Category Archives: Multicomp

Multicomp revisited.

Way back in April 2015 I wrote about building an FPGA computer based on Grant Searles Multicomp.

To be honest, it’s not had a lot of use. It has sat, largely unloved, in a cupboard awaiting redisovery. Well. in a roundabout way I have rediscovered it.

Back in September last year, I exhibited a few retro machines at the Centre for Computing History‘s Retro Festival with an old friend. There were quite a few exhibitors from all over the place and one, Neal, was demonstrating the multicomp as a fully loaded 6809 machine. The really nice feature for me was that Neal had used a PCB to build his whereas mine was using a bit of blob board I had kicking around. What was nicer was that Neal had a spare blank board that he was willing to pass on to me.

This board has been sitting on the “Shelf of good intentions” until the right time and its time has come.

Getting started.

The design files for the PCB and instructions can be found on the Retrobrew website and package the information nicely. Be careful when coosing the CF card socket and the RCA connector as there are more than one design of each out there and it’s easy to get a wrong example of each… I have found.

The first step is to solder the connectors and the components for the VGA interface and PS/2 keyboard. This can be considered a minimal system and is a good place to stop and see how you are doing.

At this stage you should have a working 6809 machine with VGA output and 2k of built-in RAM.

This is where I’m up to now. More to follow shortly.



Grant Searle’s Multicomp

IMG_20150403_144747337I have just screwed the lid down on the machine you see to the right. This is my self built Multicomp.

Grant Searle has been prolific when it comes to modern builds of retro computers. On his web site you will find examples of Z80, 6502 and 6809 based machines. All with schematics and details on how he built them.

His work has inspired other to have a go at building a simple machine with the minimum of parts (The Z80 uses 7 chips).

The only problem you really face is which to build. He has a solution. Build the Multicomp. This machine is based on a cheap FPGA board that uses free tools from Altera to build a machine that emulates any of the above processors and wraps a machine around them.

The word “emulates” is a bit of a grey term here as the machine is implemented in hardware. This is not the same as running a Spectrum emulator on your WinPC. The logic needed to “be” a Z80 is programmed into the FPGA at build time and there it sits, being a Z80. There is no other software “tricking” a Spectrum program into running. The only software there is  running on the bare metal.

IMG_20150403_145227272_HDRMine all mine.

Mine has been built as a Z80 with 2k of RAM (this saves an external chip but upto 64kb can be accessed). I have two serial ports and a VGA port. These can’t all be accessed at the same time but it’s very easy to reprogram it to use whichever of the port you need.

I’ve used a small piece of blob-board to sit on the FPGA boards I/O pins to give me something to work on. The serial outputs of the FPGA are not at RS-232 levels and so I bought two, cheap, level converters built into 9 way D-type sockets.

IMG_20150403_145144622_HDRThe whole thing is packaged into a box previously occupied by a digital TV decoder that went “POP” in a stylish fashion.

Get out there now and build your own. Please.

Screenshot from 2015-04-03 14:54:31