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J.3
Modbus PDU Description
The Modbus protocol defines a simple Protocol Data Unit (PDU) independent of the
underlying communication layer. When the Modbus PDU is parsed, the function code is
first controlled. In case of invalid data, a Modbus exception response is built. If the
function code is accepted, the request will be processed. Depending on the result of the
processing, two types of response can be built:
• A positive Modbus response: The response function code matches the request
function code.
• An Exception response:
o The objective is to provide to the client relevant information concerning
the error detected during the processing.
o The response function code is the request function code + 80h.
o The exception code is provided to indicate the reason of the error.
J.3.1 Supported Modbus Function Codes
This Section explains how to access Modbus data and how to use and understand
the Modbus subset implemented on the Hydran 201Ti. The guidelines provided below
first give an idea on the sequence required to master the communication interface.
Modbus officially defines function codes from 1 to 127, though only a small subset is
generally needed to gain access to all system data. The Hydran 201Ti's Modbus protocol
implementation only includes the function codes shown in Table J-4 below.
Page 246
is shifted into the MSbit of x, all other bits are shifted right one
location)
Table J - 3: Description of the Symbols Used in the CRC-16 Algorithm
Hydran* 201i Instruction Manual
Description
MA-033
Rev. 14.0, 12-Nov-18
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