Mystery ICs - AB 36D1024

[tony duell]

> Resistor networks.  Easy enough to take out your ohmmeter and figure out
> what's inside.

I have come across some custom DIP resistor networks in HP machines where
it's not just isolated resistors or resistors from the pins to a common pin (the
common standard configurations). With those, simple pin-pin resistance 
measurements are not enough ( I forget if it's enough in theory, but in 
practice if the resistors are wildly different values you won't do it).

I seem to remember working out a method where you (a) measure
resistance from each pin to all other pins strapped and (b) make a 
potential divider using one pin as the input, another as the output 
and all others strapped as the common. Repeat for all combinations
of pins.

Here, I think is how it works out. Suppose in (b) you apply the input to
pin x and take the output from pin y. Suppose that there is a resistor r 
between x and y, and the parallel combination of all other resistors on 
pin Y has resistance R.

Then the division ratio is clearly R/(r+R)

Now think of (a) using pin y (the pin that was the output of the 
potential divider). Resistor (r) is now included in the parallel
combination, so the resistance from pin y to all others strapped is
(R.r)/(R+r). Dividing one by the other will give you the value of r.
Repeating for all pin combinations will give you the resistor between
each pair of pins

-tony