30 May
2007
30 May
'07
4:51 p.m.
But unless you're good, that doesn't tell you
the
values of certain components. Bypass caps are usually
The time to prodcue the shcmatic is when the equipment is still working,
that is before components fail. Then if you have a capacitor with
undecipheralbe markings, you can measure it. (and if there are readable
markings, they'll not have bene obliterated by the component exploding ;-)
a typical value I guess, but inside a p/s...your guess
Actually, for decoupling caps the value is rarely, if ever, critical.
0.1uF seems to work
would be much better then mine...
You still haven't told us how you work up a schematic
either. I'm losing my patience.
I'll tell you the basic procedure, but to be honest, to produce an
easily-read scheamtic takes experience.
What you do is start by maing a list of all the main components on the
board, or at least he ICs. And separate secions within each IC. You might
end up with something like
U1 Z80A CPU
U2 74LS138
U3 a b c d 74LS00
U4 27128
U5 a b c 74LS10
and so on
Then start drawing. Start with the most obvious part, something that will
tie down a lot of signals. If there's a 'big chip' that's known -- like a
micrororcessor, start with that. The reason is that a microprocessor cna
be used in essentially one way -- the addres and data lines have known
functions, and so on. If you trace the data pins of a microprocessor to
one side of a 74LS245 buffer, and the other side of that buffer to a
backplane connecotr, it's a good first guess that those pins on the
backplance connector are a buffered system data bus.
As you draw each section of an IC, cross it off the list. That way you
don't have problems with doing the same section twice.
The difficult bits are (a) making sure you draw the circuit in the
clearest possible way (for example if you have 2 NAND gates cross-coupled
as an SR flip-flop, you want to show them in the conventional way, you
certainly don't want them on separate pages) amd (b) assigning senible
signal names to signals that go between pages, and indeed between parts
of th emachine. This is when experience helps a lot.
I thrace connectiuns usising the continuity test range of my multimeter.
It's not foold by diode junctions (inclduing protection diodes in ICs).
It is 'fooled' by components with low DE resistance like transformer
windings. I desolder those from the PCB before I start, Working out the
internal connections of a potted transforer (say a flyback) is 'fun'.
Often you have to guess based on the external circuit topology.
Boards iwth known large chips (micros, memory, LSI I/O devices) are the
easiest to do. Boards with just TTL (or 4000-series CMOS) are harder.
Boards with all discretes are harder still. Do not tackle an HP9100 as
your first project :-)
-tony