RS485 Non-Isolated Building Block Board

RS485 Non-Isolated Building Block Board#

I have now created the non-isolated version of my RS485 building block board.

This board is based on the earlier RS485 + 48 V passthrough building block board, but with the 48 V section removed. After testing and thinking through that first version, I decided that separating the RS485 communications side from the 48 V distribution side made more sense. It reduces the chance of accidentally bringing higher voltages too close to the logic and data lines, and it makes the RS485 board more focused and easier to reuse.

The process of creating this version was fairly simple. I removed the 48 V section of the original board, tightened the layout, and moved many of the supporting components down from 0805 and larger packages to 0603 and 0402 parts.

I also removed some of the labels for the passive components. Now that I am more confident in the design, I do not need quite as many visual reminders on the board itself. I have kept the labelling for the jumpers though, as those are user-configurable. That should save a bit of head scratching, both for me and for anyone else using the board.

Now that I am more confident with this part of the design, I am also less concerned about needing to swap components during testing or rework. Larger parts are definitely easier to handle during early prototyping, but once a circuit has been through an initial validation pass, using smaller parts makes sense where appropriate. It helps reduce board size and makes the design feel closer to something that could be produced in small batches.

One change I made was to remove the 33 ohm resistors that were originally connected in series with the A and B lines. They were included as a possible way to reduce ringing, but after thinking it through again, they are probably not needed on this board. With a proper termination resistor in place, adding extra series resistance could weaken the differential signal rather than improve it.

I did keep the 120 ohm terminating resistor, and changed it to a 1206 package. The RS485 bus can operate over a relatively wide common-mode voltage range, commonly around -7 V to +12 V, so I wanted the package choice for the termination resistor to be a little more conservative.

I also reduced the pitch of the screw terminals from 5.08 mm to 2.54 mm. Since this version of the board is no longer carrying the 48 V rail, the larger terminal blocks are not really necessary. The 2.54 mm screw terminals I selected are still rated well above what this board needs, so in practice they provide plenty of headroom for this design.

That is one of those small lessons that only becomes obvious after building a few boards. I could probably have saved space on the original RS485 + 48 V board by using smaller terminals from the start, although the larger terminals did make sense at the time because I was still thinking of the board as a combined communications and power distribution module.

I still wanted to use screw terminals on this board though. They are practical, robust enough for prototyping, and easy to disconnect and reconnect during testing. For a reusable building block board, that matters.

The plan is to get a few of these boards manufactured alongside some of the other building block boards I am currently designing. I have quite a healthy pipeline of possible boards at the moment, and grouping a few designs together should help reduce the impact of shipping costs, which are not trivial these days.

This board is not trying to be the final answer for every RS485 use case. It is a practical, non-isolated RS485 module that I can use in prototypes, test setups, and small internal projects. For applications where isolation is required, I will likely create a separate isolated RS485 version later.

For now, this gives me a smaller and cleaner RS485 building block that builds directly on the lessons from the earlier combined RS485 and 48 V board.

Editor’s Note#

After publishing this article, I made a small design change and removed the 33 ohm series resistors from the A and B lines. The 120 ohm termination resistor remains in place.

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