How to Address a Modbus RTU Sensor with Peakboard: A Guide from the Ground Up

Peakboard connects to Modbus devices over Modbus TCP, so a serial Modbus RTU sensor reaches it through an RTU-to-TCP gateway: wire the sensor's two data lines to the gateway, put the gateway on the same LAN as the Peakboard Box, then enter the gateway's IP address and the sensor's unit ID in Peakboard Designer. From there you pick the input registers, set a multiplier and a unit, and the reading is on screen — with no scripting involved.

What Modbus actually does

The idea behind Modbus is to put several sensors on one bus, where each sensor has its own address. Whenever the master — a PLC, or in this case Peakboard — wants a value, it sends a request packet containing the sensor's address onto the bus, and the sensor answers with its data. Schneider Electric invented the protocol in the late 1970s, and it is still in widespread use today, largely because it is simple enough to implement on almost any device.

Which register type holds your value?

Depending on the kind of answer involved, Modbus organizes the data exchange into four main structures:

• Discrete input — a read-only bit, for example a limit switch
• Coil, or discrete output — a writable bit, for example a relay
• Holding register — a writable register for command and configuration values
• Input register — a read-only register holding the sensor's actual measurement

Since we are querying sensor values, the input register is the one to use.

Modbus RTU vs. Modbus TCP and the hardware you need

Most Modbus sensors are RTU sensors that only communicate serially. That standard is called RS-485. The example here uses an XY-MD02 temperature and humidity sensor with RS-485 serial communication. Apart from the two power lines, it has exactly two communication lines.

Modbus TCP carries the identical protocol over standard Ethernet, by default on port 502. The registers, the addresses and the values are the same in both — only the transport differs.

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Why you need a Modbus RTU-to-TCP gateway

Peakboard works best when it can reach a device on a modern network instead of handling individual cables and serial communication. That is what a Modbus RTU-to-TCP gateway — a serial-to-Ethernet converter — is for: it turns the serial line into something addressable over an ordinary LAN cable. The protocol itself stays the same; the gateway is purely a physical converter. Peakboard currently connects over Modbus TCP, and this converter is what makes any Modbus sensor reachable and addressable for it. The sensor is wired to the converter, and the converter sits on the same LAN as the Peakboard Box and Peakboard Designer. One gateway serves every sensor on the same RS-485 bus, so a second or third sensor does not mean a second or third converter.

Configuring the Modbus data source in Peakboard Designer

Compared with the hardware setup just described, the software side is short. Two things go into Peakboard Designer:

• The IP address of the RTU-to-TCP converter
• The bus address of the sensor within the Modbus bus system — the unit ID. In this case: 1

You then declare that several input registers are to be read, and how many data points to expect — here two, temperature and humidity. Ideally the preview works right away and shows the live readings.

Formatting the values without a line of script

Modbus returns whole numbers, and it is up to the device how to interpret them. This sensor reports temperature in tenths of a degree and humidity in tenths of a percent, so a raw 224 means 22.4 °C. No scripting is required to correct that: the standard formatting options set the multiplier, the unit of measurement and the number of decimal places. That is the whole job.

The part no guide can do for you

As this article shows, mastering Modbus over both TCP and RTU is straightforward — as long as you know your hardware well and have a basic understanding of Modbus. What matters more is knowing how the sensor actually behaves in the context of serial communication. Unfortunately there is no way around reading the documentation for each individual sensor. There is no standard for how values are addressed and interpreted. Every manufacturer has its own implementation.

Where to go from here

Once the readings are in Peakboard, they can go on a dashboard, be combined with other sources and be written back to the device. The Modbus data connection page covers what Peakboard reads and writes over Modbus TCP. For machines that already offer a modern interface, the OPC UA data connection is usually the shorter path, and the full list of data connections shows what else can sit on the same screen. The Peakboard Designer is a free download if you want to try this setup yourself.