B-4
B90 Low Impedance Bus Differential System
GE Multilin
B.2 MODBUS FUNCTION CODES
APPENDIX B
B
B.2MODBUS FUNCTION CODES
B.2.1 SUPPORTED FUNCTION CODES
Modbus officially defines function codes from 1 to 127 though only a small subset is generally needed. The relay supports
some of these functions, as summarized in the following table. Subsequent sections describe each function code in detail.
B.2.2 READ ACTUAL VALUES OR SETTINGS (FUNCTION CODE 03/04H)
This function code allows the master to read one or more consecutive data registers (actual values or settings) from a relay.
Data registers are always 16-bit (two-byte) values transmitted with high order byte first. The maximum number of registers
that can be read in a single packet is 125. See the
Modbus memory map
table for exact details on the data registers.
Since some PLC implementations of Modbus only support one of function codes 03h and 04h. The UR interpretation allows
either function code to be used for reading one or more consecutive data registers. The data starting address will determine
the type of data being read. Function codes 03h and 04h are therefore identical.
The following table shows the format of the master and slave packetsin Modbus RTU. Modbus TCP/IP ADUs have a MBAP
instead of slave address, and CRC is in another stack layer. The example shows a master device requesting three register
values starting at address 4050h from slave device 11h (17 decimal); the slave device responds with the values 40, 300,
and 0 from registers 4050h, 4051h, and 4052h, respectively.
FUNCTION CODE
MODBUS DEFINITION
GE MULTILIN DEFINITION
HEX
DEC
03
3
Read holding registers
Read actual values or settings
04
4
Read holding registers
Read actual values or settings
05
5
Force single coil
Execute operation
06
6
Preset single register
Store single setting
10
16
Preset multiple registers
Store multiple settings
Table B–4: MASTER AND SLAVE DEVICE PACKET TRANSMISSION EXAMPLE
MASTER TRANSMISSION
SLAVE RESPONSE
PACKET FORMAT
EXAMPLE (HEX)
PACKET FORMAT
EXAMPLE (HEX)
SLAVE ADDRESS
11
SLAVE ADDRESS
11
FUNCTION CODE
04
FUNCTION CODE
04
DATA STARTING ADDRESS - high
40
BYTE COUNT
06
DATA STARTING ADDRESS - low
50
DATA #1 - high
00
NUMBER OF REGISTERS - high
00
DATA #1 - low
28
NUMBER OF REGISTERS - low
03
DATA #2 - high
01
CRC - low
A7
DATA #2 - low
2C
CRC - high
4A
DATA #3 - high
00
DATA #3 - low
00
CRC - low
0D
CRC - high
60
Summary of Contents for B90
Page 10: ...x B90 Low Impedance Bus Differential System GE Multilin TABLE OF CONTENTS ...
Page 284: ...5 166 B90 Low Impedance Bus Differential System GE Multilin 5 8 TESTING 5 SETTINGS 5 ...
Page 334: ...10 8 B90 Low Impedance Bus Differential System GE Multilin 10 2 BATTERIES 10 MAINTENANCE 10 ...
Page 338: ...A 4 B90 Low Impedance Bus Differential System GE Multilin A 1 PARAMETER LISTS APPENDIX A A ...
Page 460: ...C 30 B90 Low Impedance Bus Differential System GE Multilin C 7 LOGICAL NODES APPENDIX C C ...
Page 476: ...E 10 B90 Low Impedance Bus Differential System GE Multilin E 1 IEC 60870 5 104 APPENDIX E E ...
Page 502: ...viii B90 Low Impedance Bus Differential System GE Multilin INDEX ...