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Installation and Configuration Manual 67
Modbus Communications
Calculated Parameters Modbus CommunicationsSafety CertificationFactory Default Settings Calculated Parameters Modbus CommunicationsSafety CertificationFactory Default Settings Calculated Parameters Modbus CommunicationsSafety CertificationFactory Default Settings Calculated Parameters Modbus Communications
Safety Certification
Factory Default Settings
Appendix D
Modbus Communications
D.1 Overview
The Modbus/RS-485 communications facility on the 7829 Viscomaster
®
/ Viscomaster Dynamic
™
viscosity meter can be useful in a number of ways. It is the only means of configuring the meter, 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 RS-485 serial interface of the 7829 Viscomaster
®
/ Viscomaster Dynamic
™
viscosity meter
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.
A Modbus network can have only one Master at any one time, with up to 32 Slaves. The 7829
Viscomaster
®
/ Viscomaster Dynamic
™
viscosity meter 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 7829 Viscomaster
®
/ Viscomaster Dynamic
™
viscosity meter is fully
compliant with the Modicon Specification. All information is stored in memory locations in the 7829
Viscomaster
®
/ Viscomaster Dynamic
™
viscosity meter referred to as Modbus Registers. These store
all the data required to control the operation, calculations and data output of the 7829 Viscomaster
®
/
Viscomaster Dynamic
™
viscosity meter. Modbus communication with the meter consists of reading or
writing to these registers.
The 7829 Viscomaster
®
/ Viscomaster Dynamic
™
viscosity meter implements only two Modbus
commands:-
• Command 3: Read Modbus Register
• 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, the Micro Motion ADView or ProLink II software program
enables you to configure the meter, 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.