>> most signals (but I don't think ALL signals) go OUT though an >> inverter but they come IN without an inverter. >NPN transistors behave better and quicker for some reasons then PNPs. It has >something to do with quantum mechanics I was told so don't ask me further :-) Electron mobility in silicon is higher than hole mobility. But that has nothing to do with the 1541. The serial bus uses open-collector drivers for the outputs. In the 1541, they use a 7406 (this is from memory - might be wrong), which inverts the signals. The lines are connected directly to the inputs, so they're not inverted (there's probably a series resistor or something to protect the PIAs, but the signal still doesn't get inverted). Why do they use open collector drivers? So you can connect more than one device to the same wires. John West
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