I want to make a retro computer around a 6809E and a CPLD. It occurs to me that the physical layout/design of this board would be nearly identical for all the 40 pin, 8 bit CPUs except for power and ground. I want to use a Lattice ispMACH 4000ZE for the CPLD (3.3V I/O, 5V tolerant) and primarily I'm interested in the Z80, 6502, and the 6809E. I'm not opposed to supporting other 5V, 40 pin DIP CPUs.
As an example I know that PROM programmers implement software selectable power and ground pins.
My experience is mostly software and digital logic. I suspect that this can be fairly easily implemented with some sort of pull up/pull down transistor circuit, but I don't know enough to find an explanation / example.
Thank you in advance!
EDIT: Additional info that may be useful.
The CPLD supports 3.3V LVTTL levels with 5V tolerant inputs so I don't need to level shift signals between the CPU and the CPLD.
I'm planning on using a PC ATX power supply, so I have +3.3, +5, +12 and -12 easily available
I know I could use jumpers to accomplish this, The reason I'm looking at doing this electronically as opposed to physically is that there are a limited number of pins that I need to support (5 power, 4 ground). I'm not looking at having a completely generic solution.
The list of CPUs I look at:
- Motorola 6800
- Motorola 6809/6809E
- Hitachi 6309/6309E
- Zilog Z80
- Mostek 6809
- Mostek 65816
- Intel 8085