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The difference between common anode and common cathode is that diode conduction is in the opposite direction.
You will need fairly massive circuit modifications to use the "opposite" diode orientation. You'll likely need to invert the sense of all the outputs, including the common lines. If you're lucky enough to have 4543 drivers to drive your LEDs it could make it easier to invert sense (which does not seem to be the case). Another issue is that the common anode and common cathode pinouts are different - especially if you're retrofitting or replacing displays. The digit driver transistors may need to change from N to P channel devices depending topology.
It's best to get the correct type if you're using existing circuitry. Otherwise adding the inverters can be quite messy, especially if you have no space for them.
The pinouts are definitely not a problem - this is a circuit built from scratch, and I have the spare space, but obviously want to keep the component count as low as possible.
The maxim LED drivers commonly use a technique called Charlieplexing named after Charlie Allen at Maxim.
**broken link removed**
This technique REQUIRES common cathode displays. AFAIK there is no way around this problem. You need to get common cathode displays or come up with another type of LED driver. Good luck on that one.
A lot of it is still dependent on the circuitry. Is the circuit multiplexed? What do the digit drivers look like now? How many displays are there?
The general way to do it is:
1. put an inverter on every output that leads to the displays, including the common pins. Select an inverter that has the same drive strength as the original circuit.
2. Change all NPN digit driver transistors to PNP, or n-channel to p-channel if using FETs, and vice versa.
Change their common pin (usually GND for common emitter config or VDD for emitter follower) to the other supply.
That's the basic steps. It could be easier if more of the circuit was provided, I don't know what's in the circuit right now to make a really good judgement.
(EDIT: Since now it's know it's charlieplexed, this special mutation of multiplexing will require unavailable driver inverters. You may try to build custom tristate inverters that will somehow tristate automatically when a tristated input is detected. This can get very complicated and the previous suggestion to get CC displays stands.)
The pinouts are definitely not a problem - this is a circuit built from scratch, and I have the spare space, but obviously want to keep the component count as low as possible.
I'm using the MAX695X series - I need to use these because of the sheer number of displays I'm running. I need to run 9 x 16 segment, plus 36 x 7 segment displays, all from one 18F PIC. I can do that pretty easily if I tie 3 of the 695Xs together and connect them to the PIC.
I don't really want to change the choice of driver IC. And I can't change the segment displays I'm using either, as the specific design isn't available in common cathode.
So, just sticking plain inverters on the outputs isn't going to work...
That makes things very tricky.
Anyone got any other clues as to how I might do this?
Could you go into a bit more detail about the tristate inverter idea?
I would SERIOUSLY suggest using the correct parts for the job, bodging a solution because you've got the wrong parts is likely to be more complicated than the rest of the project.
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