Bygonebytes

Introduction

When searching for something Raspberry Pi related but nothing connected to Acorn computers I stumbled across a reference to a Raspberry Pi second processor. Bouncing around a few links brought me to Sundby.com which sold the PiTubeDirect signal converter and links for the Raspberry Pi 2nd processor emulator software. All this gained my interest so I opened my retro museum and unpacked loads of my Acorn stuff - This lot of computers to the right.
It turns out that my interest in the Raspberry Pi had led me rather belatedly to the Raspberry Pi Second Processor emulator project for Acorn BBC/Electron computers, my other obsession! So I thought that I'd buy the interface and give it a go. I ordered the Sundby PiTubeDirect interface external kit which I'll use with a Pi Zero. Learning of this project this rather late in the day made it fairly easy to put together as it's well documented, anyway I'm getting ahead of myself, I had to find out if my collection of Electrons, BBC B+ and Master were still working after being in storage for such a long time. First was the BBC Master..



BBC Master 128
A week later I switched on the BBC Master for the first time in many years, needless to say the batteries were exhaused and therefore the configuration lost. The batteries were dated 2001! amazingly after 18 years they hadn't leaked. As it's been a while since I've used an Acorn machine I had to re-aquaint myself with the *configure command and now it's all working perfectly.

Since the Master was OK I re-manufactured my 32-16 ROM board from the '90s so I could install my inter series ROMS inside. More info on the BBC Master page.




The Master 32-16 ROM Board Gerber and Schematic files.

BBC B+ 128
Another few days had passed before I got the chance to switch on the BBC B+. All was well for the first ten minutes then I could hear a faint buzzing sound, not long after the dreaded smoke genie appeared! I think I was quick enough to switch it off before there was a lot of damage. The smoke came from the power supply area so I disconnected the mains supply and removed the PSU and striped it down - the fault was fairly obvious, the suppression capacitor C2 had smoke damage as you can see in the photos. I've ordered a replacement and we'll see what happens next..

After a quick look round the internet it seems that it's a good idea to replace the smaller 10nF capacitor C1 as it's the same type. Looking at the capacitor it is showing signs of age/stress/swelling and I don't want to dismantle the supply again so it'll be replaced..

A few weeks later I received the parts and replaced both suppression capacitors, a tricky task as once the capacitors have swollen it's difficult to get them out due to the tightly packed neighbouring components - you can see the problem in the photos. I put the power supply back in the BBC case and before connecting up tested each of the power lines - all was ok. So that's the BBC B+ back up and running - good for another 40 years!

On getting the Raspberry Pi 2nd processor up and running on the Electron I thought I'd build one for the BBC B+. I've got no space to put my Acorn 2nd Processors on my desk and with the Pi 2nd processor being able to fit under the BBC it would be prefect for me. I have had some experience soldering surface mount components but I'm never very happy with the results so needless to say I am not keen on using surface mount devices. I thought I'd simplify the interface by making a Level Shifter PCB using standard DIL devices. Even when using the larger devices it still fits neatly under the BBC.

I said it would last another 40 years..it hardly lasted another 40 days until it wouldn't power up properly! Removing C9 it could be seen that it had swollen on the underside so I replaced it then rebuilt & tested the supply. All is well again but for how long...










Electron
The Electrons and the Plus 1's worked ok except for the disc drive PSU and three keys on the Electron keyboard.. The Electrons are issue 6 and 4 and my three Acorn Plus 1's are all issue 2's.

I replaced the PSU and used switch cleaner on the keys so everything is now up and running. My aim for the PiTubeDirect & Raspberry Pi Zero is to get a stable 2nd processor working on the Electron. Years ago I bought a Slogger 2nd Processor which worked fairly well but over time became too unstable to use. I only used the 2nd processor for large documents written in View and occasionally with Starword, I eventually replaced it with the Master RAM Board. I no longer have either of these add-ons so the 2nd processor emulator is an interesting replacement.

To run the 2nd processor emulator on an Electron it requires a TUBE interface. Research brought me to the Retro Hardware ATI which not only has the TUBE interface but two 16K sideways RAM banks which will come in handy for running View. I assembled the PiTubeDirect interface which was a bit fiddly due to the surface mount components. Care has to be taken mounting the 40 pin headers as it's easy to solder them in the wrong way round. Also when creating the SD card the instructions state that the card should be formated 'standard FAT' which I took to be FAT16. This can be done but it's a bit tricky with cards greater than 4Gb so I also tried FAT32 which works just fine. With everything assembled it all worked first time, I have run the test programs but yet to try View etc.

Just had five minutes to try an old EUG disc I found, issue 18, the opening screen of the EUG being munched by a dot - what an amazing speed increase with the second processor! I wasn't expecting much of a speed increase with View and I wasn't dissapointed, the only upside is that 30462 characters are available in any screen mode. So far nice and stable..

Just fitted a nice new AP6 ROM expansion board from Retro Hardware, excellent piece of hardware. Six sockets and up to 7 ROMs, I particularly like the EEPROM compatibility. Slowly getting the Electron back to it's former self.








Disc Drives
Each machine has it's own disc drive(s), the Master has twin 5 1/4" drives, the BBC B+ a single 5 1/4" drive and the Electron a 3 1/2" drive. There is a disconnect with this arrangement which is that the Electron is isolated, I cannot transfer programs/data between the Electron and the other two computers. I decided to solve this problem by adding a 3 1/2" drive to the BBC B+ giving it both sizes of drives.

I found an old 3 1/2" drive and ribbon cable from by PC spares (junk), a USB power supply and made up a USB to Floppy 5V power cable. The cable was made up from old PC drive 'Y' cable - two molex and one floppy connector, a resolderable USB connector and some 2-core 3A mains cable. I cut the floppy connector off leaving plenty of wire, soldered and sleeved it onto the 3A cable then soldered the USB connector to the other end. The use of the fairly heavy cable is to ensure minimal volt drop accross the cable when the drive is taking peak current ie when the stepping motor is stepping. Connecting the drive to the Electron PRES Plus 3 with the ribbon cable and plugging in the usb power adaptor for testing, no problems were encountered and as the drive defaults to drive 1 this makes it easy to connect to the existing BBC B+ drive which has its drive set to 0 - no link swapping or cable core twisting was required. All I need now is a 34 way IDC connector to press onto the existing 5 1/4" drive ribbon cable.




And to add to the mix is this GOTEK Floppy drive emulator - I intend to plug it into the BBC B+ along with the 5 1/4" drive so I can copy years worth of documents that have been stored away since the 80's. Then it'll move around the computers as required.



Having spent a day using the GOTEK I will consider replacing the old disc drives with these units. I've been copying data onto the GOTEK from floppy discs that have been stored in drawers for around 35 years..1984 or even earier! One paricular make, Parrot, hasn't stood the test of time. They have all suffered in such a way that the disc struggles to spin and hence shows a disc error. Once I've found the scale of the problem I'll see if it's possible to transfer the disc into a known good jacket - it may or may not work. Other makes like 3M and Verbatim, no problem at all.

TTL RGB Switch
Working with several machines at once with only one monitor is a bit of a chore, constantly swapping the RGB cable over is a real nuisance so I've decided to resurrect another old project from the early 90's - my RGB switch. The original switch had two inputs switched to one output, the new one has five into one letting me easily select which computer to display. The output selection is by rotary switch. The photos below show the build from board layout through testing then mounted and in use.



The TTL RGB Switch Gerber and Schematic files.

Battery Backed RAM modules
Now that I have the new AP6 board in my Plus 1 I thought I'd resurrect another module from the past - the BBRAM128. This module gave eight 16K RAM banks, one at a time. Bank selection was via a BCD thumb wheel switch. At first I thought that just a professionally made PCB would give this module the facelift it needed but I came across a the AS6C4008 SRAM chip which would give me thirty two 16K RAM Banks. - a ridiculous amount!

When designing this new prototype I ended up making quite a few changes, I removed most of the pull up resistors on the data & address lines, put in a smaller battery, increased the RAM size and reduced the board size. The only component that was difficult to obtain was the OA47 germanium diodes, I found equivalents but they are getting quite expensive.

To test the board I had a couple of 128K x 8 SRAM chips left from the '90s but I also ordered some from China. The old chips worked and the new ones didn't..maybe they need the pull-up's? So when it comes to the AS6C4008 I have ordered some from CPC-Farnell and a couple from China to see if I get similar problems. At this point I thought that I could use one of the 32K sockets on the new AP6 and switch two ROM's in at a time, also that there should be a newer method, one with a smaller component count, to feed the battery power to the SRAM chip. Looking around the DS1210 Nonvolatile Controller Chip seems to be the most popular but it is also now obsolete but seems to be readily available so the board is away for a second prototype. I've added some pads for SIL pull-up resistors just in case they're needed and a link to the board so it can be used in either a 16K (32x16K banks) or a 32K (16 x 32K (2x16K) banks) socket. The advantage of using the 32K sockets in the Plus 6, 5/6 or 13/15 is that you can use software lock/unlock instead of a switch to make the RAM read only.

The AS6C4008 (512Kx8) SRAM chips arrived from CPC and tested out ok with my first prototype board so I can now store and select 32 ROMs at a touch of a button..well two! I don't think I've got 32 Electron ROM's!!

The second prototype board is now built and tested and works well in the AP6 ROM sockets 5/6. I think I'll leave both boards in the Plus 1 at the moment and mount the selection switches to the right hand side in front of the cartridge slots.



The BBRAM4096 Gerber and Schematic files.