6 Oct
2005
6 Oct
'05
8:18 a.m.
No? OK, time for plan B, reverse engineering it.
Most of the chips on the
panel are straightforward 74xxx TTL, however, a number of them appear to
have a seven digit part number instead of a regular 74xxx stamp. Is there a
A number of manufacturers used house-coded ICs like this. HP are the
best-known (the 1820-xxxx numbers), but ICL, Xerox and I believe IBM did
it too. The main reason for this, apparently was that any IC with that
house code would work in the circuit. So there might be one number for a
generic 7493 and another for a specifically TI 7493 (I think that's the
chip were the TI one has a slight difference to others). TO have the
house code simplified manufacturing.
translation guide between this seven digit number and
regular 74xxx
numbers? Most of the 74xxx parts on the board also have these seven digit
numbers.
Well, with ICs marked with both, you can start to make the equivalents
list. If you are very lucky that will identify a few more ICs on the board.
Any tips on how to guess the actual function would be most appreciated.
Well, you can find the power connections, right? If you are lucky, there
will be a few ICs with odd power connections (i.e. not the corner pins)
which will be a start.
Then you might be abloe to identify some pins as definite inputs (driven
from totem-pole outputs on known ICs), others as
definite outputs (only
go to known inputs).
If all else fails you may have to desoler the IC, power it up on a
breadboard, find outputs with a logic probe, try the inputs, and so on.
-tony