Re: [CAD_CAM_EDM_DRO] CMOS/LSTTL fan out

Bill Vance wrote:

>Howdy;
>
>Looks like my potential DRO circuit is going to have to go with a mixed system.
>In part thats due to timing, availability, and/or pricing concerns. Now my old,
>perhaps I should say, ancient data says that the TTL, and LSTTL families have a
>fan out of 10, i.e., that one output will drive 10 inputs, and that the old
>standard CMOS chips should be kept under a fan out of 50. The families I'll be
>going with however are 74LS TTL, and 74HC CMOS. Everything I've seen so far
>says that one family will drive only one input of the other. Does anyone have
>more recent fan out data for these, and what it might be for either family to
>the other? I also need the under what conditions data, (pull up/down resistors,
>or using buffer gates, etc.).
>
>

To drive CMOS HC from LS or TTL, you usually need a resistor pull-up to
+5 V.
A better way is to use HCT components. Then, you have matched the
inputs of the HCT
to the output voltage levels of the LS. The outputs of HCT are the full
5 V swing.

To drive LS from HC, you can drive at least 4 LS from one HC output, but
maybe only
one or two TTL, as they source 1.6 mA in the low state. You can
actually violate these
rules quite a bit, and some HC devices have as much as 24 mA sink
capability, especially
the HC tri-state driver series, like 74HC244.

Jon