Overview

The Modbus serial data source is used to gather data from a local modbus network, accessible via RS232 or RS485 (requires adapter) communication. This is a polling data source.

Configuration

Every data source requires a Name, which can be any description. The Update period determines how often the Modbus network is polled for data. The Timeout and Retries fields determine the system behaviour in the case of a polling failure. The data source waits the given number of timeout milliseconds for a response from the network. If it is not received the request will be retried the given number of times.

Serial communication is controlled with the Baud rate, Flow control in, Flow control out, Data bits, Stop bits, and Parity values. The Echo setting can be used with RS485 networks as appropriate.

The Encoding value determines how Modbus requests are formatted. Most production hardware uses RTU formatted messages. See the Modbus documentation for your equipment to determine how to set this field.

Node scanning

Networks can be scanned for slave nodes using the scan utility. This feature iterates from slave id 1 to 240, sending each a ReadExceptionStatus (function code 7). If a response is received (using the given timeout and retries settings), the node is considered available. Note that not all equipment supports this function code, so false negatives are possible.