Hach Sigma 950 Instrument Manual page 118

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SCADA-Modbus System Guidelines
First Register (i.e., 4001)
High Word, High Byte
First Register (i.e., 4001)
Low Word, High Byte
E.11 Port Expanders and Protocol Converters
E.12 Other Reference Material
116
The complications arise because Modicon doesn't store floating point values in this
standard (IEEE) format. Modicon stores floating point values the opposite way with the
"Low-order" word in the first register and the "High-order" word in the second register.
Table 47 IEEE Floating Point Representation
High Word, Low Byte
Table 48 Floating Point Values Representation
Low Word, Low Byte
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.
In some situations, there may not be a Modbus ASCII port available for use with the 950
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.
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.
Second Register (i.e., 4002)
Low Word, High Byte
Second Register (i.e., 4002)
High Word, High Byte
Low Word, Low Byte
High Word, Low Byte