Function Code 04H - GE 350 Communications Manual

CHAPTER 8: MODBUS FUNCTIONS

Function Code 04H

SR3 SERIES PROTECTIVE RELAY PLATFORM – COMMUNICATIONS GUIDE
Modbus Implementation: Read Input Registers
3 Series relay implementation: Read Actual Values
For the 3 Series relay 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 3 Series relay 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 8-2: 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
Query:
The query message specifies the starting register and quantity of registers to be read.
Registers are addressed starting at zero: registers 1 to 16 are addressed as 0 to 15.
Here is an example of a request to read register 30305 from slave device 254:
Field Name
Slave Address
Function
Starting Address Hi
Starting Address Lo
No. of Points Hi
No. of Points Lo
Response:
The register data in the response message are packed as two bytes per register, with the
binary contents right justified within each byte. For each register, the first byte contains
the high order bits and the second contains the low order bits.
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
Hex
FE
04
01
30
00
01
FUNCTION CODE 04H
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
8-3