Modbus Slave
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OpenPLC can be configured as a Modbus slave (server). A Modbus slave is normally controlled by a master, and performs local measurement and control. The remote master initiates read and write requests to the OpenPLC slave.
OpenPLC supports all common Modbus function codes:
- Read discrete output coil (0x01)
- Write discrete output coil (0x05)
- Write multiple discrete output coils (0x0F)
- Read discrete input contacts (0x02)
- Read analog input registers (0x04)
- Read analog output holding registers (0x03)
- Write analog output holding register (0x06)
- Write multiple analog output registers (0x10)
Binding Modbus Slave to PLC Addresses
OpenPLC operates as a block for Modbus tables. PLC address binding (the Modbus map) is based on the hierarchical address value, the data width, and the direction.
| Modbus Table | Usage | PLC Address | Modbus Data Address | Data Size | Range | Access† |
|---|---|---|---|---|---|---|
| Discrete Output Coils | Digital Outputs | %QX0.0 - %QX99.7 |
0 - 799 | 1 bit | 0 or 1 | RW |
| Discrete Input Contacts | Digital Inputs | %IX0.0 - %IX99.7 |
0 - 799 | 1 bit | 0 or 1 | R |
| Analog Input Registers | Analog Input | %IW0 - %IW1023 |
0 - 1023 | 16 bits | 0 - 65535 | R |
| Analog Output Holding Registers | Analog Outputs | %QW0 - %QW1023 |
0 - 1023 | 16 bits | 0 - 65535 | RW |
Discrete output coil and discrete input contact binding are based on the the two-part PLC address, without unused Modbus data addresses. The least-significant part of the PLC address has a range 0 to 7, therefore, a little math is needed to translate between PLC addresses and Modbus data addresses.
Given the Modbus data address, the PLC address is determined as:
msp := int(modbus_data_address / 8)
lsp := modbus_data_address mod 8
hierarchical_address = str(msp) + "." " str(lsp)
For example, if the Modbus address for a discrete output coil is 22, then the
most significant part is 2 (22 / 8) and the least significant part is 6
(22 mod 8). Therefore, the PLC address is %QX2.6.
Memory PLC addresses are mapped to higher-index analog output registers.
| Modbus Table | Usage | PLC Address | Modbus Data Address | Data Size | Range | Access† |
|---|---|---|---|---|---|---|
| Analog Output Holding Registers | General 16-bit Register | %MW0 - %MW1023 | 1024 - 2047 | 16 bits | 0 - 65536 | RW |
| Analog Output Holding Registers | General 32-bit Register | %MD0 - %MD1023 | 2048 - 4095 | 32 bits | 0 - 4294967295 | RW |
| Analog Output Holding Registers | General 64-bit Register | %ML0 - %ML1023 | 4096 - 8191 | 64 bits | 0 - really big | RW |
† Access is defined from the perspective of the Modbus master. That is, the Modbus master can read and write discrete input coils, but can only read discrete input contacts.