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Chapter 12
Modbus Interface
12.1 About this chapter
This chapter discusses the following topics:
• Modbus and the Model 3711 memory structure
• Configuring the Modbus interface
• Accessing Model 3711 data via the Modbus interface
Note: This Modbus interface and these Modbus registers are not the same as the Modbus interface
and Modbus registers documented in Section 13.5. Section 13.5 describes a method for reading and
writing to the Modbus registers in the core processor. This chapter describes how to use
ROCLINK 800 to configure the Modbus interface, and how a user-written Modbus program can read
and write to memory locations in the Model 3711.
12.2 Overview
The Model 3711 can use either Modbus or ROC protocol to communicate over the LOI and COMM 2
ports. ROCLINK 800 uses ROC protocol. Customer-written programs can use either Modbus or ROC
protocol. The Model 3711 automatically detects whether incoming communications are using
Modbus or ROC protocol.
When using Modbus protocol, the Model 3711 acts as a Modbus slave device.
Note: The Model 3711 can be ordered with a special user program which enables the Model 3711 to
function as a Modbus master. If this was ordered, it is preloaded on the device. If you need to add this
functionality, contact Micro Motion to purchase the user program. See the manual entitled
Modbus
Host User Program for information on programming and use.
The Modbus interface on the Model 3711 provides access to Model 3711 points and to history data,
alarm data, and event data. The interface can be configured to scale integer values, and to convert
floating-point values to and from integer values if required by the Modbus host.
12.3 Modbus overview
The Modbus protocol supports two modes of transmission: ASCII and RTU. All devices in the same
communications network must be configured to use the same transmission mode.