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5
Ethernet Modual
3) Function Field
The function code field of a message frame contains eight bits. Valid
codes are in the range of 1~255 decimal. When a message is sent from a
client to a server device the function code field tells the server what kind
of action to perform.
Code Meaning Action
01 Read Relay Output Status Obtain current status of Relay Output
02 Read Digital Input(DI) Status Obtain current status of Digital Input
03 Read Data
Obtain current binary value in one or more
registers
05 Control Single Relay Output Force Relay to a state of on or off
16 Write Multiple-registers
Place specific value into a series of consecutive
multiple-registers
Table 3-3
4) Data Field
The data field is constructed using sets of two hexadecimal digits, in
the range of 00 to FF hexadecimal. The data field of messages sent
from a master to slave devices contains additional information which
the slave must use to take the action defined by the function code. This
can include items like discrete and register addresses, the quantity of
items to be handled, and the count of actual data bytes in the field.
For example, if the master requests a slave to read a group of holding
registers (function code 03), and the data field specifies the starting
register and how many registers are to be read. If the master writes to
a group of registers in the slave (function code 10 hexadecimal), the
data field specifies the starting register, how many registers to write, the
count of data bytes to follow in the data field, and the data to be written
into the registers.