11-4
8 SERIES PROTECTIVE RELAY PLATFORM – COMMUNICATIONS GUIDE
FUNCTION CODE 04H (READ ACTUAL VALUES)
CHAPTER 11: MODBUS PROTOCOL
ASE 2000 V2 Example
--> 00 00 00 00 00 06 FE 03 4A 8A 00 04
--> 00 00 00 00 00 0B FE 03 08 00 00 04 B0 00 00 00 64
Function Code 04H (Read Actual Values)
Modbus Implementation: Read Input Registers
8 Series relay Implementation: Read Actual Values
For the 8 Series relay implementation of Modbus, the 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 8 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 11-3: Master Packet Format for Function Code 04H
Table 11-4: Slave Packet Format for Function Code 04H
ASE 2000 V2 Example
--> 00 00 00 00 00 06 FE 04 05 95 00 02
--> 00 00 00 00 00 07 FE 04 04 00 00 27 0F
MASTER TRANSMISSION
BYTES
EXAMPLE
DESCRIPTION
SLAVE ADDRESS
1
FE
message for slave 254
FUNCTION CODE
1
04
read registers
DATA STARTING ADDRESS
2
05 95
data starting at 0595
NUMBER OF ACTUAL VALUES
2
00 02
2 registers = 4 bytes total
CRC (low, high)
2
75 24
CRC error code
SLAVE RESPONSE
BYTES
EXAMPLE
DESCRIPTION
SLAVE ADDRESS
1
FE
message from slave 254
FUNCTION CODE
1
04
read registers
BYTE COUNT
1
04
2 register = 4 bytes total
DATA 1
2
00 00
value in address 0595 (32-bit)
DATA 2
2
27 0F
CRC (low, high)
2
AF 7F
CRC error code