In my previous blog — Kan Klive’s Karnaugh Maps Be Korrect? — I mentioned that starting sometime in 2021, I’m going to be writing a series of columns for Practical Electronics magazine in the UK teaching digital logic fundamentals to absolute beginners. (As an aside, I meant that title as a joke, but you wouldn’t believe how many emails I’ve subsequently received addressing me as “Klive” — maybe the joke’s on me LOL.)
I also noted that this is going to have a hands-on component with an accompanying circuit board. We’re going to commence by constructing some simple logic gates at the transistor level, then use primitive logic gates in 7400-series ICs to construct more sophisticated functions, starting with combinatorial and working our way up to a sequential edge-triggered D-type flip-flop, all implemented out of AND, NAND, OR, NOR, and NOT gates. From there we’re going to… but once again, I don’t want to give too much away.
The reason for my wafflings here is that I have a question. We’re going to group the gates together as they would be presented in the 7400-series devices; that is, groups of six inverters, four 2-input ANDs, and so forth (I’m not talking about the pin layout, just the gathering of the functions).
Each of these primitive gates will have two LEDs — one red, one green — associated with each of its outputs. Initially, we (yours truly and the publisher of PE, Matt Pulzer) were tempted to associate the green LED with 0V (ground, grass) and the red LED with 5V (danger, power). However, we’ve come to the conclusion that it will be better to associate the red LED with 0V (off, stop) and the green LED with 5V (on, go).
The image below shows a small prototype board containing a single hex inverter. We are using this as a test case to decide the best way to annotate the device names, signal names, pin numbers, and suchlike.
Observe the way in which the two LEDs associated with each gate output are centered on the signal line. This was the original layout Matt came up with. To be honest, I wouldn’t have thought anything more about this until he came up with an alternative possibility as shown below.
As you can see (especially if you click the image to make it larger), in this new version Matt has raised the LED pairs above their associated signal lines. Furthermore, he’s modified the signal lines so that they appear to form the shape of a positive-going pulse around the LEDs.
Matt’s idea is that when the output of the gate transitions (rises) from 0 (red) to 1 (green), the light will appear to move up. Similarly, when the output of the gate transitions (falls) from 1 (green) to 0 (red), the light will appear to fall down.
I must admit that I rather like this scheme. It’s a little different from anything I’ve seen before, and I think it adds a little je ne sais pas to the proceedings. But I suggested to Matt that it might be a good idea to post this blog to ask what your feelings were about all of this. As always, I (we) welcome your comments, questions, and suggestions.