Modbus variable list
Appendix C Modbus variable list
C.1 Introduction
The Modbus protocol is a general-purpose protocol for data exchange between for instance control units,
SCADA systems, instruments, and electricity meters. It’s an asynchronous, serial Master Slave protocol. It’s
widely used, well documented and simple to understand.
A Modbus master can communicate with up to 247 slave units with the device ID 1-247. A protocol like
Modbus consists of several layers (OSI-model). The bottom layer is always the physical layer; the number of
wires and signal levels. The next layer describes the communication digits (number of data bits, stop-bits,
parity etc.). Next are the layers describing the Modbus-specific functions (number of digits per message, the
meaning of different messages, etc.).
C.2 Modbus register types
1. Discrete Input Register
2. Coils Register
3. Input Register
4. Holding Register
Supported Modbus functions:
✓ 0x01 Read Coils
✓ 0x02 Read Discrete Inputs
✓ 0x03 Read Holding Registers
✓ 0x04 Read Input Registers
✓ 0x05 Write Single Coil
✓ 0x06 Write Single Register
✓ 0x0F Write Multiple Coils
✓ 0x10 Write Multiple Registers
✓ 0x17 Read/Write Multiple Registers
C.3 Discrete Input Register
Modbus address Description
1 Not used
2 Not used
3 Presence detected
0 = Presence not detected
1 = Presence detected
Active if presence sensor is configured at terminal DI.
4 Change-over heating/cooling
0 = Change-over heating
1 = Change-over cooling
Active if Change-over sensor is configured at terminal DI.
5 Fan speed 1
0 = Fan speed 1 is not active on DO FAN1
1 = Fan speed 1 is active on DO FAN1
RCFD-230C manual, Rev. A 25
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