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

On 26/08/2014 23:27, Eric Smith wrote: The TTL Data Book for Design Engineers, 1982 ed, page 5.2 says Some possible ways of connecting unused inputs are: a. Connect unused inputs to an independent voltage supply. Preferably this voltage should be between VOHmin and 4.5V. Series 54LS/74LS devices with diode inputs may be connected directly to Vcc. b. Connect unused inputs to a used input if maximum drive capability of the driving output will not be exceeded. Each additional input presents a full load to the driving output at a high-level voltage but adds no loading at a low-level voltage. c. Connect unused inputs to Vcc through a 1k resistor so that if a transient that exceeds the input maximum rating should occur, the impedance will be high enough to protect the input. One to 25 unused inputs may be connected to each 1k resistor. Series 54LS/74LS devices with diode inputs may be connected directly to Vcc. d. Connect unused inputs to any fixed-high-level compatible output such as the output of an inverter or NAND gate that has its input(s) grounded. Maximum high-level drive capability of the output should not be exceeded. There it is in part (c). There's some more esoteric stuff about circuit parameters including worst-case testing in Chapter 3 of TI's "Designing with TTL Integrated Circuits", 1971, but it doesn't actually say to use 1k pullups. Calculation of values of various things for various circumstances is left as an exercise or two for the reader :-) -- Pete Peter Turnbull Network Manager University of York