Tuesday, May 4, 2021

Backplate Musings

My original plan for the LINK_01... my first homebrew computer... was to ultimately have it built using a single PCB. But the more I think about that decision and the limits that would impose on the machine the more I want to go another direction.

I'm heavily influenced by the RC2014 Z80 based homebrew machine which has been going since 2013... and the backplate design which the RC2014 uses goes right back to the birth of home computers. Breaking a machine down into discreet modules and making them all connect to a standard connector is a great way of managing both development and post-launch expansion. As long as the backplate supports enough signals so that you don't limit yourself the sky is literally the limit. The extra connection length may mean that you can't crank the speed up quite as high as a single-PCB machine, but maximum performance isn't something I'm aiming for with this machine... I just want to get something working which I can build upon.

I'm also considering having a second backplate system for connecting external modules to the remaining pins on the Raspberry Pi Pico. Why hardwire the VGA circuit to the Pico when I can modularise it. The final machine would have two sides to it... the 5V 8-bit side and the 3V Pico side... each with their own modules.

So that's the current plan. The first step is to get the backplate design and modules working using stripboard (veroboard)... and the limit for that is 34 lines... which should be enough to get me started with a simple 6502 based machine.

I'm still waiting on a few parts... and I've still not nailed how my shifter array is going to give the Pico R/W access to the main bus... but I'm zeroing in on a plan piece by piece.

For anyone interested here is a link to the excellent RC2014 project.
Here is a video of Steve Furber, from Acorn, talking about his old prototype backboard-based machines from the 70s....

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Backplate Musings

My original plan for the LINK_01... my first homebrew computer... was to ultimately have it built using a single PCB. But the more I think a...