perform. Slave response packet should include same function field byte as
request. The Modbus functions supported by SPM33 are listed as below:
Function
Code
Meaning
Action
0x01
Read Relay Output
Status
Obtains ON/ OFF information of one or
more relay output in SPM33 (0/1)
0x03 Read
Holding
Registers
Obtains the current value in one or more
holding registers of the SPM33.
0x05
Relay control
Write 0xFF00 to close (ON) the relay
Write 0x0000 to open (OFF) the relay
0x10 Preset
Multiple
Registers
Places specific binary values into a series
of consecutive holding registers of the
SPM33
z
Data Field
The length of Data Field is varies in length depending on its function. In general,
MODBUS supports “BIG INDIAN” mode, it means high-order byte first,
low-order byte second.
For example,
30