Careful imprinting scheme

Article By : Paul Rako

Schematic symbol preferences are like musical preferences; they are very personal. It is part of your style as an engineer.

« Previously: Do’s and don’ts in schematic symbol creation

There was some great discussion about drawing schematic symbols. Sometimes, the pre-packaged symbols in you CAD package will work, most times they won’t.

Those heterogeneous parts invented in Cadence OrCAD decades ago can be used to break connectors into different chunks as well. This too will preserve the flow of the schematic, while still making sure every wire goes to the right connector (Figure 1). Now you can ensure your schematics flow from left to right and it will make it much easier for other engineers to understand them. It will also be easier for you to understand them when you look at them five years later.

[EDN schematic fig6 (cr)]
Figure 1: If you let a connector symbol stay one part it makes the schematic very confusing (a). By using the heterogeneous part function in OrCAD, or the part “modes” in Altium/CircuitStudio, you can break the connector up so that the schematic flows better and is much easier to understand at a glance (b).

Another consideration is the clarity of complicated parts, like a switching power chip. Even when you move the inputs to the left side and outputs to the right, it still is pretty mysterious as to how the part works. For that you can draw a simple diagram inside the symbol outline to give a hint as to what the part does. It does not have to be the block diagram from the datasheet, but a simple representation to remind you and others what the part does.

There are other schematic symbol conventions that are more preferences, rather than good design practice. I have always liked transistors enclosed in a circle. Once again, it was those semiconductor folks that had to draw hundreds of transistors that dropped the circle. I think the circle has dignity. Similarly, I have always loved to have little jumps when a wire crosses another. This brings up another point: no four-way ties. I have worked on a FAXed schematic and sure enough, it was impossible to see if the wires were just crossing, rather than tied together. I guessed wrong and it cost me a day. If all schematics had jumps, the “no four-way ties” rule would not be as important. To my delight, the recent versions of Altium/CircuitStudio can show jumps as well as suppress four-way ties (Figure 2).

[EDN schematic fig7 (cr)]
Figure 2: An old-timer like me loves to show wires jumping over other wires where there is no connection. Note that four-way ties are a schematic taboo. Altium/CircuitStudio has preferences to create wire jumps and also to eliminate cross-ties by kicking the wires offset, as at the GND connection for this chip. Note the library part has outputs on the left and inputs on the right, the exact opposite of what you want.

My preference is to re-draw the part symbol, using the inputs on the left convention (Figure 3). I also used separate power and ground symbols, which reduces the tangle effect. After all it’s the signal flow we care about.

[EDN schematic fig8 (cr)]
Figure 3: Modify the 555 timer of Figure 2 to put inputs on the left and outputs on the right. The schematic flows better. Separate power and ground symbols eliminate the wire tangles.

Most engineers understand what is inside a 555 timer IC. But if you don’t, or you think the people reading the schematic won’t, then you can draw some or all of the block diagram inside the part. Altium/CircuitStudio will let you put an image into a schematic symbol, so I found a nice 555 timer block diagram online and after a little trimming and stretching, I plopped it into the schematic symbol. I had to adhere to their pin-out, so now the schematic has some jumps in it (Figure 4).

[EDN schematic fig9 (cr)]
Figure 4: You can draw a block diagram inside the part to show its function. This might be as simple as showing an open-collector output, or more involved, like showing functions inside a switching power chip. Some CAD packages let you paste an image inside the part.

This brings up a good point. You could honour the block structure of what’s inside the part at the expense of the overall schematic, or worry less about what is inside the part and keep a clean simple schematic. My thinking is to try and draw the pertinent things inside the part, like an open-collector output. But what is important is to keep the overall schematic untangled and understandable.

OK, a last silly analog guy preference. Back in college, John Kuras used to joke that a power transistor should be drawn bigger with thicker lines. We all laughed back then but now I do like to show that a transistor is a giant TO-3 monster by using a bigger symbol (Figure 5). Being analog is accepting a sense of importance, and that bigger transistor is more important, and it’s nice to draw it that way.

[EDN schematic fig10 (cr)]
Figure 5: Everyone can see the transistor on the right is a power transistor.

Schematic symbol preferences are like musical preferences; they are very personal. It is part of your style as an engineer. Some things like wire jumps and circles around transistors are less important. Things like inputs on the right and top, and outputs on the right and bottom are more important. We all get to argue how to handle buses that are both inputs and outputs. I consider the ground symbols important. There is an app note on the web that if you connect it up to earth ground as the symbol suggests, it will blow up the diodes.

This article was first published on EDN.

« Previously: Do’s and don’ts in schematic symbol creation

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