Schweitzer Engineering Laboratories SEL-701-1, Instruction Manual

Page: 295 / 452

Date Code 20011009
SEL-701-1 Monitor
Modbus
®
RTU Communications Protocol
03h Read Holding Registers Command
C.7
03h Read Holding Registers Command
Use function code 03h to read directly from the Modbus Register map shown in
Table C.18 on page C.18 . You may read a maximum of 125 registers at once with this
function code. Most masters use 4X references with this function code. If you are
accustomed to 4X references with this function code, for 5-digit addressing, add 40001
to the standard database address.
Table C.6 03h Read Holding Registers Command
Bytes Field
The Master Request Must Have the Following Format
1 byte Slave address
1 byte Function code (03h)
2 bytes Starting register address
2 bytes Number of registers to read
2 bytes CRC-16 for message
A Successful SEL-701-1 Monitor Response Will Have the Following Format
1 byte Slave address
1 byte Function code (03h)
1 byte Byte count (should be twice number of registers read)
n bytes Byte count bytes of data
2 bytes CRC-16 for message