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SCADA-Modbus® System Guidelines
Appendix E
Since the Modbus protocol doesn't define how floating point values are
handled or stored, some Modbus-capable servers incorrectly use the normal,
“High word — Low word” format for converting the Modbus message
response to the client application. Since Modicon stores the floating point
values in the opp0odbus and floating point numbers.
Port Expanders and Protocol Converters
In some situations, there may not be a Modbus ASCII port available for use
with the 980 Flow Meter. A good example might be where there is a need to
install a flow meter at a remote pump site that already has a single Modbus
line connected to a PLC that is used to control the pumps.
Port expanders are available from third party manufacturers; these allow
several Modbus slave devices to be connected to a single Modbus Master
device. Typically, a single port expander will have 3–5 separate Modbus ports
on it. Depending on the manufacturer, the user may be able to configure each
of these ports for different communications parameters. In essence, not only
does this type of port expander allow multiple slave devices to be connected
to a single Modbus master device, but it can also be configured to convert
incompatible communications parameters such as Modbus ASCII to RTU (or
vice versa), baud rate, parity, stop bits, etc.
In addition to the port expanders mentioned above, other protocol converters
from third-party manufacturers, can be used to convert other industrial
protocols to Modbus ASCII.
Other Reference Material
SCADA ANSI Specification. ANSI/IEEE Std. C37. 1–1994.
Boyer, Stuart A. SCADA supervisory control and data acquisition. Research
Triangle Park, NC: Instrument Society of America. 1993.
MODICON. Modicon modbus protocol reference guide. North Andover, MA:
MODICON, Inc., Industrial Automation Systems. 1996.
AEG Schneider Automation. Modicon ladder logic block library user guide.
North Andover, MA: AEG Schneider Automation, Inc. 1996.
Troubleshooting Tips
Problem: 980 Flow Meter responds to some Modbus messages but not all
Response: Check the Register Addresses
The flow meter will only respond to valid Modbus message requests. If a
Modbus message sent to the flow meter asks for stored register addresses for
values that are outside of the address range currently supported by the meter,
the meter will ignore the request.
The flow meter currently only supports register addresses 40001 through
40083. Consequently, a request to read the value in any register address
greater then 40083 will be ignored. If a range of registers is requested and
Table 41 Floating Point Values Representation
First Register (i.e., 4001) Second Register (i.e., 4002)
Low Word, High Byte Low Word, Low Byte High Word, High Byte High Word, Low Byte