* Posts by ianthompsonbell

2 posts • joined 1 Aug 2012

The Dragon 32 is 30


Re: Memories of the Project Leader

Hello Duncan,

Very long time no see. I tried to contact you about a year ago because I was contacted by someone who was doing a Dragon exhibit at the museum at Bletchley Park. He wanted an article on the Dragon from the developer's perspective.

Can you let me have your current email address?




Memories of the Project Leader

I was the project leader for the Dragon 32 at PAT, or Patcentre as is was then known. I thought you might like to know what I remember of that time.

I remember spending a lot of time talking to a really helpful Motorola chip salesman called Robin Saxby. Yes, the same guy who later ran ARM and is now Sir Robin.

We certianly did not copy the CoCo. It was not really available in the UK because it had an NTSC video system which would not work on UK TVs in those days. The Motorola application for the SAM chip (synchronous address multiplxer) showed a complete home computer to which the CoCo was identical. We made numerous improvements to this app note. We included a real A/D and D/A convertors for generating the FSK signals used to store programs on tape. We added a parallel printer port and used the same chip to scan the keyboard. We had a separate power supply PCB which also contained the TV modulator. It was a single sided PCB so it saved cost but also allowed variants to different TV standards to be made cost effectively. We made a SECAM variant for France and also an RGB and US version.

I do not rmemeber Motorola suppying a BIOS. Microsoft wrote the Basic interpreter, which was essentially the same as the one they licensed to Tandy but with a few add-ons, but you were expected to create all your own peripheral drivers - a situation unchanged to this day. It was these drivers that Duncan (Smeed) wrote. I do not remeber the keyboard speed up being his alone. The nromal way to scan a key board is to activate a row and read the columns to see if a key has been pressed and repeat that for each row. Of course, most of the time there was no key pressed and this routine just wasted a lot of time getting a no key preessed result. We realised that becuase we had used the same chip to scan the keyboard as drive the parallel printer port, we could do one thing the CoCo could not, and that was activate all the rows at once. If you do this and then look at the columns, in one go you get to know if there are no keys pressed, the most common situation, and you can exit straight away. If you find a key has been pressed you scan as usual to find which one. This is what saved the time.

The PAL output had nothing to do with the CoCo. PAL was essntial for it to work on UK TVs. Few if any had SCART sockets so you had to create genuine PAL. Persuading a chip designed to make 525 line 60Hz NTSC to make 625 line 50Hz PAL instead is a non trivial exercise and needed a lot of descrete logic - ASICs were in their infancy then.

Two weeks before the official launch, the Spectrun 16K came out. The piggy back RAM PCB was designed, tested and ramped up for production in that two weeks. Later we used a bunch of Siemens 32K RAM chips that consisted of two 16K RAM chips literally piggy backed on eachother and later still upgraded the main PCB to 32K then 64K.

We then worked on the disk drive unit which was abandoned when Tony Clarke left and all development work went in house. What is probably not well known is that at the same time we were working on the successor to the Dragon, code named Draconis. This used a Motorola 68K processor and a very powerful graphics chip from NEC. Along with OS/9 as a true real time executive, this would have beaten the PC hands down as a business machine. But for the vagaries of the home computer market, we might all be using Dragons today.


Ian Thompson-Bell