Mystery ICs - AB 36D1024
ard at p850ug1.demon.co.uk
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
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
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