DrDiettrich:
A seven segment display has 7 segments, or 21 segment cathodes (-) for 3 digits. These segments can be connected in parallel for all fields, to 21 TPIC outputs.Every display has a common opposite electrode, which should be the anode of every digit. There exists displays with anodes and cathodes reversed, we need these with common anodes. A transistor is needed for the high side switch, that turns on just the display anodes (+) of the currently active field. This sums up to 9 outputs for the 9 fields, and a total of 30 outputs from the 32 outputs of 4 TPICs. And all that can be controlled from 3 Arduino pins for the shift registers.
Thanks.