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DasyLab Platinum Modbus Interface
2 Modbus Interface
The Modbus interface is fully described in MODBUS APPLICATION PROTOCOL SPECIFICATION
(V1.1b3).
The Modbus specification allows accessing to up 65535 internal 'holding' registers using register
READ, register WRITE and WRITE MULTIPLE commands. Each Modbus holding register is defined
as a 16 bit entity structured as BIG ENDIAN values (most significant byte always presented first).
The Platinum Modbus interface provides access to the internal database of the Platinum product
family by internally mapping Modbus holding registers to specific database items.
Modbus is structured using a MASTER-SLAVE topology, in which there is one MASTER device and
up to 255 slave devices. All transactions are initiated by the MASTER device.
Modbus slave devices are individually accessed using a one byte SLAVE address. The MASTER
device initiates a transaction by sending a request packet to a specific slave. The SLAVE device
processes the transaction and returns either response packet indicating success or failure.
Address 0 is reserved as a 'broadcast' address, in which all slave devices will accept and process the
transaction but will not send a response.
2.1 Modbus Functions
The Platinum Modbus interface supports the following Modbus FUNCTION requests.
Function
Code
0x03
Read Holding Register
0x06
Write Single Register
0x07
Read Exception status
0x08
Diagnostic
0x10
Write Multiple Registers
0x0b
Get Comm events
2.2 Data Formats
Modbus holding registers are represented as 16 bit entities. The following encoding is used for
extended data items. Note that 'byte 0' will be the first byte received/transmitted.
For data types that can be represented in 16 bit (Boolean, byte, char, int16 and uint16) a single
register is used.
For data types that require 32 bits two consecutive registers are used. The lower number register will
represent the most significant data. The 2
2.2.1 Multiple Register Reads
When reading a dual register entity the lower order register should be used as the requested 'holdiing
register', with a request for a minimum of 2 registers. Internally the entire entity is read and data is
then built into a response packet.
Document Number
For Internal Use Only: This document contains confidential, proprietary information of Omega Engineering/Newport Electronics,
The Document may not be copied or reproduced without the prior, written permission of Omega Engineering/Newport Electronics
Mnemonic
Reads one or more consecutive 16 bit holding registers
Writes a specific 16 bit holding register
Reads structured status information
Read/Write diagnostic information
Write one or more consecutive 16 bit holding registers
Read communication event counters
nd
register represents the leas significant data.
Revision 1.0
Description
Page 4 of 11
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