CometCAD: From Symbol To PCB Layout

Priya Ravindran

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CometCAD is a tool for creating circuit schematics and printed circuit boards (PCBs). With the tiny green board inside every electronic system deciding the complete functionality of the system, creating worthy designs take priority. With this thought in mind, this month we explore designing a PCB using CometCAD.

CometCAD is built for Windows and falls under free-use licence. Design of the PCB using this software is divided into three stages: symbols, circuit and layout. Let us take a look at how each of these translate to the expected end design we are looking for. To begin with, we have a few notable features.

Look at your drawings how you want

As you work with CometCAD, you have the option to customise the settings of the screen. You might be working on a tiny-screen laptop or a wide-screen desktop, but when dealing with design, the correct zoom and optimum aspect ratio turn out to be as important as the design itself. You also have the liberty to configure the workspace for your circuit, right from the size of the sheet to spacing and visibility of grid lines.

Leave behind a footprint

A footprint is a pattern describing the arrangement of box 1 - Copypads or through-holes used to physically attach and electrically connect the components to a PCB. Effectively, it is similar to the map of the components on the board. With CometCAD, symbols can be linked to footprints, while off-page connectors can contain footprints other than for those that work as signal connectors.

The connectors then refer to the footprint for their pin configuration, which can be accessed individually, or the total number of pins can be split between sheets. Once created, symbols and footprints are stored in the library for further use. These are termed parts and can simply be added to the circuit like any ordinary symbol.

Fit in your PCB anywhere, anyhow

Often we see PCBs fitted onto other boards with nails or screws. Holes for these can be added in the design stage itself, via the tool. But the allowed size of the hole is fixed. If holes of other dimensions are needed, at design stage you can add a polygon around the region, so as not to route traces through that area. This is called a cut-out and can be milled in the final stage.

fig 1
Fig. 1: Sample of a dual-layer PCB

Architecting the board

You can choose to create a new PCB layout from scratch or update an existing PCB file, which is called forward annotation. Every PCB layout requires a circuit schematic as an input, and on importing the same, CometCAD allows you to estimate the area of the final PCB.

fig 2
Fig. 2: A PCB layout drawn with CometCAD

The layout editor allows you to edit the different layers of the PCB. The tool provides a polygon function, which can be used to set the borderline for your board and include gaps or cuts and copper planes on the PCB. These parameters will decide the fill rate area for the PCB. In other words, these will decide the area of the PCB that is available for the parts to be placed.

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