PXM PX340, Руководство пользователя

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9.2. Modbus
The controller supports the Modbus protocol. A PX340/PX345/PX710 controller operates as a
server (slave) to which an application / a device operating as a client (master) connects. The
master establishes a connection through TCP port 502 (a standard Modbus port) using the
current IP address of the controller. Information is exchanged on the following principle: question
from master → answer from slave. At any given time, only one master can be connected
to the controller.
In Modbus TCP protocol, data are transmitted in packets referred to as ADUs (application data
units). An ADU is composed of a header (MBAP header) and a PDU (protocol data unit).
According to the standard, all data sent using the Modbus protocol are represented as big endian
(most significant byte coming first).
Single packet
MBAP Header Command Data
PDU
Single packet (ADU)
Field Description Size
MBAP
Transaction ID
Packet number - for purposes of simplification it can
be be equal to 0
2B
Protocol ID For the Modbus protocol it is always equal to 0 2B
Length
The number of subsequent data (after this field)
transmitted – in bytes
2B
Unit ID It is always equal to 0 or 255 1B
PDU
Function Code
A Modbus command – supported commands are set
out in table (a)
1B
Address/Action
For standard Modbus commands, these are register
addresses – table (d), for special commands these
are action numbers - table (c)
2B
Parameters Command-dependent – available in table (c)
Size in the
table (c)
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