I'm new to Arduino - and electronics in general - and am having a frustrating problem with two serial-to-parallel 74HC595 chips trying to drive two 7-segment LED displays. The LEDs are both common cathode, and each LED input has its own 510 ohm resistor, connected up to an output of a 74HC595. If I unplug the common cathode from A, I see the correct display in B. If I unplug the common cathode from B, I see the correct display in A. But when both common cathodes are plugged in, B starts displaying rubbish (mostly it's just blank though) while A displays correctly. It's like there's not enough power to go around. I'm using the 5V power output, because that's what the 74HC595 chips need (actually it can be between 2V and 6) and I don't want to start frying chips.
If anyone can point out what I'm doing wrong (or what I could potentially try to get it working) that would be great. I am working on a hobby project where I was hoping to have 6 of these LED and 74HC595 chips hooked up.