Revision 2.6
WebRelay-Quad™
Users Manual
3.3.3 Write Multiple Coils (Modbus Function Code 15 (0x0F))
A single byte can be written to set the state of all four relays. To do this, set the starting address to
0, set the quantity of outputs to 4, and write a single byte with the appropriate value according to
the table below.
Data Byte
(hex)
LS Data Bits
(binary)
Relay 4 State Relay 3 State Relay 2 State Relay1 State
0x00
0000
off
off
off
off
0x01
0001
off
off
off
on
0x02
0010
off
off
on
off
0x03
0011
off
off
on
on
0x04
0100
off
on
off
off
0x05
0101
off
on
off
on
0x06
0110
off
on
on
off
0x07
0111
off
on
on
on
0x08
1000
on
off
off
off
0x009
1001
on
off
off
on
0x0A
1010
on
off
on
off
0x0B
1011
on
off
on
on
0x0C
1100
on
on
off
off
0x0D
1101
on
on
off
on
0x0E
1110
on
on
on
off
0x0F
1111
on
on
on
on
Alternatively, to modify the state of one, two, or three relays only, the starting address and quantity
of outputs can be set to values other than 0, and 4 respectively.
Request
Modbus/TCP:
Transaction identifier (2 Bytes): 0x0001
Protocol identifier (2 Bytes): 0x0000
Length (2 Bytes): 0x0008
Unit identifier (1 Byte): 0xff
Modbus:
Function code (1 Byte): 0x0f (write coil)
Starting address (2 Bytes): 0x0000 ~ 0x0003
Quantity of outputs (2 Bytes): 0x0001 ~ 0x0004
Byte count (1 Byte): 0x01
Output value (1 Byte): 0x00 ~ 0x0f (see table above)
Character String Example:
char write_mult_coil_mb_request[] =
{0x00, 0x01, 0x00, 0x00, 0x00, 0x08, 0xff, 0x0f, 0x00, 0x00, 0x00, 0x01, 0x01,
0x0F };
Xytronix Research & Design, Inc.
page 35
