Building an Acorn System 1 replica - Part 5 (27/04/2020)
Having a decent nights sleep is usually a good thing for most people, but for me it can open a real can of worms especially if I fall asleep thinking about a project. Last night, I fell asleep thinking about "this" project, and this morning my head was full of new ideas.
Even though this is supposed to be a replica of an existing product, I'm becoming more comfortable hacking the design and specifications around to suit my own needs. I was perfectly comfortable replacing the two 2114 RAM chips with something that was a bit (though only a bit) more modern as I had a box full of them. Likewise it seemed sensible to use the newer W65C02 variant CPU with it's much higher clock speed support (the clock speed is configurable on the board). And since any peripherals I build will be either designed from scratch or at least use my own PCB design, I was perfectly happy utilising the 96 way shell Euro connectors that I've got a box full of.
However, I've received a couple of Emails from people asking how compatible my version is with the original in terms of connectivity via the edge connector.
Well I based the connection layout on the original Acorn specification and only really added some extra connections. I think the only existing connection that is significantly different is the 9v DC input pin was re-tasked to be an external clock input. The big difference however is that my PCBs are designed to accept the 96 way Euro shell with rows A + C loaded, and, on further investigation, it looks like my Side A and B are the opposite way around to the original, and the original uses 64 way Euro connectors with rows A + B loaded.
So this morning I addressed those issues.
The CPU board will now support either a 96 way Euro Shell (with rows A + C, A + B or even A + B + C loaded ) or a 64 way Euro Shell with rows A + B loaded which I believe is what the original Acorn systems used. The board already offered isolation for the 9v / Ext. Clock pin so no changes are needed there.
I've also added a diode that will allow the 2nd 1K block of the main system RAM to be used if desired.
This should make the CPU board much more compatible with the original.
I've also been asked if I will be publishing the design files for this project. The answer is most certainly yes.
If there's interest I'll always publish plans for any of my projects, but I'd always intended from the start to release my design files for this project, so from the outset I've tried to keep accurate and up to date design documentation.
As soon as I'm happy that everything works as it should I'll upload the files and you will be able to grab them from this site.
One thing that I've very happy about is the arrival of my 3-slot powered back plane boards.
Even though this is supposed to be a replica of an existing product, I'm becoming more comfortable hacking the design and specifications around to suit my own needs. I was perfectly comfortable replacing the two 2114 RAM chips with something that was a bit (though only a bit) more modern as I had a box full of them. Likewise it seemed sensible to use the newer W65C02 variant CPU with it's much higher clock speed support (the clock speed is configurable on the board). And since any peripherals I build will be either designed from scratch or at least use my own PCB design, I was perfectly happy utilising the 96 way shell Euro connectors that I've got a box full of.
However, I've received a couple of Emails from people asking how compatible my version is with the original in terms of connectivity via the edge connector.
Well I based the connection layout on the original Acorn specification and only really added some extra connections. I think the only existing connection that is significantly different is the 9v DC input pin was re-tasked to be an external clock input. The big difference however is that my PCBs are designed to accept the 96 way Euro shell with rows A + C loaded, and, on further investigation, it looks like my Side A and B are the opposite way around to the original, and the original uses 64 way Euro connectors with rows A + B loaded.
So this morning I addressed those issues.
The CPU board will now support either a 96 way Euro Shell (with rows A + C, A + B or even A + B + C loaded ) or a 64 way Euro Shell with rows A + B loaded which I believe is what the original Acorn systems used. The board already offered isolation for the 9v / Ext. Clock pin so no changes are needed there.
I've also added a diode that will allow the 2nd 1K block of the main system RAM to be used if desired.
This should make the CPU board much more compatible with the original.
I've also been asked if I will be publishing the design files for this project. The answer is most certainly yes.
If there's interest I'll always publish plans for any of my projects, but I'd always intended from the start to release my design files for this project, so from the outset I've tried to keep accurate and up to date design documentation.
As soon as I'm happy that everything works as it should I'll upload the files and you will be able to grab them from this site.
One thing that I've very happy about is the arrival of my 3-slot powered back plane boards.
This 3-slot board is ideal for limited space applications as provides one slot for the CPU board and then two expansion slots. It also has an onboard 5v regulator allowing the entire assembly to be powered from a 9v AC/DC power source.
The above is a short video showing my development system connected to the 3-slot backplane and a project board containing 8 x LEDs running a simple binary counter program. I left this running as a simple stress test for several hours to prove that the CPU was stable.
The project card is actually a shorter version of a standard Euro card that I've designed - and the reason it's shorter is that I was embarrassed at asking JLCPCB to drill sooooo many holes. Smaller is also cheaper of course.
The project card is actually a shorter version of a standard Euro card that I've designed - and the reason it's shorter is that I was embarrassed at asking JLCPCB to drill sooooo many holes. Smaller is also cheaper of course.