Modbus frame format types

Traditional Modbus messages can be placed in RTU or ASCII frames and can be packaged with a TCP/IP interface, so it can be sent over the internet. Both RTU and ASCII contains a Checksum, so when converting to TCP/IP, it doesn't require a checksum calculation or in other words, CRC.


RTU
Modbus RTU is the most common implementation available for Modbus. A Modbus RTU message must be transmitted continuously without inter-character hesitations. Modbus messages are framed (separated) by idle (silent) periods. Each message must be transmitted in a continuous stream.
Discover how an RTU frame is built

ASCII
This is used in serial communication and makes use of ASCII characters for protocol communication. The ASCII (American Standard Code for Information Interchange) format uses a longitudinal redundancy check checksum. Modbus ASCII messages are framed by leading colon ":" and trailing newline (CR/LF).
See how an ASCII frame looks like

TCP
Modbus TCP/IP is basicly the Modbus RTU protocol with a TCP interface that runs on Ethernet. It does not require a checksum calculation, as lower layers already provide checksum protection. It defines the rules for organizing and interpreting the data independently of the data transmission medium.
Look how a Modbus TCI/IP is built

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