11 Nov
2005
11 Nov
'05
8:36 p.m.
--- Douglas Taylor <dj.taylor at starpower.net> wrote:
Any tips on how to efficiently reconstruct circuit
diagrams from inspection
of the electronics? There are two cards, with
standard TTL and analog
parts. The printed circuit has a design date of
1985, while the IC's have
88-89 dates.
Power is easy, just ohm out the connector pins with
the known power input pins of the standard TTL/analog
parts, likewise with ground. The control signals are
the hard part, even drawing out the schematic by
following the pcb traces may not tell you the
information you need, you have have a bunch of input
and output gates and have no idea which goes where
unless you understand what the circuit is trying to
do.
What I have done in the past is to figure out the
power first, hook up the power and then activate the
device (whatever input controls it had) and monitor
all the control signals with a logic analyzer. If I
had a general idea what the device was suppose to do
the purpose of control signals became pretty obvious
without even looking at the circuit details (like if I
hit a reset button I would see a pulse on one control
line, that must be the reset line, or if I saw a
constant pulse train I knew it was a clock, or if I
saw one pulse then a bunch of random data on a bunch
of pins it was probably a read signal and a data bus).
Once I figured out generally what was going on I would
look at the other device and figure out that it must
have a clock input, reset input, read/write inputs and
data lines and figure out pretty quickly where it
hooks up.
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