Tab: Modbus Server Channel

You use this tab to define server channels.

Each channel represents a single Modbus request.

Add Channel

Opens the Modbus Channel dialog. Here you can create new channels for the current Modbus Server. In the device description file of the Modbus Server, you can predefine descriptions for individual Modbus registers or entire channels.

Dialog: Modbus Channel

Table 6. Channel

Name	A string that contains the name of the channel
Access type	Read Coils (function code 01) Read Discrete Inputs (function code 02) Read Holding Registers (function code 03) Read Input Registers (function code 04) Read Single Coil (function code 05) Write Single Register (function code 06) Write Multiple Coils (function code 15) Write Multiple Registers (function code 16) Read/Write Multiple Registers (function code 23)
Trigger	CYCLIC: The request occurs periodically. RISING_EDGE: The request occurs as a reaction to a rising edge of the Boolean trigger variables. The trigger variable is defined on the I/O Mapping tab. Application: The Modbus request is triggered by the PLC application. This happens by means of the ModbusChannel function block, which is included in the respective I/O driver library. This function block provides complete control of and information about the execution of this command, for example the start time, the processing time, and the result.
Cycle Time (ms)	For Trigger = CYCLIC: Request interval Note: The request interval should be the same as or a multiple of the cycle time of the application.
Comment	Description of the channel

Table 7. READ Register

Offset	Start address where reading should start (value range 0–65535)
Length	Number of registers to be read (for word access) or number of discrete inputs to be read (for bit access)
Error handling	Defines what should happen to the data in case of a communication error Keep last Value Set to zero

Table 8. WRITE Register

Offset

Number of the register to be written to (value range 0–65535)

Length

Number of registers to be written to (= words)

The value range of the parameter depends on function code.

Reading of coils and discrete inputs / Writing of coils to overlapping register memory

The CODESYS MODBUS I/O driver allows for the reading of coils and discrete inputs, as well as the writing of coils to overlapping register memory (the Discrete Bit Areas check box not selected). In this case, the first 8 bits which are read (%IB0) or written align with the high byte of the corresponding register. The second 8 bits which are read (%IB1) align with the low byte of the corresponding register (LSB first).

Example 1. Example

In the following example, the first line defines a combined read/write operation (function code 23). It reads a word from the "holding register" with offset 16#0001 and writes two words to the register with offset 16#0003. The operation is performed as soon as the trigger variable defined on the I/O Mapping tab shows a rising edge.