Replacement for a DEC 7474 Chip

[Brent Hilpert]

On 2022-May-15, at 1:16 AM, Eric Smith via cctalk wrote:
> 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.

Per TI schematics from 1969: 74 standard, H and L series flip-flops are unbuffered. Or at least many of them are/were, in their then-original form. Including 7475, 7490, etc. The output transistors connect both to the pins and wrap back to form the FF or other purposes.

Collector-triggering was discussed a some years ago on the list in regards to a pdp8 front panel where DEC used collector-triggering on 74175's (IMO, bad design practice). From (my) empirical tests at the time, it turned out some 74S (Schottky) parts could be collector-triggered. However, between standard, LS, and S types, behaviour could vary with manufacturer and production date.


> 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.