Modbus protocols
There are three standard variants of Modbus protocols:
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Modbus RTU — Modbus RTU is the most common implementation available for Modbus. Used in serial communications, data is transmitted in a binary format. The RTU format follows the commands/data with a cyclic redundancy check checksum.
NOTE:The Modbus RTU protocol standard does not allow a delay between characters of 1.5 times or more the length of time normally required to receive a character. This is analogous to “pizza” being understood, and “piz za” being gibberish. It's important to note that communications hardware used for Modbus RTU, such as radios, must transfer data as entire packets without injecting delays in the middle of Modbus messages.
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Modbus ASCII — Used in serial communications, data is transmitted as an ASCII representation of the hexadecimal values. Timing requirements are loosened, and a simpler longitudinal redundancy check checksum is used.
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Modbus TCP/IP or Modbus TCP — Used for communications over TCP/IP networks. The TCP/IP format does not require a checksum calculation, as lower layers already provide checksum protection. The packet structure is similar to RTU, but uses a different header. Devices labeled as Modbus gateways will convert from Modbus TCP to Modbus RTU.
Campbell Scientific data loggers support Modbus RTU, Modbus ASCII, and Modbus TCP protocols. If the connection is over IP, Campbell Scientific data loggers always use Modbus TCP. Modbus server functionality over other comports use RTU. When acting as a client, the data logger can be switched between ASCII and RTU protocols using an option in the ModbusClient() instruction. See the CRBasic Editor help for detailed instruction information and program examples: 