Communication applications – Micromod MOD: Modcell 2050R MODBUS Communications User Manual

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IB-23C650M

COMMUNICATION APPLICATIONS

The following are sample applications available with the controller through the
use of Modbus communications. The RS-485 MENU is used to set up the
controller for these applications. The setup requirements are described in
detail in Table 1.

Standard Control With Modbus Communications

For this application, the instrument acts as a stand alone single loop controller
which can receive read/write commands from a host Modbus device such as a
personal computer, MODCELL Multiloop Processor, PLC, etc. Multiple
instruments and other devices can be on a single Modbus network. Each
device on a network must have a unique address. The instrument address is
assigned using the Bus Address attribute in the RS-485 MENU.

During runtime operation, the last attribute entry to the controller becomes the
active entry. For example, if the operator sets an active set-point value via the
instrument display and moments later the host sends a new active set-point
value, the controller uses the operators value until it receives the host value.
At that time the host value becomes the new active set-point value.

A prompt in the DISPLAYS MENU permits the operator to disable the host
from writing to the controller. The host can still receive data from the
instrument, but can not write to it. This is useful if the operator must maintain
manual control of the process and does not want the host to write to the
instrument. The operator can reinitiate the host write capacity via the
DISPLAYS MENU when required.

The Computer Timeout attribute is not active in this application. When
necessary, this attribute is initialized by the host as a means of notifying the
instrument of a communication failure. In this application, failure of the host or
the communications has no effect on instrument operation. The controller
continues to operate with its current attribute values as if nothing happened.

Supervisory Control

The controller receives its set-point from the host device in a supervisory
control application. All other aspects of controller operation are similar to
standard control as described above. The PID control function is provided by
the controller.