Decompose requests¶
Modbus requests and responses contain an Application Data Unit (ADU) which contains a Protocol Data Unit (PDU). The ADU is an envelope containing a message, the PDU is the message itself. Modbus requests can be sent via two communication layers, RTU or TCP/IP. The ADU for these layers differs. But the PDU, the message, always has the same strcuture, regardless of the way it’s transported.
PDU¶
Note
This section is based on MODBUS Application Protocol Specification V1.1b3
The Protocol Data Unit (PDU) is the request or response message and is indepedent of the underlying communication layer. This module only implements requests PDU’s.
A request PDU contains two parts: a function code and request data. A response PDU contains the function code from the request and response data. The general structure is listed in table below:
| Field | Size (bytes) |
| Function code | 1 |
| data | N |
Below you see the request PDU with function code 1, requesting status of 3 coils, starting from coil 100.
>>> req_pdu = b'\x01 d\x00\x03'
>>> function_code = req_pdu[:1]
>>> function_code
b'\x01'
>>> starting_address = req_pdu[1:3]
>>> starting_address
b'\x00d'
>>> quantity = req_pdu[3:]
>>> quantity
b'\x00\x03'
A response PDU could look like this:
>>> resp_pdu = b'\x01\x01\x06'
>>> function_code = resp_pdu[:1]
>>> function_code
b'\x01'
>>> byte_count = resp[1:2]
>>> byte_count
b'\x01'
>>> coil_status = resp[2:]
'b\x06'
ADU for TCP/IP requests and responses¶
Note
This section is based on MODBUS Messaging on TCP/IP Implementation Guide V1.0b.
The Application Data Unit (ADU) for Modbus messages carried over a TCP/IP are build out of two components: a MBAP header and a PDU. The Modbus Application Header (MBAP) is what makes Modbus TCP/IP requests and responsen different from their counterparts send over a serial line. Below the components of the Modbus TCP/IP are listed together with their size in bytes:
| Component | Size (bytes) |
| MBAP Header | 7 |
| PDU | N |
Below you see an hexidecimal presentation of request over TCP/IP with Modbus function code 1. It requests data of slave with 1, starting at coil 100, for the length of 3 coils:
>>> # Read coils, starting from coil 100 for the length of 3 coils.
>>> adu = b'\x00\x08\x00\x00\x00\x06\x01\x01\x00d\x00\x03'
The length of the ADU is 12 bytes:
>>> len(adu)
12
The MBAP header is 7 bytes long:
>>> mbap = adu[:7]
>>> mbap
b'\x00\x08\x00\x00\x00\x06\x01'
The MBAP header contains the following fields:
| Field | Length (bytes) | Description |
| Transaction identifier | 2 | Identification of a Modbus request/response transaction. |
| Protocol identifier | 2 | Protocol ID, is 0 for Modbus. |
| Length | 2 | Number of following bytes |
| Unit identifier | 1 | Identification of a remote slave |
When unpacked, these fields have the following values:
>>> transaction_id = mbap[:2]
>>> transaction_id
b'\x00\x08'
>>> protocol_id = mbap[2:4]
>>> protocol_id
b'\x00\x00'
>>> length = mbap[4:6]
>>> length
b'\x00\x06'
>>> unit_id = mbap[6:]
>>> unit_id
b'\0x01'
The request in words: a request with Transaction ID 8 for slave 1. The request uses Protocol ID 0, which is the Modbus protocol. The length of the bytes after the Length field is 6 bytes. These 6 bytes are Unit Identifier (1 byte) + PDU (5 bytes).
ADU for RTU requests and responses¶
Note
This section is based on MODBUS over Serial Line Specification and Implementation Guide V1.02.
The ADU for Modbus RTU messages differs from Modbus TCP/IP messages. Messages send over RTU don’t have a MBAP header, instead they have an Address field. This field contains the slave id. A CRC is appended to the message. Below all parts of a Modbus RTU message are listed together with their byte size:
| Component | Size (bytes) |
| Address field | 1 |
| PDU | N |
| CRC | 2 |
The CRC is calculated from the Address field and the PDU.
Below you see an hexidecimal presentation of request over RTU with Modbus function code 1. It requests data of slave with 1, starting at coil 100, for the length of 3 coils:
>>> # Read coils, starting from coil 100 for the length of 3 coils.
>>> adu = b'\x01\x01\x00d\x00\x03=\xd4'
The lenght of this ADU is 8 bytes:
>>> len(adu)
8