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The Hewlett-Packard HP 9000 series 300 Workstation, being used for schematic Circuit capture. (Courtesy Hewlett-Packard Australia)

drawing cxtra emphasis, by giving them    out a suitable pattem for itself, using    corner” with nowhere left to put some

a heavier linę weight (e g., transistor    your Circuit schematic (which you must * remaining connection linęs. This can

and IC symbol outlines).    obviously have fed in, first!). This is    happen with almost any of the CAD

Many of these criticisms are oyer-    generally termed aulo-routing. Needless    packages, including the very expensive

come by the newer and morę elaborate    to say, you generally also have to give it    ones.

packages, which give you a wider choice    a bit of guidance, by    telling    it things    In this kind of situation you    generally

of icons in their “library”. In some    like the size and shape    of the    PCB you    have to look at the attempt    it madę,

cases they also allow you to customise    want — and often where you’d like    to    and see where it struck problems in the

the library icons yourself, so if you want    put the various IC packages.    way of connection linę “bottlenecks”.

a stronger outline around your IC sym-    Even when you give it this basie in-    This will usually give you a clue    as to

bols, you can give them one. Similarly if    formation and tell it to get cracking, a    the kind of changes in position    (and

you find the resistor zig-zag icons too    typical CAD package running on a per-    perhaps orientation) of some of    your

“stretchcd out”, you can make your    sonal Computer can take literally hours    main IC packages, that sęem likely to

own one with tighter spacing.    to produce a trial PCB routing pattem,    ease the problem. Then you need to

Notę that even if the supplier of the    particularly if the PCB    is a fairly com-    make these changes and tell    it to “try

CAD system software doesn’t supply a    plex one with quite a few ICs    and a lot    again”. Sometimes it can be    necessary

variety of different sets of symbols,    of interconncctions. Even an AT-level    to make these kinds of changes a    num-

you’ll often find that other companies    machinę fitted with a numeric co-    ber of times, until a satisfactory    PCB

do supply altemative symbol libraries.    processor chip can take quite a while,    routing solution can be achieved.

For example the firm Edutech Produc- while a minicomputer might not be Of course designing a PCB isn’t just a tions in Melbourne (see product section    much faster.    matter of fitting all of the ICs and other

following this article) can supply icon li-    Still, you have to compare these times    devices into the required number of

braries for the well-known package Au-    with the time you’d take yourself to do    square millimetres, together with all of

toCAD, designed to tailor it for a vari-    the same job. There’s an awful lot of    the necessary interconnections. There

ety of different applications such as ih-    work in designing a complex PCB pat-    are other rcquircments, necessary so the

dustrial electronics, electrical contract-    tem — referring to the Circuit, looking    resulting Circuit will perform correctly

ing, hydraulics and pneumatics.    up the connections for ICs and other    from an electronic point of view. Criti-

Of course so far weTe talking about    devices, working out how to route each    cal signal lines must be    kept as short

what is still essentially manuał drafting:    connection from A to B without cross-    and direct as possible, inputs must be

for example with a PCB pattem, youTe    ing any others (or hopefully requiring    routed away from high-level signal lines

still deciding on where the IC pad sym-    any links), and so on. With a complex    so there will be minimum coupling, sup-

bols go, how theyTe connected up and    board, this can all take days, even    ply lines must have as Iow an imped-

where the connecting lines will run. The    weeks by the manuał method — so the    ance as possible for minimum noise,

CAD system is still morę or less acting    fact that a CAD program may take    and so on.

as a fancy drafting machinę. The next hours is neither surprising nor unreason- Generally speaking, CAD packages step is to use it to help you morę in the    able. Generally youTe still going    to    be    still aren*t capable of helping much in

actual design work itself.    well ahead!    these respects (at least, as far as the

Now we’re getting into the morę gen-    Mind you, with comp!ex PCBs, the    packages available for PCs are con-

eral CAD area, of course. Staying with    CAD program can still reach a    point    cerned). They can do all the hack work

the design of a PCB pattern, you can    where it is literally unable to complete    of producing a solution    to the basie

get the CAD package to actually work    the pattern, having “routed itself into a    routing problem, but it’s    still up to the

. ELECTRONICS Australia. September 1987    99