Chapter 3 - Device Overview
UNITROL 1020 User Manual 3BHS335648 E82 75
3.4.6 Modbus for Remote Access
Remote Access is a feature that allows monitoring and control of the AVR using the
MODBUS protocol. It can be suitable for applications where the control from remote
locations is preferred (i.e. offshore) and/or a custom user interface is required.
The following sections explain the Remote Access software and configuration. The
electrical connections are described in Chapter 3.3.9 Communication Ports. It is
recommended to read Chapter 3.4.7 Access Levels prior to this section, in order to fully
understand the software capabilities.
3.4.6.1 Overview
A remote terminal configured as MODBUS master can access the device through RS485
or Ethernet TCP/IP using Remote Access. The communication choice is up to the
application requirements but since VDC needs a dedicated RS485 connection, Ethernet
TCP/IP is the only choice when VDC is used. When VDC is not needed, Remote Access
can be performed using the RS485 interface. More information about the communication
interfaces can be found in 3.3.9 Communication Ports.
The Remote Access feature allows the user to read and write registers (measurements,
setpoints and other information) from the AVR. The document that describes the
MODBUS Registers Table is normally included in the CD-ROM that comes with new
devices. It can also be ordered from ABB. The manufacturer details and contact
information can be found in Chapter 1.5 - Manufacturer’s Address.
The feature supports up to one MODBUS remote terminal and it must be assured that
requests are sent from only one remote terminal using the same communication interface
(RS485 or Ethernet TCP/IP). If more than one request comes from different interfaces,
they might be answered via the same interface from where they come. This method is
however not recommended and should not be used.
In order to communicate with the device, the remote terminal should gain a determined
access level in the AVR according to the operation desired: Monitor Access for reading
only and Control Access for reading and writing to registers. More information about
Access Levels can be found in Chapter 3.4.7 - Access Levels.
3.4.6.2 Compatibility
The Remote Access feature over RS485 cannot be used together with VDC. If both
Remote Access and VDC are required, Remote Access should be used via TCP/IP using
an external gateway. When VDC mode is not required, Remote Access can be used via
either RS485 or Ethernet. For more information see Chapter 3.3.9 - Communication
Ports.
3.4.6.3 Configuration
Remote Access over RS485 has a number of parameters that should be configured:
Protocol (Modbus RTU or TCP), Activation (of the feature over RS485), Baud Rate,
Answer delay and others shown in Chapter 6.4.7 Communication Menu.
The Modbus Slave ID value is used for both Remote Access over RS485 and Ethernet
TCP/IP; there is no differentiation of the physical interface. This number can be
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