7474 Chip Replacement QUESTIONS

From: Wirehead Prime <wirehead_at_retrocomputing.com>
Date: Thu Mar 12 19:27:57 1998

> considering the low speed of a 6800, an LS (or even an HCT)
> should work...but, are you sure it isn't the case of a fast
> pulse on the D input when CLK is hit (on the falling edge if
> memory is correct)? and you aren't seeing it (using a logic
> probe or a triggered scope)? did you check the voltage in to D?
> maybe it's in no mans land (i.e. around 2v). Maybe its the IC
> driving the flip flop D input thats bad, or a fast pulse is
> hitting the R* input

OK, here's the pins, what the logic probe says and what the voltmeter says:

Pin Correct Name Rough Name State Voltage
8 2NOTQ Not Q Output High 3.93v
9 2Q Q Ouput Low 0.13v
10 2Set Preset High 4.99v
11 2Clk Clock Pulsing High 4.34v
12 2D Input High 5.02v
13 2Cr Clear High 5.00v

D is driven directly from the bus so there's no chip driving it and the
bus voltage looks fine.

According to "Interfacing Microcomputers to the Real World" by Sargent
and Shoemaker (the only thing I have that talks about the 7474 in detail,
"The basic idea is this: with the clear and preset inputs high (not
active), a transition of the clock input from low to high (called a
positive edge) "clocks" the value of D into Q and the inverse into
NOT Q. The clock transition is required; D can do anything it wants to,
but nothing will happen to Q and NOT Q unless the clock has a positive edge."

And the clock input is pulsing high so that's supposed to be positive edges.

[THIS IS SO MUCH FUN!!!! SERIOUSLY!!! =-) ]

And I don't see any fast pulse on any of the inputs that are supposed to
stay stable.

But what doesn't make sense to me is why the flip flop would fail in
such a way that the outputs would be reversed with appropriate voltages.
I feel I must be missing something but the poor thing is being driven
in a pretty direct fashion...IE not alot of chips in the way and all the
inputs look correct. I think I'm going to socket the thing and replace
it with a 74LS74...if after two or three swaps (just for giggles) I get
the same response, well then I guess I did miss something somewhere else.

=-)

In either case, I guess it's never a bad idea to socket things when you
have an excuse in case something DOES go goofy.

Anthony Clifton - Wirehead
Received on Thu Mar 12 1998 - 19:27:57 GMT