Replacement for a DEC 7474 Chip

[Brent Hilpert]

On 2022-May-15, at 3:53 PM, Eric Smith wrote:
> On Sun, May 15, 2022, 13:03 Brent Hilpert via cctalk <cctalk at classiccmp.org> wrote:
> 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.

> I specifically said 74x74. Early TTL flipflops were very crude by comparison.


S = { "", "L", "H", "S", "LS", "F", "AS", "ALS", "AC", "ABT", etc. }

x ∈ S

"", "L", "H", "S" ∈ S

"L", "H", "S" != ""

7474: x == ""

pre-TTL != early TTL