UNS0007A-P V1 HIEE305098R0001 HIEE410730P201 module

Two kinds of error-checking methods are used for standard Modbus networks. The error checking field contents depend upon the method that is being used. ASCII When ASCII mode is used for character framing, the error checking field contains two ASCII characters. The error check characters are the result of a Longitudinal Redundancy Check (LRC) calculation that is performed on the message contents, exclusive of the beginning ’colon’ and terminating CRLF characters. The LRC characters are appended to the message as the last field preceding the CRLF characters.

When RTU mode is used for character framing

 the error checking field contains a 16-bit value implemented as two 8-bit bytes. The error check value is the result of a Cyclical Redundancy Check calculation performed on the message contents. The CRC field is appended to the message as the last field in the message. When this is done, the low-order byte of the field is appended first, followed by the high-order byte. The CRC high-order byte is the last byte to be sent in the message. Additional information about error checking is contained later in this appendix.

Error Checking Methods

 Standard Modbus serial networks use two kinds of error checking. Parity checking (even or odd) can be optionally applied to each character. Frame checking (CRC) is applied to the entire message. Both the character check and message frame check are generated in the master device and applied to the message contents before transmission. The slave device checks each character and the entire message frame during receipt.