 6522 breakout board which fits neatly into IC69 and can be used to connect to a 99p microSD interface :)

Build pictures below.
IMG_0790.jpg
Step 1, test fitting the breakout PCB with an intermediate 40pin dual wipe socket between PCB and IC69 socket
20200516_214907.jpg
Step 2, solder header pins, and snip the tops off to make space for the socket.
20200516_220814.jpg
Step 3, ready to test-fit
20200516_220829.jpg
Step 4, fitted in IC69
IMG_0797.jpg
Step 5, confirm enough space under keyboard
20200517_131634.jpg
Step 6, wire up the mircoSD adaptor

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Re: What to do with a Model A?

Post by steve3000 » Sun May 17, 2020 1:46 pm

...it was after this I discovered a problem. The BASIC ROM was in IC52 (left most ROM socket), but wherever I tried placing the SuperMMC EPROM (in any of the three remaining ROM sockets) it was not recognised. So I tried moving BASIC into one of the other ROM sockets... and it was not recognised (just Language? on startup).

Scouring the manuals this morning, lead to the following revelation: Model A's (at least early model A's) don't support >1 sideways ROM! Only a single ROM in IC52 is supported.

Code: Select all

S12. CLOSED ties ROM select line A to 0V.
OPEN ROM select line A driven by IC76.
- On model A's, IC76 is not fitted because sideways ROM'S are not
used. ROM 0 (IC52) is permanently selected. Do not fit IC76 with this
link closed.

S13. CLOSED ties ROM select line B to 0V at IC20.
OPEN ROM select line B driven by IC76.
- Do not fit IC76 with this link closed. See comments on link 12.
Next step, open links and find a 74LS163 to fit to IC76.

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Re: What to do with a Model A?

Post by KenLowe » Sun May 17, 2020 2:04 pm

Is there already a socket in IC76 position? If there isn't one there, and you don't want to solder one in (to keep the board authentic), one other option is to install an expansion ROM board that does its own ROM bank decoding. For example, my IntegraB board has an onboard 74159 which decodes all 16 available ROM banks. It plugs into the 6502 socket to pick up all the necessary control, data and address signals. I'm sure other boards will do similar. It's not the cheapest of options, though - for the sake of a socket.

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Re: What to do with a Model A?

Post by steve3000 » Sun May 17, 2020 2:34 pm

KenLowe wrote:
Sun May 17, 2020 2:04 pm
Is there already a socket in IC76 position? If there isn't one there, and you don't want to solder one in (to keep the board authentic), one other option is to install an expansion ROM board that does its own ROM bank decoding.
The issue 2 has everything socketted, so IC76 fitting will be easy, but that's a great suggestion about the expansion board - I was thinking about adding sideways RAM...and have an original IntegraB in my beeb box... :)

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Re: What to do with a Model A?

Post by steve3000 » Sat May 23, 2020 1:55 am

So it turned out that fitting IC76 and opening links S12 and S13, to enable the sideways ROM sockets, while keeping everything easily reversible, was not quite as easy it sounds.

S12 and S13 are actually soldered wire links, so it looked for a moment that keeping to my plan of not soldering or making irreversible changes (like cutting the links) to this model A PCB was going to be a problem.

Thankfully, there was a solution. From the circuit diagram you can follow IC76 pins 13 and 14 through to links S12 and S13 (which are wired to 0v) and also to the north pin of links S20 and S22 which are jumpered to their respective central pins. So the solution is simply to remove jumpers on S20 & S22 then lift the legs of pins 13 & 14 on the new IC76, and wire these via flying leads to the centre pins of S20 & S22 respectively, bypassing S12 and S13 completely. No soldering to the PCB or breaking of wire links required!

See photo below - with this solution, I can now successfully move the BASIC ROM to any sideways ROM position :)
20200522_210259.jpg
IC76 with pins 13 & 14 routed to centre of S20 & S22

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Re: What to do with a Model A?

Post by trixster » Sat May 23, 2020 9:26 am

I’m loving reading this thread, fascinating! That breakout pcb for the mmc is very clever

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Re: What to do with a Model A?

Post by steve3000 » Sat May 23, 2020 2:59 pm

trixster wrote:
Sat May 23, 2020 9:26 am
I’m loving reading this thread, fascinating! That breakout pcb for the mmc is very clever
:D and it works!!!

More coming soon, I'm currently distracted playing JCB digger from the SD card!

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Re: What to do with a Model A?

Post by e668ecp » Sat May 23, 2020 5:56 pm

A very interesting read.. love that you are keeping the Model A original!

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Re: What to do with a Model A?

Post by steve3000 » Sun May 24, 2020 12:53 am

I'm very pleased to confirm that my Model A now has a fully working microSD card adaptor, via Ken's breakout board PCB :)

I only hit one minor error along the way - I misread the beeb circuit diagram and noted down the wrong pin for PB0 & PB1 (be ware of previous photos above - they're wrong - I'd wired up PA0 & PA1!). The fix should have been simple, but unfortunately the position of the breakout board pin for PB1 (6522, IC69 pin 10) almost perfectly overlaps with the centre pin of S20, where I'm routing pin 13 from IC76... so it's a very tight squeeze to get both patch connectors in. I suspect it may be possible to adjust MMFS to use a different pin or even PA0/PA1, and therefore avoid this pin clash completely, but I'm not that concerned as it all works fine now :)

Pins I wired up are as follows:

Code: Select all

UserP  6522              SD

    1  Pin 20 +5v        +5v
    2  Pin 18 CB1(->PB1) SCK
    3  Pin 20 +5v        +5v
    4  Pin 19 CB2        MISO  
    5  Pin  1 0v         GND/CS
    6  Pin 10 PB0        MOSI
    7  Pin  1 0v         CS/GND
    8  Pin 11 PB1(->CB1) SCK
Photos of the final install below:
20200523_093050.jpg
Breakout board microSD card adaptor added to Model A
20200523_123326.jpg
All working!
Next steps... Swap out BASIC 1 for BASIC 2, and add some sideways RAM :)

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Re: What to do with a Model A?

Post by steve3000 » Sun May 24, 2020 5:54 pm

It's great using the "model A" in it's reversibly upgraded state, but having just singed my finger on the PSU - I realise how hot this linear PSU gets! I have just measured 70 C after 30 minutes use... Is this "normal" for these 7805s to get quite so hot?
20200523_132241.jpg
Hot hot hot

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Re: What to do with a Model A?

Post by 1024MAK » Sun May 24, 2020 7:52 pm

Yeah any series pass element voltage or current regulator will get hot if delivering substantial amounts of current.
Look at the heatsinks on various other 1980s computers where they are using less power.

1980s computers with big heatsinks:

ZX Spectrum 128 Image

Enterprise Image (heatsink on the right)

The Memotech MTX used the large aluminium case it was housed in as a heatsink.

Inside and less obvious, the Commodore PET.

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Re: What to do with a Model A?

Post by gfoot » Sun May 24, 2020 7:58 pm

It might be worth reversibly-upgrading the PSU - if the newer switched ones fit in the same case space? You could always swap back again at another time, if/when you restore the computer to its original state.

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Re: What to do with a Model A?

Post by steve3000 » Sun May 24, 2020 8:15 pm

gfoot wrote:
Sun May 24, 2020 7:58 pm
It might be worth reversibly-upgrading the PSU - if the newer switched ones fit in the same case space? You could always swap back again at another time, if/when you restore the computer to its original state.
That'd work, but not really what I'm trying to do - I don't want to take anything away form the computer (especially after the effort taken to fix this PSU) ...although if I started down the route of swapping out large parts, I suppose I could also swap the Issue 2 PCB for a nice Issue 4 with all the ports and floppy interface... ;)

No, I'm only adding to this one, not taking away - I really just to see what can be done using an original Issue 2 model A base, without doing anything that isn't easily & quickly reversible. :)

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