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Modbus functions
11.4.7
Standard
(i)
The functions of the Modbus model are divided into four types: [1] Coil, [2] Discrete input, [3]
Input Register, and [4] Holding resister. Each function type is designed to read/write the IED
signal† (memory in slave) from the substation computer request (i.e., master query).
#
Function types
1
Coil
2
Discrete input
3
Input register
4
Holding register
Note: Chapter
†
Data ID (i.e., the bit length of that) by seeing the codes. Default signal mapping
information is shown in
diagnostic register
The master packs a function code (FC) in a Modbus message frame; thus, the slave can identify
and know the message by reading the FC of message frame (see Figure 11.4-7 and Figure
11.4-10).
Function codes
(Decimal)
0x01 (1)
0x02 (2)
0x03 (3)
0x04 (4)
0x05 (5)
0x06 (6)
0x0F (15)
0x10 (16)
[Type1] Coil (Function codes: 0x01, 0x05, and 0x0F)
Table 11.4-2FC types and addresses in the IED
Data type (bits length)
1-bit
1-bit
8/16-bits
32-bits
64-bits
8/16-bits
32-bits
64-bits
Introduction
(Table 1.7-2) illustrates how to know the data length of the
Appendix Modbus address, exception status, return
(→p. 1110).
Table 11.4-3 Function codes
Function type
Coil
Discrete input
Holding Register
Input register
Coil
Holding register
Coil
Holding register
The Coil type is provided to read/write the memory in the IED (slave) from
the substation computer (mater). For example, it can be used to write an
operation signal (e.g., On[1:True decision] or Off[0:False decision]) in the IED,
and it is used to read the operated signal (e.g., Operated[1:Running] or
Halted[0:Stopping]). The functions of the Coil type is designed accessing 1-
bit information. The addresses for the Coil type do not have default settings.
Function 0x0F(15) is supported in the IED (slave), hence the substation
Read / Write
Modbus addresses†
Read / Write
Read-Only
Read-Only
Read /Write
R/W mode
Master query notes (signal length)
Read
Read some signals at Data IDs (1-bit)
Read
Read some signals at Data IDs (1-bit)
Read
Read some values at Data IDs(8~64-bits)
Read
Read some values at Data IDs(8~64-bits)
Write
Write On (or Off) at Data IDs(1-bit)
Write
Write a value at Data IDs (8~64-bits)
Write
Write On/Off states at Data IDs(1-bit)
Write
Write values at Data IDs (8~64-bits)
- 927 -
6F2T0207 (0.01)
Function codes
00000~00999
0x01,05,15
01000~09999
10000~10999
11000~12999
13000~16999
17000~17999
18000~19999
0x03, 06, 16
20000~23999
GRE200 (5,6)
0x02
0x04
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