GE Industrial Solutions Entellisys Integrator’s Guide - Manual (Page 15)

Modbus ® protocol implementation 15
1.1.5.5 Exception responses
Programming or operation errors usually happen because of illegal data in a packet. These
errors result in an exception response from the slave. The slave detecting one of these errors
sends a response packet to the master with the high order bit of the function code set to 1. The
following table shows the format of the master and slave packets. The example shows a master
device sending the unsupported function code 39h to slave device 11.
Master and Slave Device Packet Transmission Example
SLAVE RESPONSE
PACKET FORMAT EXAMPLE (HEX)
SLAVE ADDRESS 11
FUNCTION CODE 10
DATA STARTING ADDR - hi 40
DATA STARTING ADDR - lo 51
NUMBER OF SETTINGS - hi 00
NUMBER OF SETTINGS - lo 02
CRC - lo 07
CRC - hi 64
MASTER TRANSMISSION
PACKET FORMAT EXAMPLE (HEX)
SLAVE ADDRESS 11
FUNCTION CODE 39
CRC - low order byte CD
CRC - high order byte F2
SLAVE RESPONSE
PACKET FORMAT EXAMPLE (HEX)
SLAVE ADDRESS 11
FUNCTION CODE B9
ERROR CODE 01
CRC - low order byte 93
CRC - high order byte 95