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Operation X-317™ Users Manual 5.3.4 Read Holding Registers – Modbus Function Code 03 (0x03) The Read Holding Registers function is used to read the analog output values. 32-bit values are read from 16-bit register pairs. Consequently, the addresses and registers must be even numbers. 5.3.4.1 Request Modbus TCP Request Frame Example – Read analog output 1 and 2 Field Name Length Function Example Data Transaction Identifier 2 bytes Synchronization number. 0x0001 Protocol Identifier 2 bytes Zero for Modbus/TCP 0x0000 Message Length 2 bytes Number of bytes in frame (below) 0x0005 Unit Identifier 1 byte Slave Address (0xff if not used) 0xFF Function Code 1 byte Read Holding Register 0x03 Start Address 2-bytes Start Address 0x0510 Quantity of Registers 2 bytes Quantity of Registers 0x0002 5.3.4.2 Response Each value is returned as two registers in IEEE 754 floating point formats. The four data bytes are treated as two individual big-endian 16-bit words with the least significant word being sent first. In other words, the 32-bit floating point number represented as '1234 ABCD' is sent as an 'ABCD 1234.' The device returns a value of 0xFFFFFFFF (NaN) if there isn't a valid reading or when the output is in an alarm condition. X-317 Response Frame Example – Read Sensor 1 and 2 Page 46 Field Name Length Function Example Data Transaction Identifier 2 bytes Synchronization number. 0x0001 Protocol Identifier 2 bytes Zero for Modbus/TCP 0x0000 Message Length 2 bytes Number of bytes in frame (below) 0x000C Unit Identifier 1 byte Slave Address (0xff if not used) 0xFF Function Code 1 byte Read Holding Registers 0x03 Byte Count 1 byte Byte Count 0x08 Analog Out 1 4-bytes Data Value (0x00000000 – 0xFFFFFFFF) 0x999A41B1 Analog Out 2 4-bytes Data Value (0x00000000 – 0xFFFFFFFF) 0xB85142D2 Xytronix Research & Design, Inc.
