Extension surprises

A project log for MO5 interface boards

I'm a Thomson MO5 fanatic and despite its bad rep, it's a very nice machine that is easy to extend.

Yann Guidon / YGDESYann Guidon / YGDES 03/31/2018 at 18:082 Comments

As I struggled to get the right connector for the extension slot, I tried to get whole modules with the intention to unsolder the female connector.

Let's start with the Nanoréseau controller. I don't use this type of network (based on the MC6854) so let's rip that module up.

But wait, there is a board with strange pads. Actually it's not pads but machined female sockets for the MMI PAL (address decoder ?) and a 24-pins EPROM... So this means that I can burn my own programs and just plug a Flash (?) with almost no modification to the board ? There is even a DIP switch at the other end so I could select a region of the Flash to use, and create a multicart system...

And I discover only now that this hack is already used by others, see at

OK so the Nanoréseau is out of the way... I also have a communication port, that looks like a Centronics over DB25 interface, just like on the PC. Let's rip it up !

Oh wait... It's a 6821 inside and apparently, one port (8 bits ?) is directly tied to the DB25 connector ! So it could be used for a direct, rough output for prototyping.

With CB1 available, there is one interrupt input available.

With CB2 also available, both interrupt input and bit output are possible.

And each of PB's bits can be configured as input or output, so it's about 10 GPIO...

I have no idea why there is RS232 level converters because bitbanged asynch serial would be SLOW and SW-heavy. But it could be useful as high-voltage I/O...

So these old circuits are not so lousy after all. There is a lot of potential for hacking and reuse... but this doesn't solve my initial problem, and I don't want to solder anything to the extension fingers of the main unit...


Dylan Brophy wrote 04/01/2018 at 00:19 point

I have seen these kinds of sockets in the hard drive for this really old mac I had.  I actually was able to download the firmware to my computer with my eeprom programmer!

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Yann Guidon / YGDES wrote 04/01/2018 at 00:43 point

Yes this opens quite a few new perspectives :-)

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