Data Packet Format A complete request/response sequence consists of the following bytes (transmitted as separate data frames):
1.A Master Query Message consisting of: a 1-byteSlave Address, a 1-byteFunc- tion Code, a variable number of Data Bytes depending on the Function Code, and a 2-byteCRC code.
2.A Slave Response Message consisting of: a 1-byte Slave Address, a 1-byte Function Code, a variable number of Data Bytes depending on the Function Code, and a 2-byte CRC code.
The terms Slave Address, Function Code, Data Bytes, and CRC are explained below:
•SLAVE ADDRESS: This is the first byte of every transmission. This byte represents the user-assigned address of the slave device that is to receive the message sent by the master. Each slave device must be assigned a unique address and only the addressed slave will respond to a transmission that starts with its address. In a master request transmission the Slave Address represents the address of the slave to which the request is being sent. In a slave response transmission the Slave Address represents the address of the slave that is sending the response. The RS232 port ignores the slave address, so it will respond regardless of the value in the message. Note: A master transmission with a Slave Address of 0 indicates a broadcast command. Broadcast commands can be used for specific functions.
•FUNCTION CODE: This is the second byte of every transmission. Modbus defines function codes of 1 to 127. The 489 implements some of these functions. In a master request transmission the Function Code tells the slave what action to perform. In a slave response transmission if the Function Code sent from the slave is the same as the Function Code sent from the master indicating the slave performed the function as requested. If the high order bit of the Function Code sent from the slave is a 1 (i.e. if the Function Code is greater than 127) then the slave did not perform the function as requested and is sending an error or exception response.
•DATA BYTES: This is a variable number of bytes depending on the Function Code. These may be actual values, setpoints, or addresses sent by the master to the slave or vice-versa. Data is sent MSByte first followed by the LSByte.
•CRC: This is a two byte error checking code. CRC is sent LSByte first followed by the MSByte. The RTU version of Modbus includes a two byte CRC-16 (16-bit cyclic redundancy check) with every transmission. The CRC-16 algorithm essentially treats the entire data stream (data bits only; start, stop and parity ignored) as one continuous binary number. This number is first shifted left 16 bits and then divided by a characteristic polynomial (11000000000000101B). The 16-bit remainder of the division is appended to the end of the transmission, LSByte first. The resulting message including CRC, when divided by the same polynomial at the receiver will give a zero remainder if no transmission errors have occurred.
If a 489 Modbus slave device receives a transmission in which an error is indicated by the CRC-16 calculation, the slave device will not respond to the transmission. A CRC-16 error indicates than one or more bytes of the transmission were received incorrectly and thus the entire transmission should be ignored in order to avoid the 489 performing any incorrect operation. The CRC-16 calculation is an industry standard method used for error detection. An algorithm is included here to assist programmers in situations where no standard CRC-16 calculation routines are available.
