Brad Velander said:
Mike,
I don't know about you but I can readily switch gates in a
multigate
part without introducing errors, been doing it for many years
including prior to CAD, thanks. What would you ever do without CAD? Or
how do you get your gates right in the first place, if you can't
manually swap them and bring them forward into the PCB later? What is
manual back-annotation, what is manual forward annotation? Part of
DESIGN! What is the alternative, Button Pushers? Computer operators?
Maybe you could explain what differences there would be with my
comments
you quote below, verses the original design and how it would be any
more risky than the original processes that got you to the initial PCB
design in the first place?
Brad,
Maybe you are perfect, but the people I have to hire certainly are not.
They get distracted, bored, tired, interrupted, hungry, have to go
pee, develop tunnel vision, and all the other human traits that lead to
mistakes.
The main concern in back annotation is to swap components to eliminate
noise problems or parasitic oscillation caused by long traces or bad part
placement. The parts may be different sections of an op amp, or high-speed
ecl or cmos gates. Noise-sensitive circuits, such as oscillators or low-
level amplifiers need to be kept separate from high-speed or high-power
circuits.
You can see this easily while viewing the pcb. I mark each critical node on
the schematic, then manually route the trace on the pcb. However, a better
route can often be obtained by swapping to a different part.
Swapping means back-annotating to the schematic. If this is not done
correctly, and you do the final board route without checking carefully, the
board may have problems that only show up with certain parts or operating
conditions. This can be difficult or impossible to debug when the board is
shipped. Sure, you can claim it would be caught in a design review, but
that is not certain due to the human frailities mentioned above.
Due to the complexity of the problem, I would not accept software that
required manual back-annotation. But if you can find different vendors that
can import/export the data to different formats, and not make errors in the
conversion, that would be fine.
Regards,
Mike Monett
Antiviral, Antibacterial Silver Solution:
http://silversol.freewebpage.org/index.htm
SPICE Analysis of Crystal Oscillators:
http://silversol.freewebpage.org/spice/xtal/clapp.htm
Noise-Rejecting Wideband Sampler:
http://www3.sympatico.ca/add.automation/sampler/intro.htm