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SCADA-Modbus® System Guidelines
D.14 Port Expanders and Protocol Converters
In some situations, there may not be a Modbus ASCII port available for use with the 940
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 has 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.
D.15 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.
D.16 Troubleshooting Tips
Problem: 940 Flow Meter responds to some Modbus messages but not all
Response: Check the Register Addresses
The flow meter only responds 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 ignores 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 that range includes register addresses
greater then 40083, the entire request will be ignored
Response: Check the number of registers being polled
Additionally, the 940 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 ignores 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.