Read an SDM630 over Modbus RTU and provide the values over Modbus TCP

Scenario

An Eastron SDM630-Modbus V2 energy meter is connected to IOZER basic through RS485. IOZER periodically reads the measurements as a Modbus RTU master and provides them on an IP network as a Modbus TCP slave.

A Modbus TCP client can therefore access the energy meter measurements without being connected directly to the RS485 bus.

In current Modbus terminology, the roles “master” and “slave” correspond to “client” and “server.” This article uses the terms shown in the IOZER user interface.

Goal

After completing the configuration:

• IOZER basic reads twelve electrical measurements from the SDM630,
• the datapoint overview shows current and valid values,
• IOZER provides the values in holding registers over Modbus TCP, and
• a computer on the network can read the values using mbclient.

The register addresses on the TCP side can be selected freely. This example retains the zero-based start addresses of the SDM630. The measurements move from the SDM630 input register area to the IOZER holding register area. This simplifies commissioning and subsequent mapping.

Hardware used

IOZER components

• IOZER basic (IOZ-B-10)
• Suitable 7–30 V DC SELV/PELV power supply

Additional components

• Eastron SDM630-Modbus V2
• Twisted bus cable suitable for RS485
• Ethernet or Wi-Fi network
• Computer with mbclient from ModbusTools

Requirements

• IOZER basic is installed according to the safety information and connected to a suitable power supply.
• IOZER can be reached over Ethernet or Wi-Fi and has a known IP address.
• The SDM630 has been installed correctly and is connected over RS485 to the IOZER terminals A, B, and, if required, SHLD.
• The bus cable has the correct polarity. For longer cable runs, terminate the bus only at its two physical ends.
• The Modbus address of the SDM630 is known. The following steps use Unit ID 1 as an example. Adjust this value if the meter uses a different address.
• The following communication parameters are configured on the SDM630:

Implementation

1. Configure Modbus RTU

Open the IOZER web interface and navigate to Settings → Modbus → Modbus RTU.

Configure the RS485 interface as follows:

Then add the SDM630 as an RTU slave:

Save the configuration. Interface and status information is available under Modbus.

2. Configure Modbus TCP

Navigate to Settings → Modbus → Modbus TCP and configure IOZER as a Modbus TCP slave:

The TCP Unit ID can be configured independently of the RTU device address. This example uses Unit ID 1 on both sides. On the TCP side, the mapping occupies the holding registers with zero-based start addresses 0 through 23.

The screenshot shows the complete interface configuration used in this example. A higher value for Max. registers is permitted; the twelve Float32 values require only addresses 0 through 23.

3. Configure registers and datapoints

The SDM630 measurements shown here are located in input registers and are read using Modbus function 04. Each measurement occupies two consecutive 16-bit registers and is transmitted as a 32-bit IEEE 754 floating-point value. By default, the SDM630 provides the high-order register first. No additional scaling is required for these measurements.

Under Settings → Datapoints, create one datapoint of type Float for each measurement. IOZER address fields use the zero-based protocol address. Configure each datapoint as follows:

• Use the SDM630 on the Modbus RTU master as the data source.
• Select function 04 – Read Input Registers.
• Enter the relevant zero-based RTU address.
• Select data type Float32 with the high-order register first.
• Set scale to 1 and offset to 0.
• Use the Modbus TCP slave with function 03 – Read Holding Registers as the data target.
• Enter the relevant zero-based TCP address.
• Also select Float32 with the high-order register first for the TCP target.

The following mapping distinguishes between the address notations used: the SDM630 column follows the five-digit notation in the official SDM630 Modbus protocol documentation, while the mbclient column uses the six-digit standard view of ModbusTools. Enter only the zero-based values in the IOZER address fields.

Save the datapoints and check the Value, Status, and Updated columns in the overview. The values should update regularly and have status OK. The overview shows the mapping in a compact form: RTU #1 / F4 / 0 reads the first SDM630 input register; TCP #1 / F3 / 0 provides the same value in the first TCP holding register.

4. Read test using mbclient

Install and start mbclient from the serhmarch/ModbusTools project. Then create a new project.

1. Add a port of type Modbus TCP.
2. Enter the IOZER IP address as the host and 502 as the port.
3. Add a device with Unit address 1 to the port.
4. Create a data view entry for a holding register.
5. Select address 400001 and the Float32 or Float representation with the high-order register first.
6. Start communication.

ModbusTools uses six-digit register notation in its default view. The holding register at IOZER address 0 is therefore entered as 400001 in mbclient. Alternatively, select zero-based IEC 61131-3 notation under Tools → Settings → View → Modbus adr. notation; the same holding register is then shown as %MW0.

For an individual test, you can also use Tools → Send Message:

The expected result is the voltage between L1 and neutral. The value should be plausible and approximately match the SDM630 display. Additional measurements can be checked using the start addresses in the mapping table.

The screenshot shows all twelve values at mbclient addresses 400001 through 400023. Status Good confirms successful TCP communication and reading with function 03.

Troubleshooting

Result and extension

After a successful test, IOZER basic operates as a protocol gateway: it reads the SDM630 measurements over Modbus RTU and makes them available on the network in Modbus TCP holding registers, independently of the RS485 bus.

The same principle can be used to provide additional Modbus RTU devices over Modbus TCP. Add each device as another RTU slave and create corresponding datapoints with a Modbus TCP target for its values. Observe the following:

• Each RTU device requires a unique Unit ID.
• All devices on the same RS485 bus must use the same baud rate, parity, and number of stop bits.
• Assign each datapoint its own non-overlapping TCP register range.
• Set Max. registers high enough to fully include the highest TCP address used.

IOZER combines the values from the connected RTU devices in one shared Modbus TCP register map. A TCP client can read all provided values through the IP connection to IOZER and does not require direct access to the RS485 bus.