Datakom DKM-409, Руководство пользователя

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J38D01 - 27 -
9. MODBUS COMMUNICATIONS
9.1 DESCRIPTION
The unit offers serial data communication port allowing it to be integrated in automation
systems.
The serial port is of RS-485 MODBUS-RTU standard. It is fully isolated from power supply
and measurement terminals for failure-free operation under harsh industrial conditions.
The MODBUS properties of the unit are:
-Data transfer mode: RTU
-Serial data: 9600 bps, 8 bit data, no parity, 1 bit stop
-Supported functions:
-Function 3 (Read multiple registers)
-Function 6 (Write single register)
-The answer to an incoming message is sent with a minimum of 4.3ms delay after
message reception.
Each register consists of 2 bytes (16 bits). Larger data structure contain multiple registers.
Detailed description about the MODBUS protocol is found in the document “ Modicon
Modbus Protocol Reference Guide” . This document may be downloaded at:
www.modbus.org/docs/PI_MBUS_300.pdf
Data Reading
The function 03 (read multiple registers) will be used for data reading. The MODBUS
master will send a query. The answer will be one of the below:
-A response containing the requested data
-An exceptional response indicating a read error.
The maximum number of registers read in one message is 123. If more registers are
requested, the unit will send only the first 123 registers.
The query message specifies the starting register and quantity of registers to be read. The
message structure is below:
Byte Description Value
0 Controller address 1 to 253
1 Function code 3
2 Starting address high See below the description of available
registers
3 Starting address low
4 Number of registers high always 0
5 Number of registers low max 7Bh (123 decimal)
6 CRC low byte See below for the checksum calculation
7 CRC high byte
Here is the sequence to read 16 registers starting from address 20h (32 decimal):
01 03 00 20 00 10 45 CC (each byte is expressed as 2 hexadecimal characters)
The checksum value in the above message may be used for the verification of checksum
calculation algorithm.