C.7 Message Formatu Message ContentIn MEMOBUS/Modbus communications, the master sends commands to the slave, and the slave responds. The messageformat is configured for both sending and receiving as shown below, and the length of data packets depends on the command(function) content.SLAVE ADDRESSFUNCTION CODEDATAERROR CHECKu Slave AddressThe slave address in the message defines the note the message is sent to. Use addresses between 0 and FF (hex). If a messagewith slave address 0 is sent (broadcast), the command from the master will be received by all slaves. The slaves do not providea response to a broadcast type message.u Function CodeThe three types of function codes are shown in the table below.FunctionCode Function NameData Length (bytes)Command Message Response MessageMinimum Maximum Minimum Maximum03H Read MEMOBUS/Modbus registers 8 8 7 3708H Loopback test 8 8 8 810H Write to multiple MEMOBUS/Modbus registers 11 41 8 8u DataConfigure consecutive data by combining the MEMOBUS/Modbus register address (test code in case of a loopback test) andthe data the register contains. The data length changes depending on the command details.A drive MEMOBUS/Modbus register always has a data length of two bytes. Data written into drive registers must also alwayshave a length of two bytes. Register data read out from the drive will always consist of two bytes.u Error CheckThe drive uses a CRC-16 (cyclic redundancy check, checksum method) for checking data validity. Use the procedure describedbelow when calculating the CRC-16 checksum for command data or when verifying response data.n Command DataWhen the drive receives data, it calculates the CRC-16 checksum from the data and compares it to the CRC-16 value receivedwithin the message. Both must match before a command is processed.An initial value of FFFFH (i.e., all 16 bits equal 1) must be used for CRC-16 calculations in the MEMOBUS/Modbus protocol.Calculate the CRC-16 checksum using the following steps:1. The starting value is FFFFH.2. Perform an XOR operation of this value and the slave address.3. Right shift the result.4. When the overflow bit of the shift operation becomes 1, perform an XOR operation of the result from step 3 aboveand the fix value A001H.5. Repeat steps 3 and 4 until eight shift operations have been performed.6. After eight shift operations, perform an XOR operation with the result and the next data in the message (function code,register address, data). Continue with steps 3 to 5 until the last data has been processed.7. The result of the last shift or XOR operation is the checksum.C.7 Message Format516 YASKAWA SIEP YAIP1U 01C AC Drive - P1000 Technical Manual
