ABB C250 User manual - 4.2
8
4.2
MODBUS Function Codes
The function code instructs the addressed slave which function to perform. Table 4.1
shows the function codes, and describes the action they initiate.
…4
MODBUS Protocol
Table 4.1 MODBUS Function Codes
*NAK = Negative Acknowledgment
Function
Code
Function
Title
Description
01
Read
Coil Status
Read up to 16 consecutive discrete (Boolean) points
from a specific starting point. The COMMANDER
250/V250 returns zeros for points which do not
contain defined data and NAKs* any request for point
numbers greater than 60.
Read up to 8 consecutive registers from a specific
starting register. The COMMANDER 250/V250 returns
zeros for points which do not contain defined data
and NAKs* any request for point numbers greater
than 90.
Write one discrete (Boolean) point. The COMMANDER
250/V250 NAKs* this if the point is not currently
writeable.
Write one register. This code also applies any existing
limits to the register before storage in the instrument.
The COMMANDER 250/V250 NAKs* if the register
is not currently writeable.
Echo the message. Only ‘Return of Query’ is
supported.
Write up to eight consecutive registers from a specified
starting register. The COMMANDER 250/V250 NAKs*
if any of the registers are not currently writeable, but
carries out all the writes which are valid, applying any
existing limits to the value before storage in the
instrument.
03
Read
Holding Register
05
Force
Single Coil
06
Preset
Single Register
08
Loop Back
16
Preset
Multiple
Registers