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Appendix E
Page 141
SCADA-Modbus® System Guidelines
that range includes register addresses greater then 40083, the entire request
will be ignored
Response: Check the number of registers being polled
Additionally, the 980 Flow Meter checks all Modbus messages to see if the
correct number of registers is requested for the type of data being returned.
The meter will ignore the request if the number of registers requested does
not coincide with the correct number of registers needed to accurately display
the data. For example, Velocity is a floating point value stored in register
40009–40010. Because all floating point values require two registers, the
meter would ignore a request to read just the data in register 40009, yet it
would respond correctly to a request to read the data stored in both registers
40009 AND 40010. Consequently, if the meter received a single request to
read both Level 40007–40008 and Velocity 40009–40010, the request would
have to be for an even number of registers for the meter to respond.
Problem: 980 Flow Meter does not respond to any Modbus message requests
Note:
Note: It is imperative that the DTR
be asserted prior to the
communication session and that it
remains asserted throughout the
entire communication session.
Response: Check the DTR Signal/Line
The 980 Flow Meter will not respond to any Modbus messages until the
device connected to the RS232 port asserts (raises) the DTR line (DB-9,
Pin 4 on the 1727 cable).
Response: Check the Baud Rate
The baud rate of the 980 Flow Meter is configured from 1200–19,200 and
must match the baud rate of the device communicating with the meter.
Response: Check the Communication Parameters
The communications parameters of the 980 Flow Meter meter are fixed
(except for the baud rate) and can not be changed. The device
communicating with the flow meter must be configured with the exact same
communication parameters as the meter. These parameters are as follows:
Response: Check the Modbus Device Address assigned to the
980 Flow Meter
Modbus devices, including the 980 Flow Meter, have a unique configured
device address in the range of 1 to 247. This address is embedded in the first
two characters of the Modbus message. The flow meter will only respond to
messages encoded with the same address as the meter. If the meter receives
a valid Modbus message with an encoded device address other than the
address the meter is configured for, it will ignore that message.
Response: Check the Modbus mode
There are two different forms of Modbus: ASCII and RTU. Currently the
980 Flow Meter only support Modbus ASCII. Consequently the device
communicating with the meter must be setup for Modbus ASCII. The meter
will not respond to Modbus RTU messages.
• 7 Data Bits • 1 Stop bit
• 1 Start bit • Even parity