Symbols – The Lexicon of Schematics

The first engineering class typically starts with the idea of “abstraction”, which in itself is a pretty abstract concept. As one goes through the curriculum and starts building real systems, abstraction is probably never mentioned again but exercised all the time. For myself, the most impactful abstractions start with basic circuit elements like resistors, capacitors, inductors, etc. We rarely pause and appreciate just how elegant these all too familiar symbols are – the zigzags in a resistor , the coil turns in an inductor and the parallel plates in a capacitor. These symbols came about when someone decided to wrap Maxwell’s equations into these pictures so we don’t have to see integrals and derivatives all the time. They provided more efficient ways to communicate and think about the problem, and from there new designs were created.

Then came the active devices – vacuum tubes, diodes, transistors, etc. Their symbols continued the tradition of either resembling the physical objects or highlighting their functions/physics. Examples are the directionality in a diode, the grid and filament in a vacuum tube, the gate capacitor in a MOSFET, the direct contact between the base and the device in a BJT, the arrows showing where the current flows, and so on. These subtle details are truly artistic and there is a good reason why they survived the test of time.

When the digital era came, more symbols were needed as logic gates, latches and flip flops were introduced, but to me the symbols themselves became more “abstract”. The logic symbols’ origin was actually military (MIL-STD-806B). It was a well-documented problem, which resulted in more task forces and standards (e.g. IEEE Std 91-1984) under the Graphic Symbols Committee (yes, that was a thing) from the 1960s to 1990s. It literally required an army of people, pun intended. However, till this day I still don’t know why AND and OR gates look the way they do (maybe AND looks like a “D”?). Nevertheless, it allowed all players in the field to agree upon a “style guide” to carry out more complicated design.

Creating symbols that represent circuits well was treated as a serious and urgent problem, and somewhere along the line we all seem to have forgotten the importance of this exercise. If you do a google search on “circuit schematics”, here is one example image that could induce fear and nightmares:

Perhaps the purpose of the picture is to show the complexity of the circuit, in which case it does an excellent job, but I would avoid having a schematics like this in the database at all cost. The silver-lining in this picture is that elements are already grouped and boxed together. One only needs to take another step in creating a MEANINGFUL symbol for each section, but unfortunately most of the symbols end up looking like this

This symbol conveys little information on the block’s function. The cell name won’t help if it’s not readable when the symbol sits in a larger schematic. We have all taken the concept of “abstraction” too far in this case. Symbols are supposed to be the way we communicate with whoever reads the schematic later, not that different from caving painting actually. Imagine alien archeologists finding a schematics containing these symbols in our digital caves. I think one might understand the design for lower level schematics with primitives, transistors and logic gates, but completely loses it at mid and higher levels.

Symbols make up the lexicon of schematics. Drawing good symbols can be both fun and helpful when designing. I am sure all circuit designers are visual people and love to draw, but the tape-out treadmills may have buried our passions somehow. Let’s pay more respect to our symbols, build up our vocabularies and rediscover the most fundamental joy when drawing symbols and schematics.