Modbus TCP/IP Functions
General Description
The Modbus messaging service provides a client/server communication between devices connected on an Ethernet TCP/IP network.
The client/server model is based on four type of messages:
-
Modbus Request, the message sent on the network by the client to initiate a transaction.
-
Modbus Indication, the request message received on the server side.
-
Modbus Response, the response message sent by the server.
-
Modbus Confirmation, the response message received on the client side.
The Modbus messaging services (client/server model) are used for real time information exchange between:
-
Two device applications.
-
Device application and other device.
-
HMI/SCADA applications and devices.
-
A PC and a device program providing on line services.
A dedicated header is used on TCP/IP to identify the Modbus Application Data Unit. It is called the MBAP header (Modbus Application Protocol header).
The MBAP header contains the following fields:
|
Fields |
Length |
Description |
Client |
Server |
|---|---|---|---|---|
|
Transaction Identifier |
2 bytes |
Identification of a Modbus Request/Response transaction |
Initialized by the client |
Recopied by the server from the received request |
|
Protocol Identifier |
2 bytes |
0 = Modbus protocol |
Initialized by the client |
Recopied by the server from the received request |
|
Length |
2 bytes |
Number of following bytes |
Initialized by the client (Request) |
Initialized by the server (Response) |
|
Unit Identifier |
1 byte |
Identification of a remote server connected on a serial line or on other buses |
Initialized by the client |
Recopied by the server from the received request |
Table of Modbus Functions
The following table describes in detail the functions supported by the wireless devices of the PowerTag Link gateway:
|
Function Code |
Function Name |
|---|---|
|
01 |
Read n output or internal bits |
|
02 |
Read n input bits |
|
03 |
Read n output or internal bits |
|
05 |
Write 1 bit |
|
06 |
Write 1 word |
|
08 |
Modbus diagnostic data (refer to Function 8: Modbus Diagnostics) |
|
15 |
Write n bits |
|
16 |
Write n words |
|
43-14 |
Read identification (refer to Function 43-14: Read Device Identification (Basic)) |
|
43-15 |
Read the date and time (refer to Function 43-15: Read Date and Time) |
|
43-16 |
Write the date and time (refer to Function 43-16: Write Date and Time) |
|
100-4 |
Read non-adjacent words where n ≤ 100 (refer to Function 100-4: Read Non-Adjacent Registers) |