290 • Common MODBUS functions
Function code FC15 (Force Multiple Coils)
Modular I/O System
ETHERNET TCP/IP
6.2.8
Function code FC15 (Force Multiple Coils)
Using this function a number of output bits are set to 1 or 0. The maximum
number is 256 bits.
Request
The first point is addressed with 0.
The inquiry message specifies the bits to be set. The requested 1 or 0 states are
determined by the contents of the inquiry data field.
In this example 16 bits are set, starting with the address 0. The inquiry con-
tains 2 bytes with the value 0xA5F0 or 1010 0101 1111 0000 in binary format.
The first byte transmits the 0xA5 to the addresses 7 to 0, whereby 0 is the low-
est value bit. The next byte transmits 0xF0 to the addresses 15 to 8, whereby
the lowest value bit is 8.
Byte
Field name
Example
Byte 0, 1
Transaction identifier
0x0000
Byte 2, 3
protocol identifier
0x0000
Byte 4, 5
Length field
0x0009
Byte 6
unit identifier
0x01 not used
Byte 7
MODBUS function code
0x0F
Byte 8, 9
reference number
0x0000
Byte 10, 11
Bit Count
0x0010
Byte 12
Byte Count
0x02
Byte 13
Data Byte1
0xA5
Byte 14
Data Byte2
0xF0
Response
Byte
Field name
Example
.....
Byte 7
MODBUS function code
0x0F
Byte 8, 9
Reference number
0x0000
Byte 10, 11
Bit Count
0x0010
Exception
Byte
Field name
Example
.....
Byte 7
MODBUS function code
0x8F
Byte 8
Exception code
0x01 or 0x02