Isolated RS485 Building Block Board

Isolated RS485 Building Block Board#

Today I worked on the next version of the RS485 building block board, this time moving from the simpler non-isolated version to an isolated RS485 design.

The non-isolated RS485 board is useful for short runs, bench testing, cabinet wiring, and situations where all of the connected equipment shares a sensible ground reference. It is a good practical board, and it has already been useful in testing with the Raspberry Pi Pico boards, the Digital I/O Expander board, and the EC11 rotary encoder setup.

However, RS485 often ends up being used in less tidy environments than that.

Once cable runs get longer, equipment is powered from different supplies, or boards are installed in different parts of a system, isolation starts to make a lot more sense. The aim with this board is to keep the logic side and the RS485 cable side separated, while still keeping the same general building block board philosophy: simple connectors, clear labelling, configurable options, and practical protection.

What Changed#

The main change is the addition of isolation between the controller side and the RS485 bus side.

The board now has:

• A logic-side interface for the host controller
• An isolated RS485 transceiver
• An isolated DC-DC converter for the bus side
• Separate logic ground and isolated ground areas
• Input-side protection using a PTC and clamp diode
• Configurable termination
• Configurable bias resistors
• Screw terminals for the isolated RS485 bus connections
• Status LEDs for basic power and signal visibility

The input side is marked clearly as requiring a 3.3 V supply with enough available current for the isolated power stage. That is important, because this board is not just powering a small transceiver directly from the host logic supply. It also needs to run the isolated DC-DC converter that powers the isolated side of the interface.

Input Protection#

Because this board includes an isolated DC-DC converter, the 3.3 V input side needs a bit more care than a very simple logic breakout board.

The input supply is protected using a resettable PTC fuse and a clamp diode. The intention is not to make the board indestructible, but to give it a better chance of surviving common mistakes and fault conditions.

The 500 mA hold-current PTC helps limit current if something downstream goes wrong, or if the input protection clamp has to conduct. The clamp diode then helps prevent the input rail from rising too far above the expected 3.3 V level.

This protection is mainly for the logic-side supply input. It does not replace proper RS485 bus-side protection, correct wiring, or sensible grounding. It is simply a practical layer of protection for the board’s own power input.

Isolation Is Not Magic#

One thing I wanted to be careful about with this design is not treating isolation as a magic fix for every problem.

Isolation helps break direct ground connections between the controller side and the RS485 cable side. That is useful when different parts of a system may sit at slightly different ground potentials, or when I want to reduce the chance of noise and fault currents travelling directly back into the controller.

The input protection helps protect the board’s local supply side, while the isolation helps separate the controller side from the RS485 cable side. They solve different problems, and both are useful.

But isolation does not remove the need for sensible layout, protection, grounding decisions, and cable-side design.

The isolated side still needs a reference point. The RS485 lines still need appropriate protection. The board still needs enough spacing between the isolated and non-isolated sections. The termination and biasing still need to be configured correctly for the bus.

So the design is intended to be more robust and flexible than the non-isolated board, but it still needs to be used properly.

Indoor Operation#

For this revision I am mainly thinking about indoor and protected-environment use.

That means bench setups, cabinets, workshops, test rigs, control panels, equipment enclosures, and short-to-medium cable runs inside a building. It is not intended to be treated as a bare outdoor board, and it is not designed as a weatherproof or fully industrialised field device on its own.

The isolation makes the board more suitable for real wiring than the non-isolated version, but it does not automatically make it suitable for rain, condensation, direct outdoor cable runs, lightning exposure, or harsh electrical environments.

For outdoor use, this board would need to be part of a bigger system design. That would likely mean a suitable enclosure, cable glands, environmental sealing, proper earthing decisions, additional surge protection, and a much more careful look at how the cable enters and leaves the equipment.

So the practical target for this board is:

• Indoor systems
• Protected enclosures
• Workshop and lab equipment
• Prototype control systems
• Building-scale wiring where the environment is controlled
• Installations where isolation is useful, but full outdoor hardening is not required

That feels like the right scope for this building block. It is more capable than a simple non-isolated breakout, but it is still a small prototype-friendly board rather than a sealed industrial field module.

Configurable Termination and Biasing#

As with the non-isolated RS485 board, I wanted the termination and biasing to be configurable rather than permanently fixed.

The board includes jumper-selectable options for the termination resistor and the A/B bias network. This allows the board to be used in different positions on an RS485 bus without needing to modify the PCB.

For example, a board at the end of a bus may need termination fitted, while a board in the middle of a longer bus generally should not. Biasing also needs to be considered at the system level rather than blindly enabled on every node.

This is one of those areas where making the board slightly more configurable avoids building in assumptions that are only correct for one specific installation.

Logic Side and Bus Side#

The logic side of the board is intended to connect to a microcontroller or host board using simple digital signals:

• RO
• DI
• DE
• RE
• 3.3 V
• GND

The bus side then exposes the isolated RS485 connections using screw terminals.

Keeping these two sides visually and electrically distinct is the whole point of the board. The controller side belongs to the local embedded system. The isolated side belongs to the RS485 cable and whatever is connected to the far end of it.

That separation should make the board more useful for real installations than the earlier non-isolated design.

Why This Board Matters#

This board fits nicely into the wider building block board range.

The non-isolated RS485 board is still useful. It is simpler, cheaper, and perfectly fine for many controlled situations. But the isolated version gives me a better option for projects where the wiring leaves the immediate bench or enclosure.

I can see this being useful for:

• Remote sensor nodes
• Small industrial-style control systems
• Multi-board test setups
• Longer cable runs
• Mixed-power systems
• Prototype equipment where I do not fully trust the grounding arrangement yet

It also gives me another useful board for demonstrating the difference between a quick prototype interface and something that is more suitable for deployment in the real world.

The important thing is scope. This board is a more robust indoor/protected-environment RS485 building block. It is not trying to be every possible RS485 interface at once.

Current Status#

At this stage the board is designed and rendered, and the layout is looking much closer to something I would be happy sending for manufacture.

There are still details to double-check, especially around isolation spacing, input protection behaviour, RS485-side protection, jumper labelling, and the split between the logic-side and isolated-side ground areas. These are the kinds of details that matter more on this board than on a simple non-isolated breakout.

But overall, this feels like the right next step.

The original RS485 board proved the basic communication side. This isolated version takes the same idea and makes it more appropriate for messier, more realistic indoor environments.

Assuming the final checks go well, this will probably be one of the next boards I send off for fabrication.