Instruction manual

MODBUS System Set Up
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rapidly changing signals could be polled more often than less
important or slowly changing signals.
Timeout Periods
The timeout period should be the same for both poll-only and
report-by-exception systems. The period should be set to minimum
time that it takes for a poll to go out over the radio to an RTU and
for the response to return to the master. A typical value is five
seconds. However, if the RTUs are doing store-and-forward, or
there is a lot of traffic on the radio channel, a longer timeout may be
needed.
Retries
The master can usually be configured to retry a poll for a specified
number of times before giving up and signaling a communications
failure for that RTU. The appropriate setting for retries is really
system dependent, but a value of 2 or 3 is usually a good starting
place.
BASIC MODBUS I/O ADDRESSING
The basic I/O capability of the Model 1700 controller consists of four digital outputs. These
are open-collector outputs capable of sinking up to 100 mA and dealing with voltages up to
50 V
dc
.
In the MODBUS protocol, each I/O point is given a unique address. This address not only
identifies a unique point within the system, it also tells the master what type of I/O point the
address identifies.
The MODBUS protocol also has its own nomenclature for the various I/O types. It also has
its own set of ranges for data. Table 14 shows the relationship between MODBUS
nomenclature for I/O addressing and the Zetron nomenclature used in the Model 1700,
Model 1708, and the Model 1716.
Table 14. MODBUS versus Zetron I/O Addressing
Zetron Values MODBUS Values
I/O Type
Address
Range
Value
Range I/O Type
Address
Range
Value Range
(Raw)
Digital Outputs 1 to 16 OFF, ON Coils 1 to 16 0 (off), 1 (on)
Digital Inputs 1 to 16 Open Circuit,
Grounded
Input Status 10001 to
10016
0 (open circuit)
1 (grounded)
Analog Inputs 1 to 8
0 to 5 V or
0 to 20 mA
Input Registers
30001 to
30008
0 to 32767
Analog Outputs 1 to 4 0 to 5 V Holding
Registers
40001 to
40004
0 to 32767