6926475826

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

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