Replacement for a DEC 7474 Chip

[Eric Smith]

On Sat, May 14, 2022, 16:09 ben via cctalk <cctalk at classiccmp.org> wrote:

> On 2022-05-14 11:50 a.m., Nigel Johnson Ham via cctalk wrote:
> > AFAIR LS can only drive one unit TTL load.
> >>     paul
> LS is 4 TTL, 4 ma low.
> Was there a trick of forcing the output of D flip flip
> to clear it? I was wondering if this is what kills all
> the 7474's?
>

I don't think that worked on any TTL (or CMOS) 74x74 flip flops, except
maybe by accident if you shorted the output enough to draw Vcc down (or
ground up) enough to disrupt the FF, and then you have other problems.

Despite the logic diagram showing feedback from the outputs, all 74x74 have
buffered outputs. The recent TI data sheets show an equivalent schematic
only for the 74LS74. I can't at the moment find one for the 7474.

It seems likely to me that early pre-TTL logic families like RTL might have
had FFs with unbuffered outputs, but I haven't checked.