Instruction manual
MODBUS System Set Up
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RTUs. In a large system with lots of I/O activity, this can result in frequent communications
failures.
Which Is Better - Polled-Only or Report-By-Exception?
A polled-only system is easier to configure. Problems in a polled-only system can be much
easier to diagnose. For example, if a communications failure occurs, it can not be due to data
collisions, but is probably a true indication of a problem with the RTU, radio or
communications path. Therefore, a polled-only system should be favored over a report-by-
exception system.
A report-by-exception configuration may be needed if the system is very large so that the
polling loop takes a long time, or if some signal changes are short duration and need to be
reported automatically by the RTU. For a small system where a radio channel is also used for
voice communications, report-by-exception can reduce the amount of airtime used for data
transmission.
MODBUS MASTER REQUIREMENTS
In a MODBUS system, the “master” will typically be a PC running a software application
that is compatible with the MODBUS protocol. The following list contains some of the major
requirements for this “master” to be compatible with the Model 1700 controller and the
Model 1708/1716 RTUs.
• The serial port(s) on the PC must be capable of supporting a three-wire version of RS-
232 communications (Tx, Rx, ground). The communication parameters used are: 4800
baud, 8 data bits, no parity, and 2 stop bits.
• The application must have a configurable polling timeout period. The range must
allow enough time for a radio transmission to be sent to an RTU and for the response
to be sent back (3 to 5 seconds minimum).
• The application must limit itself to the command codes listed in Table 10.
• The application must support the MODBUS RTU protocol (as opposed to the
MODBUS ASCII or MODBUS Plus protocols).
Fortunately, most PC software packages that support MODBUS protocol follow these rules.
In some software packages, MODBUS protocol support is a standard feature. For others, the
MODBUS protocol must be purchased separately as an option. As long as the software meets
the above requirements, it will work with the Model 1700 controller.
Some of the software packages that work with the Model 1700 MODBUS option are listed
on page 36.