Function Code 04H - GE 350 Communications Manual

FUNCTION CODE 04H

Function Code 04H
9–2
SLAVE RESPONSE
SLAVE ADDRESS
FUNCTION CODE
BYTE COUNT
DATA 1 (see definition above)
DATA 2 (see definition above)
DATA 3 (see definition above)
CRC
Modbus Implementation: Read Input Registers
350 implementation: Read Actual Values
For the 350 implementation of Modbus, this function code can be used to read any actual
values ("input registers"). Input registers are 16 bit (two byte) values transmitted high order
byte first. Thus all 350 Actual Values are sent as two bytes. The maximum number of
registers that can be read in one transmission is 125.
The slave response to this function code is the slave address, function code, a count of the
data bytes to follow, the data itself and the CRC. Each data item is sent as a two byte
number with the high order byte sent first.
For example, request slave 17 to respond with 1 register starting at address 0008. For this
example the value in this register (0008) is 0000.
Table 9-2: MASTER/SLAVE PACKET FORMAT FOR FUNCTION CODE 04H
MASTER TRANSMISSION
SLAVE ADDRESS
FUNCTION CODE
DATA STARTING ADDRESS
NUMBER OF ACTUAL VALUES
CRC
SLAVE RESPONSE
SLAVE ADDRESS
FUNCTION CODE
BYTE COUNT
DATA (see definition above)
CRC
BYTES EXAMPLE
1 11
1 03
1 06
2 02 2B
2 00 00
2 00 64
2 54 83
BYTES EXAMPLE
1 11
1 04
2 00 08
2 00 01
2 B2 98
BYTES EXAMPLE
1 11
1 04
1 02
2 00 00
2 78 F3
350 FEEDER PROTECTION SYSTEM – COMMUNICATIONS GUIDE
CHAPTER 9: MODBUS FUNCTIONS
DESCRIPTION
message from slave
17
read registers
3 registers = 6 bytes
value in address
006B
value in address
006C
value in address
006D
CRC error code
DESCRIPTION
message for slave 17
read registers
data starting at 0008
1 register = 2 bytes
CRC error code
DESCRIPTION
message from slave
17
read registers
1 register = 2 bytes
value in address
0008
CRC error code