Solartron Mobrey 7828, Техническое руководство

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78285000_AA
D-1
D
Modbus Communications
D.1 INTRODUCTION
The Modbus/RS485 communications facility on the 7828 can be useful in a number of
ways. It is the only means of configuring the transmitter, and also gives access to
diagnostic information not available on the analog output. Digital representations of the
measured and calculated parameters are also available which lead to higher accuracy,
and greater integration in digital networks and systems.
The RS485 serial interface of the 7828 Density Transmitter communicates using the RTU
Modbus protocol, which is a well established system used in many industrial applications.
The protocol defines the way in which messages will be transmitted between Modbus
devices, and details how the data will be formatted and ordered.
It is beyond the scope of this manual to give a full description of the protocol, but a useful
reference on Modbus is the "Modbus Protocol Reference Guide" (PI-MBUS-200 Rev.D).
(1992) published by Modicon Industrial Automation Systems Inc., Massachusetts.
A Modbus network can have only one Master at any one time, with up to 32 Slaves . The
7828 acts as a slave device, and only communicates on the network when it receives a
request for information from a Master device such as a computer or a PLC.
The implementation used on the 7828 is fully compliant with the Modicon Specification. All
information is stored in memory locations in the 7828 referred to as
Modbus Registers.
These store all the data required to control the operation, calculations and data output of
the 7828. Modbus communication with the transmitter consists of reading or writing to
these registers.
The 7828 implements only two Modbus commands:-
•
Command 3 Read Modbus Register , and
•
Command 16 (10
16
) Write Modbus Register .
Any number of registers can be read with Command 3, but only one register can be
written to for each Command 16. This restriction does not limit the performance of the
system, since all functions are mapped into the register structure in one way or another.
In most cases, it is unnecessary to understand the detail of the protocol, as this is taken
care of by the application program. For example, Solartron’s ADView software enables
you to configure the transmitter, and even read or write to individual Modbus registers,
without you needing to know about Modbus.
However, if you are using a proprietary software package, or developing your own
application software, the information given in this section will be invaluable.