Up until now, I've been working with the breadboarded cpu ISA proto card. It is a pain, as it takes up 2 slots in the bus with all the wires sticking out.
It is now time to make a pcb for the cpu.
In addition to the cpu, I want to add line drivers for the address and data bus, and the cpu control lines. The cpu is only rated for one TTL fanout. The buffer chips provide improved noise rejection and high fan out outputs.
I am using two 74541 for the 16 address lines, I chose the 74541 over the 74244 because of the chip layout. For the data bus, it needs to be bi-directional, so I chose the 74245. There are four cpu control lines, so a 74541 would be overkill, I chose the 75125, which has four buffers. The cpu signals are E, VMA, R/W and BA.
BA (Bus Available) is used to put these buffer chips into a high-impedance state.
The cpu board uses a DS1813R reset management chip.
- Automatically Restarts a Microprocessor After Power Failure
- Monitors Pushbutton for External Override
- Maintains Reset for Typically 150ms After VCC Returns to an In-Tolerance Condition
The RJ14 plug is used to control the cpu externally.
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