5 Feb
2015
5 Feb
'15
8:24 p.m.
On 2015-Feb-05, at 5:41 PM, Noel Chiappa wrote:
TP4 looks to be an 'input' test point, rather than an output. The pull-down
resistor value (150 or 180 ohms) has been selected such that it is low enough to allow
enough current to flow through the tri-state control input E4.1 to pull it low normally,
but high enough to allow one to connect TP4 high, to enter tri-state, without unduly high
current flow. What the point of going to tri-state is, is not clear, considering that W1
is there for an external clock.
A conjecture is there may have been some external test fixture that tri-stated it for some
sort of synchronous single-step clocking.
In testing input E4.4, you could just send it directly to +5 and GND, however it would be
a little safer to use a 100-or-so ohm resistor in series for both directions, to limit
current in the event the input is shorted internally.
Could also check the V reading on the open E4.4 input.
From: Brent
Hilpert
Well, try removing W1 to isolate the output of
the xtal oscillator E15
from inverter input pin E4.4.
Thanks! Too late to try it today, but first thing tomorrow..
It seems like every time you answer one of my queries, I smack my forehead
and say 'That's an excellent idea! Why didn't _I_ think of that!?!?' :-)
If you're not familiar with them, E15 is an
integrated xtal oscillator,
not just a crystal - the crystal and oscillator logic are both in that
can
I'm not familiar, so thanks for the tip. Wonder where I'll find one if
that's
what's bad... :-)
Could also (with W1 removed) pull E4.4 hi/lo
while watching the
inverter output E4.16 to see if it responds correctly with the
inversion.
Since it's a tri-state, I may need to hit the enable somehow. Oh, wait, I
see, it's hard-wired on via a pull-down. (I wonder why they needed a
pull-down? And I wonder what the test point is there for?) And I guess I need
like a 5K series pull-up resistor to +5, right? (Ground I can just tie
directly, right?)
Anyway, thanks again for the help - very, very useful.
Noel