Land 4500 MKII Premier, Operation And Maintenance Manual

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Model 4500 Premier Dust and Opacity Monitor
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B2-3 MODBUS Communications
MODBUS is a communications system based on the RS232/RS485 three wire half duplex hardware
standard and provides direct access to the Model 4500 Premier system data via the instrument serial
port. MODBUS can be used to:
a) read any of the parameters measured by the Instrument
b) check the instrument status
c) initiate a check cycle calibration
NOTE
Changing the calibration constants may fundamentally affect the operation of the instrument.
The instrument functions as a MODBUS slave device in accordance with the MODBUS protocol (as
defined by Modicon)
Details of the MODBUS standard are available at www.modbus.org.
Before communicating with Model 4500 Premier via the MODBUS interface, it is necessary to set the
‘Station No.’ (the MODBUS slave address) and the ‘Link’ options to an appropriate value. The two
options are located in the ‘Miscellaneous’ menu under ‘Setup’ on the menu spine (see Setup Screens
section C4-4).
The system data for the Model 4500 Premier is accessible via register numbers as shown on the
‘System Data Register Table’ shown in this section.
NOTE
There is a possible source of confusion in the definition of “Register Numbers” used in
MODBUS functions. For historical reasons, user reference numbers are expressed as
decimal numbers with a starting offset of 1; so a Modbus message requesting a read at
Address Offset 0 would return the value in Register 1. The System Data Register Table
given uses Register Numbers rather than address offsets.