Turbo MMC BBC Micro adapter
After the success of repairing my BBC Micro’s PSU I decided the act of shoving 30 year old floppies into an equally old drive, while amusing and all was a bit rubbish. 100K floppy disks are a bit small and the modern way to play with your retro computers is to fit them with some form of flash media so off to eBay I went.
A few days later a BBC Micro MMC adapter arrived in the post. Compared to the masses of electronics required to interface CF cards to my ZX Spectrum, the BBC MMC board is very very simple, with most of the hard work being done inside the BBC. They built the Beeb to have lots of stuff plugged into it, so plugging stuff in is really simple!
The instructions were reasonably obvious – the kit contains a PCB, an MMC card, some ribbon cable and an EPROM. The PCB, MMC and cable obviously fit together, and there’s only one socket on the bottom of the BBC for the cable to go in. The EPROM goes inside the Beeb in a ROM socket – this is the old school way of supplying drivers.
My BBC, being a bit antique had a few problems though; the EPROM worked fine, but it couldn’t find the MMC card at all. After a lot of confused unplugging, replugging and checking I’d not put something in upside down I emailed the eBay seller to ask for help.
Within 10 minutes or so I had a helpful response, and also got to find out just how helpful the BBC Micro’s manual is. I learnt the user port on the BBC Micro is connected to a MOS Technology 6522 VIA chip (or compatible 3rd party clone) and that this could be faulty. An easy way to test this is to swap the 6522 VIA controlling the user port with the other one in the BBC that controls other important things. The user VIA is optional, the system VIA isn’t, so swapping them around will soon tell you if one is broken.
Mine wasn’t, so the next task was to check the continuity between the relevant pins on the user VIA and the pins on the user port. This took a loooong time because some of the pins didn’t have continuity. I eventually worked out the socket holding the user VIA to the BBC’s motherboard was faulty and decided to desolder it and put a new socket in. Having now desoldered a 40 pin chip socket I’m quite good at using a solder sucker 
I also discovered a strange problem with the user port where one of the pins wasn’t always connecting inside the plug of the MMC board’s ribbon cable. A bit of wiggling fixed that too.
So now I have a nice modern(ish – it’s an MMC card!) storage device for my Beeb and got to do a bit of soldering and learn things about the hardware in it.
BBC 40T SS 100K 5.25 inch floppy drive with BBC Micro MMC Interface board on top
BBC 40T SS 100K 5.25 inch floppy drive with BBC Micro MMC Interface board on top
BBC Micro showing Turbo MMC software
BBC Micro showing Turbo MMC software
BBC Micro open with removed motherboard
BBC Micro open with removed motherboard
BBC Model B EPROM sockets with EPROMS
BBC Model B EPROM sockets with EPROMS
EPROM containing Turbo MMC software
EPROM containing Turbo MMC software
Turbo MMC BBC Micro adapter
Turbo MMC BBC Micro adapter
