7.2.2 Modbus protocols
There are three standard variants of Modbus protocols:
l 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.
l 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.
l 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
7. Communications protocols 94
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