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Modbus Protocol

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Modbus is a serial protocol commonly used in industrial automation applications. Digi XTend radio modems support Modbus-ASCII and Modbus-RTU with no special configuration. In fact, the radio modems are designed to be transparent (the application is not aware of the wireless connection) to applications that include addressing protocols such as Modbus.

Digi radio modems support Modbus-ASCII networks with no special configuration. There are some considerations however, when using radio modems with Modbus-RTU:

  • The only difference between Digi wireless modems and a multi-drop wire network is that the wireless modems introduce some additional latency (delay) into the system. Modbus-ASCII is perfectly compatible with Digi radio modems.

  • Some protocols have short response timeout requirements that may not be compatible with radio modem latencies. Modbus-RTU is compatible with the latencies but does have inter-character delay requirements that must be met.

  • To satisfy the inter-character delay timing requirements for Modbus-RTU, Digi recommends that the radio modem be selected to transfer data over-the-air at twice the serial interface baud rate. For example, when the Modbus data rate is 9600 baud, use Digi part number X24-019 modems which transfer data over-the-air at 19200 baud or similar RF modem.

  • If using the XStream radio, and a data rate of 19200 is needed, then firmware versions with packetization control should be used to satisfy the intercharacter timing requirement.

Please contact Digi Technical Services for more details.


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