co1compmg- Schematic Symbols for Multi-gate Devices

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Schematic symbols for multi-gate devices

For a simple device like a resistor, there's nothing clever about its schematic symbol.

For multi-gate devices, like, for instance, a 7400 TTL quad NAND, the situation becomes more complex. Complex for good reasons, but complex.

A 7400 consists of four "copies" of a NAND gate. The schematic might be something like….

However, it would be a nuisance if that's what we got when we tried to put a "7400" device on a schematic.

Instead, we get just one of the NAND gates, we get a "part" of the device. Don't let sloppy thinking get the better of you… one of those NAND symbols is not "the device"… it is just a part of one. A useful part, and one that you will work with, in some respects, as if it were a discrete entity. But it isn't. It is just one of the gates in a multi-part device. What I've called a "part" is called a "unit" in KiCad.

If we wish to, we can subsequently place the 2nd, 3rd, and 4th units of the device on our schematic, too. (Some or all of them.) Or, we can elect to add second 7400 schematic symbol on our drawing, which will, in due course, result in two chips being needed on the PCB. (We would have to do this if we needed 5 or more NAND gates.)

If, however, we only need, say, two NAND gates, it would be silly to put two chips on the PCB. We would use just one. On our schematic, there would be two NAND gates drawn. They would both have the same value in their "reference" property, but if you looked closely, you would find in the upper left hand corner of each gate's properties, a listbox called "unit". By setting the first NANDs "unit" property to, say, "1", and the second's to "2", you would have set things up to use different "parts" which are both within one physical entity in the real world.

Confusing? I hope not.

The drawing will appear to have two NANDs on it. What it has is two parts, or units from a single schematic symbol. (And two more parts remain unused in this case, and do not show on the schematic drawing… but they will be present on the PCB, even if unused.)

One of the reasons this is a Good Thing, even if perhaps a little tedious for the beginner, is that you can place the separate parts whereever they make sense on the schematic. They do not have to be all together, even though that's exactly what they are on the PCB.

You assign whatever you wish, "U1" might be suitable, to the schematic symbol's "reference" property. The reference shown on the different parts (aka "units") will be "U1A", "U1B", etc, if you have set the device's reference property to U1.

Is this confusing? I suspect I haven't explained all of that brilliantly. For the four gates of the quad NAND, there is actually just ONE schematic symbol. But it is a CLEVER schematic symbol, and it looks like 1, 2, 3 or 4 "symbols" in the diagram, depending on how many of its units you use.

And another thing…

Something like a 7400 needs, 4 times over, pins for 2 inputs and 1 output. It ALSO has two more pins: One for the chip's Vcc, one for the GND connection. Not 4 Vcc pins/ 4 GND pins. KiCad can handle it! There's provision to hide or not hide these "invisible" pins. They are the sort of thing that "everyone knows about". Well. Maybe every expert!

More help

A more extensive essay also exists here at KiCadHowTo that goes into working with multi-gated devices.

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