RS485 Breakout with 48 V Passthrough Building Block Board — Part 7#

Now that I have tested the first batch of RS485 breakout boards from JLCPCB, I have been thinking about whether the board should remain as one combined design or be split into two separate boards.

The original idea was to make a simple RS485 breakout more useful by also allowing 48 V power to be distributed alongside the RS485 signals. On paper, that made a lot of sense.

A lot of distributed control systems need both communication and power. If a node is sitting some distance away from the controller, it is useful to have a way to get power and data to that node without needing a completely separate wiring arrangement.

However, after building and testing the first revision, I am now leaning towards separating those two concerns.

The more I look at the board, the more I think the potential to accidentally short 48 V onto the RS485 data lines, or onto the logic side of the board, is a real enough risk that it should be designed out rather than simply worked around.

That does not mean the first board was a failure. Quite the opposite. It did exactly what a prototype should do: it turned an idea into a physical object, allowed me to test the assumptions behind the design, and highlighted where the next version should be improved.

The Original Idea#

The original RS485 breakout board combined three useful features:

  1. An SP3485-based RS485 interface for 3.3 V logic systems.
  2. Screw terminal connections for the RS485 A/B pair.
  3. A 48 V passthrough section so that power could be carried along the same general wiring path.

The intention was to create a small building block board that could be used in distributed control systems, sensor networks, automation experiments, and other projects where multiple nodes might need to communicate over a reasonable distance.

RS485 is well suited to this kind of thing. It is simple, robust, and widely used in industrial and embedded systems.

The 48 V side was intended to make the board more useful in real installations, where distributing a higher voltage and regulating it locally can be more practical than trying to distribute 5 V or 3.3 V over longer cable runs.

That part of the idea is still valid.

What I am less comfortable with now is having the 48 V distribution and the RS485 communication circuitry so closely tied together on the same small board.

The Problem With Combining Power and Data#

The concern is not that the board does not work.

In testing, the 48 V side behaved largely as expected. The reverse polarity protection worked when power was applied through the intended input, and the RS485 side successfully communicated over a length of Cat 6 cable.

The concern is more about misuse, mistakes, and edge cases.

If a board has 48 V present on it, especially on screw terminals, then there is always the possibility that it could be connected incorrectly. A loose wire, an incorrect terminal position, a field wiring mistake, or someone misunderstanding the board could result in 48 V being applied somewhere it should not be.

On a prototype bench, that is annoying.

In a real system, that could destroy connected electronics.

For a board that I might publish, share, sell as a sample, or use as a reusable building block, I need to think beyond whether it works when I wire it correctly. I also need to think about how likely it is to survive when something is wired incorrectly.

That is where I am becoming less comfortable with the combined design.

The Isolation Question#

If 48 V is going to be provided alongside RS485 signals, I am increasingly of the opinion that the RS485 side should be genuinely isolated.

That would mean using an isolated RS485 transceiver, or adding isolation between the logic side and the RS485 bus. It would also mean thinking properly about isolated power, creepage, clearance, grounding, protection, and how the cable shield or reference ground is handled.

At that point, the design becomes more than a simple RS485 breakout board.

It becomes an isolated industrial communication board with power distribution.

That is a valid product idea, but it is a different class of board. It needs more care, more testing, and a more deliberate design approach.

For the current building block board series, I want each board to be useful, understandable, and easy to validate. I do not want to accidentally create a board that looks simple but carries hidden risks.

Splitting the Board in Two#

The direction I am now considering is to split the original concept into two separate boards:

  1. A dedicated RS485 communication board.
  2. A dedicated 48 V distribution board.

The RS485 board would focus purely on communication. It would expose the RS485 A/B lines, logic-level control pins, power input for the transceiver, termination options, biasing options if needed, and a sensible way to handle the RS485 reference ground.

The 48 V distribution board would focus purely on power. It could include input protection, reverse polarity protection, fusing, transient suppression, passthrough terminals, and possibly options for local conversion to lower voltages.

That feels cleaner.

It also means each board has a clearer purpose.

If someone needs RS485, they can use the RS485 board.

If someone needs 48 V distribution, they can use the 48 V distribution board.

If they need both, they can use both boards together, with the wiring and risk boundaries being much more obvious.

Why This Fits the Building Block Board Approach#

This also fits better with the overall building block board idea.

The goal of these boards is not to make one board that does everything. The goal is to create reusable, understandable modules that solve specific problems and can be combined when needed.

A combined RS485 and 48 V passthrough board is useful, but it starts to blur the boundary between communication and power distribution.

A separate RS485 board and 48 V distribution board would be more modular. It also makes the documentation easier, because each board can be explained, tested, and validated on its own terms.

For example, the RS485 board documentation can focus on:

  • RS485 A/B wiring.
  • Logic-level connections.
  • Driver enable and receiver enable behaviour.
  • Termination.
  • Bus topology.
  • Reference ground handling.
  • Basic communication testing.

The 48 V distribution board documentation can focus on:

  • Input polarity.
  • Reverse polarity protection.
  • Fuse or resettable fuse behaviour.
  • TVS diode selection.
  • Load current.
  • Cable sizing.
  • Power passthrough.
  • Safe wiring practices.

That separation makes the designs easier to reason about.

It also makes them safer to hand to someone else.

What I Would Change in the RS485 Board#

If I revise the RS485 board as a communication-only design, I would likely remove the 48 V passthrough section entirely.

That would leave a smaller and simpler board built around the RS485 transceiver.

I would still want to keep the things that worked well:

  • Screw terminals for the RS485 bus.
  • A simple logic header for connection to a microcontroller.
  • Clear labelling.
  • Optional termination.
  • A way to connect a bus reference ground when required.
  • Protection suitable for the RS485 lines.

I would also likely improve the silkscreen and labelling based on the first manufactured batch. Seeing the board in physical form always reveals things that are easy to miss on the screen.

The first revision had a few small errata, but nothing that prevented testing. A second revision would be a chance to tidy those up properly.

What I Would Put on a 48 V Distribution Board#

The separate 48 V board could be much more focused.

It could have:

  • A clearly marked 48 V input.
  • Reverse polarity protection.
  • Over-current protection.
  • Transient suppression.
  • Multiple output terminals.
  • Possibly separate fused outputs.
  • A clear indication of input and output current limits.
  • Test points for voltage checks.
  • Generous spacing and trace widths.

This board could then become a general-purpose 48 V distribution building block, rather than being tied specifically to RS485.

That may actually make it more useful.

It could be used with RS485 nodes, small automation systems, lighting controllers, low-power industrial devices, or other boards that need a distributed DC supply.

What This Prototype Has Taught Me#

This is exactly why I like building physical prototypes.

A schematic can look reasonable.

A PCB layout can look reasonable.

A KiCad render can look very convincing.

But once the assembled boards are in your hand and you begin testing them, the design becomes more real. You start thinking not just about whether the circuit works, but about how it will be wired, how it could be miswired, how someone else might interpret it, and what failure modes are acceptable.

In this case, the first board has pushed me towards a better architecture.

Not because the original board was useless, but because the physical prototype made the trade-offs clearer.

Current Thinking#

At the moment, my current thinking is:

  • Keep the first RS485 and 48 V passthrough board as a useful prototype and test platform.
  • Do not treat it as the final version of the building block board.
  • Split the design into two separate boards for the next revision.
  • Create a simpler RS485 communication board.
  • Create a separate 48 V distribution board.
  • Consider a future isolated RS485 plus power board as a more advanced design.

That last point is important.

I am not ruling out a combined RS485 and 48 V board forever. I just think that if I do combine them again, the design should probably include proper isolation and be treated as a more serious industrial-style board rather than a simple breakout.

Conclusion#

The first RS485 breakout with 48 V passthrough board has been a valuable prototype.

It allowed me to test the RS485 side, test the 48 V side, validate parts of the protection circuit, and think more seriously about how this kind of board would be used in practice.

The main conclusion is that combining communication and 48 V distribution on one small non-isolated board may not be the best direction for a general-purpose building block.

Splitting the design into a dedicated RS485 board and a dedicated 48 V distribution board feels like the cleaner and safer next step.

That is not a step backwards. It is the design becoming more mature.

Sometimes the most useful thing a prototype can tell you is not “this works”, but “this should be two separate things”.

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