SmartParallel: Centronics breakout board

Creating a full breadboard version of the SmartParallel board is turning out not to be a five-minute affair. It has to be done meticulously and methodically if it is to have any value as a prototype.

Fitting it all on to a single breadboard is proving challenging too. And that got me thinking.

What about replicating at least part of it on a PCB?

One of the more difficult parts of the breadboard build is wiring up the DB25 socket. There are eight data lines, nine signal wires and eight ground connections. It would be a lot easier to have a DB25 breakout board to handle that.

It’s easy enough to buy simple breakout boards on Ebay – in fact, I’ll probably do that anyway. But with some unexpected time on my hands, I decided to make one of my own.

It’s a lot simpler than the SmartParallel board. There’s no microprocessor here – all the signals need to be supplied or managed by the host system (or the breadboard circuit in my case). It’s not quite a straight breakout from the DB25 socket: I’ve thrown in a shift register, doing much the same job as on the SmartParallel board – that is, using just three signals to create the eight data lines. Otherwise, though, everything goes straight through.

All the lines that have pullup resistors (and one pulldown) on SmartParallel also have them here. Of course, it’s possible to leave off the resistors and handle the pulling up on the breadboard, which is what I’ll do at first.

Also missing are buffering and blinkenlights (sad).

I’ve used 1812 size SMD resistors because I happen to have a bunch hanging around, and surprisingly few of my preferred size of 1206.

What’s the point?

This was a very spur of the moment thing. And the value having this board is yet to be confirmed. However – and this is really the point of this post – my feeling was, why not do it?

I’m by no means expert with KiCad. But I feel the improvements in version 5, while not bringing it fully into the 21st Century in terms of UI and UX, have certainly reduced the reluctance I felt about using it. In other words, I’m more inclined now to hack something out in KiCad rather than cobbling together something on stripboard. (Not that the DB25 socket will work with stripboard anyway because of the pin pitch.)

Then there’s the low cost of PCB fabrication these days. I’m using JLCPCB (and no – unlike half the world’s YouTubers, I’m not sponsored by them). The cost of having five PCBs fabbed, including postage, was just $7.24. That’s less than I’ve paid for a single IC on occasions. True, there’s the two- to three-week wait for the boards to arrive from China, but I’m getting used to that now.

I plan to put the KiCad files on GitHub and will update the post when that’s happened.

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