RJ45 Connector Board for Modbus RS-485

In this post, we want to show how we used a RJ45 connector block for multi-point serial Modbus connections.

Modbus RS-485 is often used in industry for data connections with industrial controls and instrumentation, such as VFD’s, flow meters, energy meters, etc.

There are several ways of distributing the RS-485 wiring, and some OEM’s provide connection methods, but often at a high cost.

Many of the VFD’s and controls that we use have an RJ45 connector for the Modbus connection, so it makes sense to use an RJ45 distribution method, using RJ45 connectors, and shielded CAT5 cable.

After a few years of thought and trial and error (we are slow thinkers!!) …..We have figured out how to use a low cost RJ45 connector block for this purpose. It looks like this.


Modbus RS485 RJ45 Connector Block

Modbus RS485 RJ45 Connector Block


This is a nicely made RJ45 connector block, with shielded connectors, and mounts on din rail. It can be purchased from several online marketplaces for well under $50

Now that we have located a RJ45 connector block, the next step is to figure out how to connect it to our controls.

On many of the websites that sell this item, it will have a picture of a schematic, such as this one.


Modbus RS485 RJ45 Schematic

Modbus RS485 RJ45 Schematic


This schematic picture shows that all of the connector pins are wired in parallel. So, in theory, you could use pins 1, 2, and 3 for a typical three wire Modbus connection.

However, after testing, this does not work. Here is why. Take a look at the following picture. This is a visualization of the circuit board that the RJ45 connectors are soldered onto.

You will notice that the flow of the pins is alternate, in other words, pins 1, 3, 5, and 7 flow one direction around the board, and pins 2, 4, 6 and 8 flow the other direction around the board. This creates a signal flow mismatch for serial wiring. This does not work.


Modbus RJ45 block wiring flow

Modbus RJ45 block wiring flow


 

The solution, is to use pins that have wiring that flows the same direction. So, we use:

Pin 1 for Modbus Signal +

Pin 3 for Modbus Signal –

Pin 5 for Signal Ground

After testing this configuration, it works perfectly, there is no problem resulting from the opposite flow directions.

We hope this little visual tutorial will help others who have a faced problems using these connector blocks with Modbus RS485 wiring.

Following is a picture of an installation using this method.


Modbus RS485 RJ45 VFD Connections

Modbus RS485 RJ45 VFD Connections

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