Matthew,
I agree about 4-way connections,I have not seen anything to state that
they are illegal, Just not preferred. However in certain types of
circuits some Engineers will argue the need to have 4-ways to understand
the circuit.
But generally I would say 99% of time 4-ways should not be used. In the
analog or RF world maybe needed but in any strictly digital design &
most analog/RF there is no logical reason to use 4-way connections.
In most cases the 4-way connection can certainly be conveyed with the
4th line .10 away while still being able to see what is happening
circuit wise.
Looks like your going to have to be the traffic cop on this one!
Bob
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of
Brooks,Bill
Sent: Monday, November 19, 2007 1:09 PM
To: [log in to unmask]
Subject: Re: [DC] Current Schematic drawing standards?
Matthew,
While I am completely unfamiliar with what the UK is doing for specs, I
do
have a lot of info available on schematic rules and practices here in
the
USA. Some or all of which may be applicable to your desire for some
standardization in the drafting of schematics.
The IPC publishes a document that covers this subject... IPC-D-325.
The copy I have was published in 1995.
Section 10 covers Schematics and Logic Diagrams. Here's a brief
synopsis:
Formats should be as small as they can and still comfortably fit the
data to
be drawn... where there are multiple sheets all of them should be the
same
size...
Abbreviations according to MIL-STD-12
Schematics should be read from left to right, and or top to bottom.
Terminations, connections, etc. should be located on the outer edges of
the
drawing...
Connecting lines shall be drawn with as few crossovers and direction
changes
as possible, min spacing between lines at 1:1 scale is .25 inches
(that's
pretty big actually...)
4 way connection junctions are acceptable but 'not preferred'... (but I
say
outlaw them... )
They recommend showing pin numbers for things like resistors and caps
where
most of us do not... the rational explanation given says it helps with
CAD
data.
It basically mirrors what has been around for years and years that the
industry has used for guidance in creating schematic diagrams...
These specs for drafting schematic diagrams derived from MIL-STD-100 are
in
the standard drafting manual published by Global Engineering entitled
"Drawing Requirements Manual" in section 22-4 Electrical and Electronic
schematic diagrams.
It basically says the following:
1) Graphic symbols must comply with ANSI Y32.2/IEEE 315
2) The position of the symbol does not alter its meaning (remember the
ground symbol ?)
3) Symbols may be drawn to any proportionate size or line thickness to
suit
reproduction (and thus readability) of the document
4) Electrically operated Symbols shall be shown in the 'power off' or
non-energized state or you will provide a note indicating the otherwise.
5)The terminal symbol (o) may be attached to any wire connection on the
symbol..(I think they are referring to the inversion of the state of a
digital signal as in the case of an inverted input or output on a logic
gate
for example...)
6) Symbols that show a relationship between parts - in the case of
relays
for example, the separate parts of the relay can be shown on the
schematic
in different locations as long as the reference designations tie them
together... i.e. U1-A, U1-B are two parts of a single package and can be
drawing in different sections of the schematic...
7) Switches are shown in a position with no operating force applied
8) Symbols are drawn in a heavier line weight that connecting lines
9) Reference Designations shall comply with IEEE STD 200 (ANSI Y32.16)
10) Complete reference designation - if you had a cap on a board in a
unit
that was part of a rack assembly its complete reference designation
would
indicate its type of part, and position in its membership to the entire
assembly... for example C4 on your board would really have a complete
ref
des of U2-A1-C3 or equivalent. Typically we don't show those complete
ref
des on the schematic or the parts list but in a more formal
documentation
system that might be required and we are supposed to know that C3 on one
board is not the same as C3 on another board in an electronic assembly
unit... it actually has an address like your home number... it's a
unique
location with its 'zip code'... even if there are two streets with the
same
name, the post office finds your house don't they... same principle
really.
C3 identifies a capacitor on board A4, in housing A1, in rack U6 so the
complete designation is U6A1A4C3. All this stuff is explained in ANSI
Y32.16/IEEE STD 200
11) So... the abbreviated ref des C3 is a 'Partial Reference
Designation' by
definition (see above)
12) Reference Designation is NOT and abbreviation for the name of an
item.
13) Reference Designations shall be assigned in a consecutive manner
starting with the number 1 and proceed to be assigned in a left to
right,
top to bottom sequenced manner.
14) Additional information can be added to the symbols like type
designation, value, watt rating, tolerances, etc... as needed.
15) Numerical values are expressed as whole numbers and decimals.
Use .092uF not 92000pF
Use 5mH not 0.005H
Use 2.5K not 2500 OHMS
16) Don't use the comma in numerical values.
17) Repetition of values should be eliminated through the use of notes:
Unless otherwise specified:
1) Capacitance values are in Microfarads, 10%
2) Resistance Values are in K Ohms, 5%, 1/4 watt
18) Schematics should omit equipment lists... and if used for reference
only, they should not be co-mingled or in the same area as the parts
list to
avoid confusion.
Beyond that... the user can use his/her imagination to make the
schematic
understandable... notes are good to clarify anything non-standard... and
if
there is no standard symbol for your part, use a box with pins going in
and
out of it, inputs on the left, outputs on the right...
One other thing I do is give a general direction to power and ground
symbols, power up, ground down... there are exceptions to that based
upon
special circuit diagram reading practices, symmetrical circuits might
show
the grounds in an upward direction if they have 'counter parts' that are
on
the opposite side of a symmetrical RF circuit diagram going down... for
example...
Well that's more than I planned to write, I guess this turned into
Schematics 101... sorry about that.
If you want to dig deeper, there are some 'out of print' books on the
subject that I like... you might find used copies of them from Barnes
and
Nobel or Amazon.com...
Electronics Drafting and Printed Circuit Design by James M. Kirkpatrick
Drafting for Electronics by Lamit/LLoyd
Electronics Drafting and Design by Nicholas M. Raskhodoff
The Design and Drafting of Printed Circuits by Darryl Lindsey
Best regards,
Bill Brooks
PCB Design Engineer, C.I.D.+
DATRON World Communications, Inc.
"Performance you require - Value you expect"
http://www.dtwc.com
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
-----Original Message-----
From: Matthew Lamkin [mailto:[log in to unmask]]
Sent: Monday, November 19, 2007 3:24 AM
To: [log in to unmask]
Subject: [DC] Current Schematic drawing standards?
Hello, can anyone (preferably from the UK) point to what current
schematic
drawing standards there are?
I'm not talking about what symbols should look like as I have EN60617-2
for
that, but more
for how to present a drawing I.E. not to have more than 3 lines into a
junction dot.
Or does a standard not exist for this?
Basically I'm getting completely peed off with the crappy drawings that
engineers are sending me
with what I see as many drawing faults in that are not to what I'd call
standard ways of doing things.
And being engineers they wont conform to damn all unless I can show them
a
written standard that
they should conform to.
Grrrr...
Thank you,
Matthew Lamkin.
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