Mystery ICs - AB 36D1024

tony duell ard at
Sun Jan 18 13:26:17 CST 2015

> 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


More information about the cctalk mailing list