On Sun, 13 Jan 2013 19:00:46 -0500, "Charles P. Steinmetz" <charles_steinm...@lavabit.com> wrote:
>Ed wrote: > >>On opening it up I found that the circuitry includes a 74LS73 dual >>JK FF, and a 74LS140 - very obscure - apparently a dual 4-input gate >>of some sort. > >AFAIK, the '140 was only supplied in the "S" series (74S140). It's a >dual 4-input NAND 50 ohm line driver. That is my understanding. Driving 50 ohms is going to take so much power that producing a slower and lower power version would hardly be worthwhile. There are a couple of other TTL 50 ohm drivers, the 74128 quad 2-input NOR comes to mind, but I think the 74S140 was the most popular. Both of those are still produced by Texas Instruments. If you want a lower power then the other dual 4-input NAND gates (7420, etc) use the same pinout. Cleverly, and I have seen at least one design that did this, the 74S140 pinout can also be compatible with the quad 2-input NAND gates (7400, 7437 buffer, etc) if you treat them as 2-input devices. The extra 74S140 inputs can either be tied high or the extra '00 gate inputs and outputs can be tied in parallel for extra drive. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
