Parameters
Common Parameters
Listed below are the common parameters for all types of Modbus devices.
General
Slave ID: indicates the slave device ID in the Modbus network.
Inter-Request Delay: sets the delay for sending a request (in milliseconds) after receiving the response.
Writes per Cycle: indicates the number of write requests sent after read request completion.
Blocks
Output Coils Block Size and Input Coils Block Size: determines the maximum number of bits that can be read in one request from Output and Input Coils. The value of this parameter must be in the range from 1 to 2000.
Internal Registers Block Size and Holding Registers Block Size: determines the maximum number of registers that can be read for one request from the Internal and Holding Registers. The value of this parameter must be in the range from 4 to 120.
Data - Functions
Use function 06 for single register writes: indicates which function should be used when writing one register (06 or 16).
Use function 05 for single coil writes: indicates which function should be used when writing one bit (05 or 15).
Data - Encoding
Use Big-Endian Byte Order: sets the byte order (Big-Endian or Little-Endian) when organizing the word.
First word low in 32-bit values: sets the word order (from low to high or high to low) when organizing 32-bit values.
First DWord low in 64-bit values: sets the order of double words (from low to high or high to low) when organizing 64-bit values.
Reverse Bit Ordering: indicates the order of bits used when working with register bits (0 bit is MSB or 15 bit is MSB).
Swap Every Two Bytes in a String: indicates that every two bytes in a string must be swapped.
Modbus TCP Parameters
Listed below are the parameters specific to the Modbus TCP.
Host: sets the IP address or hostname of the device.
Port: specifies the port number of the device. The standard port number for Modbus protocol is 502.
Transport: indicates which transport mode must be used (TCP, RTU over TCP or UDP).
Retries: sets the number of attempts after which request is considered failed.
Request Timeout: sets the time to wait for the request to be executed (in milliseconds) after which the request execution error occurs. An error occurs after the time specified in Request Timeout parameter multiplied by Retries.
Connection Timeout: sets the connection timeout (in milliseconds) after which connection error occurs.
Close TCP Socket: specifies whether to close the TCP connection if a timeout occurred while sending a request to the server.
Transaction Threshold: specifies the maximum allowed difference between the sent and received transaction ID, after exceeding which the reconnection to the server is performed.
Operation Mode: specifies the master or slave operation mode.
Max Connection Idle Interval: specifies the maximum silence interval (in milliseconds) for each incoming connection before it is closed.
Modbus Serial Parameters
The following are the settings specific to Modbus Serial.
COM ID: indicates the symbol name of the serial port. For example: COM1, COM10, /dev/ttyS0, /dev/ttyUSB0.
Mode: sets the mode of serial protocol (RTU or ASCII).
Baud Rate: specifies data transfer rate.
Parity: defines the method of detecting errors in transmission.
Data Bits: indicates the number of used data bits.
Stop Bits: indicates the number of stop bits used.
Retries: sets the number of attempts after which request is considered failed.
Request Timeout: sets the time to wait for the request to be executed (in milliseconds) after which the request execution error occurs. An error occurs after the time specified in Request Timeout parameter multiplied by Retries.
Operation Mode: specifies the master or slave operation mode.
Most of the parameters described above are available as OPC UA items. More information about editing server object properties via OPC UA is described here.
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