Sunday 21 May 2023

Multitech Micro-Professor MPF-I Repair

This was a Patreon preview last year, but for some reason it never made it onto the main blog. I saw an advert for the MPF posted on twitter earlier and the week and was going to reply with a link to this post. It was at that point I realised it hadn't actually been posted. So now it has.

Today I am looking at a 1980s single board computer with built in keyboard and display, named after a diminutive senior academic, the Multitech Micro-Professor MPF-I.

Not to be confused, with the other 1980s single board computer with built in keyboard and display, named after a diminutive senior academic, the Little Professor.

An educational toy, sort of a calculator in reverse. It prints up sums and the user has to type in the answer. I had one of these, and it was probably somewhere between 78 and 80. Before I got my first ZX81 this was my games console.

4 + 5 = ? anyone?

The Micro-Professor MPF-I, from Multitech (which later changed it's name to Acer) is a little larger.

This is a boxed unit in very good condition for being over 40 years old.

It is housed in the same sort of fake book as you used to be able to buy to store your collection of VHS tapes along side your hardback books.

Inside the left hand side is reserved for expansion modules, and the unit itself is on the right.

This is a single board computer, based on a Z80 running at 1.79MHz. There are two ROM chips, one 2K, the other 8K (an upgrade maybe, the original would have been 2K?), and a 2K RAM chip. The bottom row has a Z80 PIO dual port input / output chip, a Z80 CTC timer chip and an 8255 dual port I/O chip (why didn't they use two PIOs or two 8255s?)

Beneath that is a six digit, seven segment LED display and a 36 key keyboard.

It comes with a nice thick manual, although this one has seen better days.

Inside it has been heavily annotated, someone clearly made a lot of use of this.

It runs from 9V DC, so I powered it from a bench supply, with current limiting for protection.

The current was within range, 450mA (it came with a 9V 600mA supply), but when I measured the 5V rail, I was getting a little under 2V, something was pulling the rail down.

The easiest option here would be one of the seven socketed chips, so I removed those to start with.

Powering it on again, it was down to 50mA, and the 5V rail was reading 5.1V. So it was one of the chips.

I reinstalled them one by one and checked each time to see that the 5V rail was still present.

As soon as I refitted the 8255 and powered it on, the current shot up and the 5V rail went back down to 2V.

Removing the 8255 and trying again, it was back to 5V.

So that's a bad 8225 by the looks of it, and also the CRTC seems to be missing a couple of pins. They aren't in the socket, so they probably came of due to some past misdemeanour.

With a replacement 8255, we're running again. The 5V rail is at 5V and the display is now active.

I checked on the CTC pinout and one of those pins is an enable pin. It may be that it was enabled when it should not have been, at the same time as the 8255, and the conflict finally wore out the 8255? I'm not sure. The Z80-CTC is still in production, so I removed it and ordered a new one.

For the moment I will continue with it removed. The Z80-PIO and Z80-CTC appear to be optional extras, only the 8255 is used by the monitor program to drive the display and keyboard.

Pressing some random buttons made it beep, so it seems to be working. Time to read the manual and find out how to use it.

If they only knew in 1981 what an impact it would continue to have.

Following the simple example in the manual, it's a simple monitor arrangement. Press ADDR, enter address, and it shows the data at that address. Press DATA and enter new data. Press + to increment the program counter. Use ADDR to go back to the start and then press GO. Use ADDR to go to the place in memory where the result is stored and there is the result:

So now we know that 4 + 5 = 9.

I'll let the Little Professor know.

I have much past experience of entering hexadecimal data, on all sorts of systems over the years, but I have to say I am not a fan of the keyboard layout on the Micro-Professor.

Normal 4x4 hex keypads use a variation of the phone (123 on the top row) or numeric keypad (789 on the top row), but that layout would take a while to get used to.

Even this guy has a more reasonable layout.

Not much of a repair I'm afraid, but it was interesting to finally have a go at using the Micro-Professor.

Back in the 1990s, there was a wonderful company called Greenweld Electronics. They used to take multipage adverts in Everyday Electronics advertising all sorts of surplus electronic parts. (the name still exists, not sure if it is the same company, but it's not the same content)

I remember buying several of the EPROM programmer module for the MPF-I purely for the parts (there were two or three 6116s, an EPROM or two and a ZIF socket, and they were probably only a few pounds each). I am pretty sure I left one intact, in case one day I happened across a Micro-Professor. Today is that day, and can I find the EPROM programmer?


Whilst I was looking for that, the post arrived.

One brand new Z80 CTC chip, still in production 40 years after the one above.

It doesn't look too out of place with the new old stock 8255 and the other original parts.

Still haven't found the EPROM programmer, but this board did come with a printer.

All the accessories came in the same box marked "Microprofessor Option", so you sometimes see these on ebay and you have no idea what is inside.

And within a thermal printer interface.

This is connected to the main board via a 40 way ribbon cable. This bus is pretty much pin for pin connected to the Z80 (other than the 5V pin which is not connected).

It is tempting to connect this us to an RC2014 backplane. The signals are in a different order, so it would need one of those cables with 1 way connectors on. That would open the Micro-Professor up to a whole range of RC2014 modules.

I couldn't find any thermal paper, but I found some old till receipts. It sort of worked, I wrote a little program to print out TEST, and it looks like it did the T and the E, but got stuck. Not sure what the issue is there, it would need to be tested with proper paper.

The owner says the don't plan to use the printer, they just use it for the ROM sockets that are on that board.

I haven't looked into it, but there is Micro-Professor BASIC on there.

That rest of it is all working as far as I can see. I wrote a few little test programs to check the CTC and the IO and it seems to be working, so that is ready to go back.


I have now found the MPF-IBP EPROM programmer module. Looks like I was using it (or at least trying to use it with something. Don't remember what, or when. Maybe one day I will try to fire it up again.


Minstrel 4D

A good platform to try the EPROM programmer module out with would be as an RC2014 module with the Minstrel 4D. Available in Kit and pre-build form from The Future Was 8 bit

More info in a previous post:


You can support me via Patreon, and get access to advance previews of posts like this and behind the scenes updates. These are often in more detail than I can fit in here, and some of these posts contain bits from several Patreon posts. This also includes access to my Patreon only Discord server for even more regular updates.