Thanks Guys
Hi Rob 
- This is in parallel with the conversation we had offline
Its good to see my question has been understood.
CrossRoads - yes you have it bang on, that is what I want to achieve.
I was talking to Rob (Graynomad) offline, and the question came up about multiplexing and what effect that has on the brightness of LED's like this. Basically if I have 4 displays in parallel, switching between them really fast, they effectively have a duty cycle of 25% - correct?
How does that effect the brightness, compared to powering the displays individually.
As Rob pointed out, the headroom is slim - 11.6V to 12.4V. If they are multiplexed, effectively PWMing the 12V source, will the voltage have to be boosted in order to keep the average within the required limits, to then keep the display at full brightness?
This is just one of the problems. 12 wires for 4 displays is the aim, but will a simple 12V source be sufficient, or will it required a variable regulator or something in order to ensure the displays are full brightness?
RE the MAXIM 8 digit driver, that is what I used first off. But since its common cathode designed, I had to use transistors (or FETS) obviously, plus the fact they require 12V. Transistors caused the voltage drop which dimmed the digits which I want to get around and FET's would fix this. I then thought about just using an IO expander or a 2nd processor and just powering each individually. That made the 36 wire solution though. The final version I want to fit in the same footprint as other baords I have made, which means I can only fit 12 wires.
All good solutions but a number of questions too.
Thanks