Motorola 6800 and S50 bus

SWTP was a small manufacturer of electronic hobby kits  that got on the small computer bandwagon soon after the Altair but it did things in a completely different way.

The first big difference was that it based its machine on the Motorola 6800 rather than the Intel 8080 or Zilog Z80 processor. There were many engineers at the time who admired the 6800’s simplicity and logical design. It also had a good range of easy to use support chips that enabled a full computer system to be put together.

From the electronics point of view, a great deal of the simplicity in designing a 6800 system stemmed from it not using a multiplexed address/data bus. A multiplexed bus saves pins on the chip by using the same set for address and data - a separate control line signals when data is present and when address is present. To build a system with a multiplexed bus processor like the early Intel CPUs you needed another big chip to separate them out.

The 6800 had separate data and address buses and this made timing and overall construction much easier.

The 6800 address, data and control bus naturally led to the definition of a new external system bus. The Altair had introduced the S100 bus which was essentially a demultiplexed form of the 8080 bus structure.

The 6800 could be forced onto the S100 bus but it wasn’t a natural fit. SWTP introduced the S50 or SS50 bus with their first design and it was not only simple but cheap. Instead of using the expensive edge connectors specified by the S100 bus, they used a set of “Molex” connectors along the edge of the PCB. Molex connectors were never intended for this application and the whole construction looked very home made - but it worked and it was cheap.

Notice the rows of pins rather than a PCB connector.

The PCBs carrying the processor and the memory were plugged into a motherboard which had  rows of 50 pins (hence S50) sticking up for each one. The motherboard looked like more like a bed of nails than part of a computer.

The interface boards had a special bus all of their own - the S30. This again was designed to save money. Each peripheral “slot” had a fully decoded address pin which selected the card and. The motherboard contained the electronics to do the decoding for each of the eight slots and again this saved money.. The idea of fully decoded I/O ports was something that turned up in the Apple II and while I have never been able to discover a connection between the two machines I have my suspicions!

The whole lot was placed inside a bolt-together metal box with an expanded metal covering. In theory the expanded metal cover was held in place by aluminum trims - but in practice not many units had them fitted. Most of the time you needed to get inside to fiddle with the hardware. The power supply was also a work of art. Two huge capacitors and an even bigger transformer provided lots of current for the power hungry memory cards.

These were the days before switch mode power supplies made small cool computers possible. Even so the MMP-68 didn’t use a fan just convection cooling.

The first machines were available only as kits. SWTP was a kit manufacturer and had no assembly facilities. You could buy an MP-68 with 4K of memory and a serial interface for £275.

If you wanted it assembled then this would be done for you by one of the Computer Workshop’s staff for £75 or so. A bigger cost was the requirement for a serial I/O device of some type. Affluent users often settled for an ASR 43 Teletype costing around £900 - expensive but you got a printer of sorts as well.

If you were not so well off then you had little choice but to buy a VDU or build the CT-64 for £90. This was an amazing design which used hundreds of chips on a single huge PCB - again to keep costs down. It also looked a bit like the VDU equivalent of the VW Beetle - all curves.

A modern look

The MP-68 was a very modern machine in that it didn’t have rows of switched and lights to enable the user to set it up. Instead there was a ROM on the processor card that communicated with a serial device.

Using this you could load hex codes into the memory and eventually run a program. If you were really serious about  computing you could buy an AC30 cassette interface, for £65 not including cassette recorder, and save and load programs onto audio tape. If you didn’t have an AC30 then you typed in your machine code program every time you wanted to use it - hence the machine was often left on permanently.