Hach Sigma 950 Instrument Manual page 120

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SCADA-Modbus System Guidelines
Problem: The data values being returned by polling the meter with Modbus
are not the same as the data values displayed in the current status screen of
the meter.
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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 950 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.
Response: Confirm that the correct register addresses are being polled.
Check to make sure the register address being polled corresponds to the correct data
channel. For example, if polling for FLOW, make sure the server or MMI is requesting
data from registers 40033–40034.
If polling for several values at the same time, try changing the polling so that only one
value is polled at a time. Then check to see if the polled value matches a different data
channel in the meter. For example, if polling for Level and it appears that you are getting
the data for Velocity instead, you probably are polling the wrong registers.
Response: Check the data format of the Modbus server.
When configuring a Modbus server or MMI application to poll a 950 Flow Meter, it is
absolutely essential that the correct data format is selected for that particular data
channel (register). For example, when polling for Flow, Level or Velocity, which are all
floating point values, the Modbus server or MMI must be configured to read these values
as floating point values. If the server or MMI is formatting this data as a data type other
then floating point, the values will not be read or displayed correctly.
Likewise, if polling the meter for engineering units, which are represented by integer
values, such as Flow Units of Measure or Level Units of Measure, the Modbus server or
MMI must be configured to read these values as Integers. If the server or MMI is
formatting this data as any data type other than Integer, the values will not be read or
displayed correctly.
Different Modbus servers and MMI manufacturers have different methods for configuring
the application to the appropriate data type contained within the register. Contact the
server or MMI manufacturer for details on how to configure the application to read the
data in the correct format.