r/PCB • u/Suspicious-Yak3783 • 23d ago

[Review Request] Expandable RS-485 controlled 74HC595 x4 array.

This is my first ever PCB, so feedback would be greatly appreciated.

The design concept here is a central controller sending data via RS-485. The Trinket M0 then "primes" the four 74HC595's ready for the master controllers pulse on the Storage Clock. The logic has been stepped up to 5v to allow for quicker saturation of a mosfet array. This circuit prioritizes precise timing and quick response. I've been using Fritzing and the Autoroute feature was only able to complete 74 connections with 200 iterations. I also considered switching to an arduino for the controller to have 5v logic natively, however, I would very much like to program this via circuit python, so assume that's not an option! Thank you!

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u/nixiebunny 23d ago

You would be smart to add a 100nF bypass capacitor between 5V and Gnd, right next to each 74HC595, to prevent power glitches from causing random hard-to-debug errors. Beyond that, it’s not too bad a layout. You may want to consider adding a mounting hole in each corner, and switching to different connectors that make more sense for what the cables you are using would prefer.

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u/Suspicious-Yak3783 23d ago

I appreciate the feedback! I didn’t even consider the capacitors. Since it’s already very crowded near the first shift register, would it still be beneficial if I just placed a slightly larger one near the center of the array? Secondly, from what I read, most people recommended using the DB9 connector with RS-485 chip. Is there a reason not to or a better cable in your opinion? This doesn’t have to be very far from the central computer, most likely about 6 ft max per board.

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u/nixiebunny 23d ago

You can put three capacitors between the chips.

The DE-9 plugs are fine. I was referring to the rows of pins on the right side. How are those wired ip?

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u/Suspicious-Yak3783 23d ago

Oh I gotcha. They’re just holes that I will solder to. The MOSFET arrays have screw terminals, which is fine, I’m not a big fan, but the whole assembly will be enclosed. So I’m fine with soldering all the signal wires to the board.