MODBUS System Set Up
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COMMUNICATIONS
In a MODBUS system using radio or wireline communications, only the controller speaks
MODBUS. For queries addressed to RTUs, the controller translates back and forth between
the MODBUS protocol and the RTU radio protocol.
There are two ways to set up a MODBUS system using the Model 1700 controller with the
MODBUS option: Polled-only and report-by-exception.
Polled-Only System
In a polled-only system, the “master” PC or PLC controls all communication in the system.
When a MODBUS query is sent to an RTU through the Model 1700 controller, the controller
always translates the query to the RTU FFSK protocol and sends it out over the radio. When
the RTU responds, the response is translated from RTU FFSK protocol to MODBUS
protocol and sent back to the master through the serial port of the Model 1700 controller.
Configuring a system as polled-only gives the master complete control over communications,
particularly use of the radio channel, which can be a bottleneck. Radio communications
occur only when the master polls an RTU. The RTUs never transmit except in response to a
poll. There is no possibility of data collisions, which completely eliminates one cause of
communication failures.
On the other hand, there is one major disadvantage to a polled-only system. The master will
not be able to see changes in RTU input status that occur between polls. It only sees the
current input status at the time each poll response is sent. Any RTU input changes that occur
between polls are lost.
Report-By-Exception System
In a report-by-exception system, RTUs report changes in I/O status as they occur without
waiting to be polled by the master. The Model 1700 controller stores this status within its
own memory. When the master polls for status from an RTU, the Model 1700 controller
builds the poll response from this stored status, and sends it back to the master immediately.
A radio transmission occurs only if the master tries to control an RTU output or asks for
RTU status that the Model 1700 controller does not have. In this case, the Model 1700 must
try to obtain the RTU status over the radio before it can build the response to send back to the
master.
A report-by-exception system should also use the Model 1700 controller autopoll feature.
This causes the Model 1700 to periodically poll the input status for all RTUs in the system. If
an RTU or its radio fails, the Model 1700 will not get a response to the poll and it will
invalidate the status of the RTU. The next time the master tries to poll that RTU, a
communications failure will occur unless the RTU has been fixed.
The advantage to a report-by-exception system is that the master sees changes in RTU I/O
status almost immediately. The main disadvantage is that data collisions can occur between
polls from the master and exceptions from RTUs or between exceptions from different
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