TTL source/sink currents and LEDs (was Re: Some Altair questions,

Actually, I've never known it not work. My first computer had an 8 digit (or was it 9 digit?) 7 segment display. Common cathode multiplexed. The (digit) cathodes went to the outputs of a '45 open-collector decoder, which also scanend the keypad. The (segment) anodes were drvien from totem-pole TTL outputs (I think something like '08s usd as buffers), which therefroe were sourcing current. No limiting resistors IIRC. THat was the machine which used '157s as the latches for the display data. No that is not one of my typos. They were not '175s. And yes, it was a Sinclair, with all the poor design that implies... [...] Is it? I thought that for plain TTL and (I think) LS it was good practice to pull unused inputs up via a resistor and not link them straight to Vcc. Something about supply line spikes causing breakdown of the input transistor. -tony