Which Modbus standard is supported?
Our RTD and FDP3 interfaces support Standard RTU Modbus protocol over RS485 (this is the port used for Modbus communication).
- Note that no custom protocol can be uploaded to our interfaces.
Details of the Modbus Protocol can be found in the Modicon Modbus Protocol Reference Guide available on the internet.
Preparing the interface for the Modbus network: RS485
The RS485 requirements are detailed in this article.
Preparing the interface for the Modbus network: Communication parameters (Baud rate, parity)
Please see following articles for following product lines:
- For our FDP3 interfaces: FDP3 Baud Rate and Parity
- For our RTD interfaces: RTD Baud Rate and Parity
Preparing the interface for the Modbus network: Modbus address
Modbus interfaces require a Modbus network address. The address has to be unique, i.e. no other device on the Modbus network can have the same address. The address is set using the DIP switches ('above' the RS485 port) of our interfaces. The modbus address range of our interfaces depends on the interface used and the operation mode it has been configured for (see respective installation instructions for details).
Note that some Modbus masters require Modbus slaves to have an address > 0.
Our interfaces only offer a limited Modbus address range. If Modbus addresses are needed which cannot be set with the DIP switches due to the limited Modbus address range covered by the used interface then a Modbus gateway can be used (we recommend our Modbus gateway RTD-DSX) to map the available address range to a higher Modbus address. For an explanatory example please see page 3 of the RTD-DSX installation instructions (click here).
Modbus Master or Slave?
ONLY 1 Modbus master (usually a BMS system or software) is allowed on the Modbus network. Whether an interface acts as Modbus master or slave depends on the interface in use, as well as the operation mode it has been set to. Please assign your Modbus slave device an address >0.
If an interface is a Modbus master and access to it Modbus registers by an external BMS is required then a Modbus gateway is needed (we recommend our Modbus gateway RTD-DSX - click here for further information).
'Mixed Modbus network', 3rd party devices on the same Modbus network?
Any device that supports the Modbus RTU protocol can be placed on the same network (e.g. RTD-NET, RTD-10, FDP3-BMS and 3rd party devices can be 'mixed' on the same network), as long as there is only one Modbus master (some of our interfaces act as modbus master when operated in specific modes, e.g. RTD-10 in Duty/Standby mode).
Which Modbus software to use?
Any software supporting the Modbus RTU protocol can be used to communicate with our interfaces. Therefore, please make sure that your BMS system (e.g. Tridium JACE, Loxone, Trend, KNX, Wago PLC etc.) support theModbus RTU protocol.
For testing purposes our free software ModTool can be used (click here for further information).
Modbus commands
Registers are accessed using standard Modbus functions. The following four functions are supported by the RTD interface.
| Function Code (hex code) | Function Name | Register Count |
| 03 (03h) | Read Holding Registers | 1..10 |
| 04 (04h) | Read Input Registers | 1..10 |
| 06 (06h) | Preset (write) Single Holding Register | 1 |
| 16 (10h) | Preset (write) Multiple Holding Register | 1..10 |
In the table above 'Register Count' means the maximum number of registers that can be read/written to with the specific function.
Note that Input registers can only be read with function code 04 for Input registers which are read-only.
List of available Modbus registers?
Details of our interfaces' Modbus registers can be found in the respective installation instructions. The register addresses in the datasheet are in Decimal.
Register Offset
Register Offset is '0'. However, do note that some BMS systems may use an offset of 1.
Are the values stored in volatile memory
When our interface is powered Off and then powered ON again the indoor unit does not reload the parameters to their settings prior to powering off, because the parameters are stored in volatile memory (only exception are registers related to the run hours timers which are available in some of our interfaces like RTD-LT/CA and RTD-W).
Refer to the indoor unit's installation instructions or manufacturer's technical support for information on which parameters their devices store in non-volatile memory.
Is there any risk for register's life cycle? Do I have to take care about writes on my master?
All registers in the RTD are stored in RAM and so have no write limitations. Some values transmitted to the indoor units/remote controllers may be stored in non-volatile storage. Typically this would only be updated if the value changes, so continuously varying a parameter such as setpoint at a high rate may not be advisable, however we do not have information on which values are stored in non-volatile memory and if there are any limitations on the write frequency. If there are concerns in this respect, please contact the Technical Support of the equipment manufacturer
Some of our interfaces have run hours timer registers. These are the only registers using EEPROM. We would advise against continuously transmitting reset run hours command, unless necessary.
Modbus Sample codes?
The RTU Modbus protocol format is used. Details of the Modbus Protocol can be found in the Modicon Modbus Protocol Reference Guide available on the internet.
A Modbus string will consist of following bytes:
Address: 1 Byte
Function Code : 1 Byte
Register Starting Address: 2 Bytes
Quantity of Registers: 2 Bytes
CRC: 2 Bytes.
We do not offer any examples of Modbus requests and we do not customize your BMS software! However, for comparison of the communication over Modbus you can use our free software ModTool which allows you to see the Modbus commands communicated between our interfaces and a Modbus master (click here for further information).
Testing
You can use our free software ModTool for testing an interface (click here for further information).