Building an Acorn System 1 replica - Part 6 (27/04/2020)
Once again, a lot has been happening with this project since my last update.
Yesterday I sent another new PCB order to JLCPCB. This order contains new CPU boards, Euro development boards, enhanced display boards, extender boards, 8514 replacement piggyback boards and backplane boards. I'll go through each of these in more detail below.
As soon as the boards arrive I'll assemble and test them and if all works as hoped, I will then release the full set of plans for everything.
You can find a list of everything that's available here. This list will evolve over time.
The first piece of news I want to talk about is I've now got a replacement solution for the INS8514 chip. The standard Acorn board has provision for two of these ICs and whilst one is optional, the other is needed if you want to connect the Acorn keypad and LED display board.
After some experimentation I've managed to persuade the more common 6522 VIA chip to take over the role with only a few limitations.
Firstly the 6522 doesn't have the 128 bytes of onboard RAM, but apparently this isn't really a problem as there seems to be only one piece of commercial software that ever used it. The other problem is that the current sink/source capabilities of the 6522 are a bit less than the original INS8514 resulting in the LEDs of the display being dimmer. Actually a lot dimmer. The brightness situation can be improved if the 6522 is replaced with the modern W65C22 device, which is completely pin compatible with the older 6522, and runs the display brighter; though it's still not brilliant.
The downside is that you can't just plug one of these 6522 IC's into the INS8514 socket as the pinout is completely different.
Yesterday I sent another new PCB order to JLCPCB. This order contains new CPU boards, Euro development boards, enhanced display boards, extender boards, 8514 replacement piggyback boards and backplane boards. I'll go through each of these in more detail below.
As soon as the boards arrive I'll assemble and test them and if all works as hoped, I will then release the full set of plans for everything.
You can find a list of everything that's available here. This list will evolve over time.
The first piece of news I want to talk about is I've now got a replacement solution for the INS8514 chip. The standard Acorn board has provision for two of these ICs and whilst one is optional, the other is needed if you want to connect the Acorn keypad and LED display board.
After some experimentation I've managed to persuade the more common 6522 VIA chip to take over the role with only a few limitations.
Firstly the 6522 doesn't have the 128 bytes of onboard RAM, but apparently this isn't really a problem as there seems to be only one piece of commercial software that ever used it. The other problem is that the current sink/source capabilities of the 6522 are a bit less than the original INS8514 resulting in the LEDs of the display being dimmer. Actually a lot dimmer. The brightness situation can be improved if the 6522 is replaced with the modern W65C22 device, which is completely pin compatible with the older 6522, and runs the display brighter; though it's still not brilliant.
The downside is that you can't just plug one of these 6522 IC's into the INS8514 socket as the pinout is completely different.
The above image shows the INS8514 replacement piggyback board fitted to my development system. This board plugs into the existing INS8514 socket and then two wires need to be soldered to the PCB. The 6522 IC then plugs into the top and hey-presto you have an INS8514 replacement module.
This is a simple hardware only upgrade. The original Acorn Monitor firmware or any of your own software that uses the display/keyboard should work without any alteration. I will be covering how this all works in more detail at a later time.
This is a simple hardware only upgrade. The original Acorn Monitor firmware or any of your own software that uses the display/keyboard should work without any alteration. I will be covering how this all works in more detail at a later time.
CPU Board (Version 1.2.0)
I covered most of the changes to this board revision in Part 5.
The big ticket items were the reversing of the pins on the edgeway connector to make it more compatible with existing Acorn boards. To be fair, it makes no real difference to me as I don't intend using this with anything original, but some feedback I had made me think that if I was going to change things, it was now or never.
The other change that made it into this board revision was to support the 6522 natively so the board no longer supports the legacy INS8514 chip. This should make it a lot simpler for people to construct and get working.
The one change that I didn't make was to support a CAN module oscillator crystal. This would have been a pretty simple change and would have removed one of the 74LS04 ICs from the board, but these CAN modules can be harder for some people to find, and my personal opinion is it removes some of the flexibility of the design as right now, you can use any suitable crystal. I may change it in the future but right now, I don't see the need.
I covered most of the changes to this board revision in Part 5.
The big ticket items were the reversing of the pins on the edgeway connector to make it more compatible with existing Acorn boards. To be fair, it makes no real difference to me as I don't intend using this with anything original, but some feedback I had made me think that if I was going to change things, it was now or never.
The other change that made it into this board revision was to support the 6522 natively so the board no longer supports the legacy INS8514 chip. This should make it a lot simpler for people to construct and get working.
The one change that I didn't make was to support a CAN module oscillator crystal. This would have been a pretty simple change and would have removed one of the 74LS04 ICs from the board, but these CAN modules can be harder for some people to find, and my personal opinion is it removes some of the flexibility of the design as right now, you can use any suitable crystal. I may change it in the future but right now, I don't see the need.
I now have a page dedicated to maintaining a list of all the boards and schematics related to this project here.
Check it out often to see what's happening.
Check it out often to see what's happening.