Thanks Eric.
The longest cable run is about 50ft. Is that a problem?
Not a problem, the ULN will be OK.
Also I thought of using a SPDT SSR to control the anode lines but searching for one I could only find one that has an in of 1.2v. The thought was to eliminate the mechanical relays because they can be problematic.
Are the lines at pin 8 and 11 of the ULN2803A really necessary?
Pin 8 is the input for the anode select invertor, pin 9 is the output.
What power source of 5v is really supplying the power for the 7 seg display?
The source of supply for the displays are the ZTX anode lines, how you power the inputs to the ZTX's is upto you.
Just want to make sure it's not the computer +5v.
Thats your choice, just keep the 0V lines connected together if you use two different +5V supplies.
There's one more option that I would like to explore. Since the display actually has 8 cathodes, one more for the decimal point which is currently unused, would it be possible to use the first method with only the ULN2803A to control the 8 cathodes and use a separate circuit for the anode select?
Yes that would be possible, I would add an extra transistor to do the anode inversion drive to the 2N2222's.
Would it be possible to use your ZTX450 circuit separately or a simple relay?
I dont follow what you mean.?