User manual

NIC Setup and Testing

Function Code:

The function code field tells the addressed slaves what function

to perform. Function codes are specifically designed invoke a

specific action by the slave device. The function code range is

from 1 to 127. However, the OpenComms NIC module

primarily uses Function Code 3 (Read Holding Registers) and

Function Code 6 (Preset Single Register).

Data Field(s):

The data field varies in length depending on whether the

message is a request or a response to a packet. This field

typically contains information required by the slave device to

perform the command specified or to pass back data to the

master device.

Error Check Field:

The Error Check Field consists of a 16-bit (2 byte) Cyclical

Redundancy Check (CRC16). It allows the receiving device to

detect a packet that has been corrupted with transmission errors.