This was a mammoth project and isn’t actually finished although it does work. I’m not going into the background of the Acorn System 1 as those who know about this microcomputer are well aware of it’s history.
Originally this rebuild project was a series of many blog entries describing how I got to a working system, but whilst moving to the new site I’ve decided to condense things.
I’ve wanted one of these computers ever since I first saw an advert back in the 1970’s, but as a kid on pocket money it just wasn’t going to happen.
Now I can afford it, deciding to build an Acorn System 1 from scratch in the 21st century proved to be a harder undertaking than I imagined. Firstly, whilst there is quite a lot of documentation available, I found some inconsistencies in the original circuit diagrams that needed to be corrected. However, the biggest issue was parts availability as the System 1 uses some parts that are now rather difficult and expensive to obtain.
Initially I decided I wanted to be as true to the original version as possible whilst using contemporary parts as much as possible. One thing that I was insistent on was that it would be compatible with the original system and would run the same Monitor Program firmware as the original, unmodified.
The System 1 consists of two boards. The top being the keypad, display and cassette interface and the bottom containing the CPU along with memory, I/O, support logic and PROMs and 5v regulator.
Right from the outset however I decided that I would make some “improvements” over the original.
The 5v regulator has been dropped. The system utilises a more modern W65C02 CPU which is faster and has some other benefits whilst still being fully software compatible with the original. The onboard Firmware PROMS/EPROM have been changed for something more modern; and available, as have the RAM chips.
The cassette interface was dropped as I don’t see the point of it in these modern times, but I’ve left provision so one can be added if needed.
I’ve maintained compatibility with the existing expansion connector pinouts but I have made use of some of the previously unused pins.
The original had a small configuration area where wire links were used to configure the hardware in different ways. This area still exists but works in a slightly different way now. There are also several configuration jumpers elsewhere on the board.
The original had 1Kb of RAM on board though the memory map allowed for 2K. The full 2K is now available.
For many people the big ticket item is the removal of the INS8154 ICs. The original required one to work, and could support a second. These IC’s are difficult to find and expensive. This IC has now been replaced with the W65C22 and support for the second IC has been removed.
A notable difference here is that the original INS8154 also had 128 bytes of RAM. That isn’t available on the replica but apparently there was only ever one piece of commercial software that made use of it. It would be pretty simple to create an expansion card that held this 128 bytes if it was deemed necessary.
Using the new W65C02 and W65C22 IC’s means the CPU can now run at up to 8 MHz and there is now a configurable clock that allows for 1, 2, 4 or 8MHz operation. Actually the CPU can to 14 MHz, it’s the support logic, EPROMs and RAM’s that cant keep up.
The NMI and IRQ buttons that were originally on the CPU board have been relocated to the KeyPad.
IDC connectors now allow the keypad to be easily disconnected from the CPU board without the need of a soldering iron.
The big take-away from all this is that the replica machine will run the original, unmodified version of the Acorn System 1 monitor Firmware. It is pin compatible on the expansion connector and the cards are still in the Euro card size so is board size compatible. It’s just faster (if you want it to be), has more memory (if you want it to) and can be built from available parts.
I’ve broken the project down into a number of pages, and that number will grow as time moves on.
One important thing that needs to be stated is that whilst I’ve done as much as possible to make the replica compatible with the original, that compatibility stops after the CPU board and KeyPad/Display board.
My expansion cards and especially the backplane are not compatible with the original expansion cards. There is no reason why you cannot design a backplane that is fully compatible with the original, it’s just I decided I didn’t want to go that route.