18 Jun
2011
18 Jun
'11
12:26 p.m.
On 18 Jun 2011 at 11:55, Brent Hilpert wrote:
I have a schematic from the 60's that used, in
essence, 4 types of
gate symbols: one each for +/- AND/OR, but you still have to scratch
your head some when it comes to interpreting signal levels at the
different types of gates.
Well, as I said, it's mostly a state of mind. I prefer a gate used
as an "AND" to be represented as such, regardless of the logic
polarity (a better term?). In fact, it's too bad that we can't use
colors to say "this stuff is negative polarity logic" and get rid of
the gratuitous bubbles. Like musical notation, there can be
problems in getting an idea across.
One thing that still throws me is the use of IEC symbols in
schematics. I really have to stop and pause over each symbol as I
read a schematic. Granted, the IEC symbols can represent logic that
would otherwise have to be drawn as a box with a number in the
distinctive-shape style (e.g. priority encoder, 3-of-5 gate, etc.),
but that doesn't make it any easier to parse.
However, I increasingly run across schematics where a number of
packages are drawn with tags on almost all leads. Somehow, to get
an idea of the logic flow, you're expected to map out the connections
in your head, or use colored ink to connect the points until a real
schematic emerges. About half of the time, I simply give up and draw
my own schematic.
Inevitably, these are the result of some CAD schematic-with-PCB EDA
tool. Curiously, I've never seen an EDA tool to convert this stuff
into more conventionally human-readable schematic form.
Maybe they're not intended to be read by people, but merely serve as
a reference from which to construct netlists, but I'd still like to
see the guy who figured it was a good thing get his come-uppance.
--Chuck