Hach Sigma 950, Инструкция по эксплуатации инструмента

Страница: 117 / 146

115
SCADA-Modbus
®
System Guidelines
com port reply time-out must be set to 12 seconds. This is the amount of time that the
meter will be given to reply to Modbus queries via this serial port.
Communication Handshaking
The 950 Flow Meter contains minimal communication handshaking. For the meter to
identify an RS232 connection from an outside source, and to keep the RS232 hardware
active once communicating, the Data Terminal Equipment (DTE) must assert and hold
high the DTR line of the DB9 connector (DSR of meter). The 950
Flow Meter does not
support RTS/CTS hardware handshaking. Note that DTE must be capable of handling a
12-second maximum response lag.
E.10 Complications with Floating Point Values
The manufacturer’s implementation of the Modbus protocol was based on the idea that
we would enable our flow meters to emulate a Modicon
®
, Compact 984 PLC.
Consequently, we follow the exact same format that Modicon uses for the storing and
processing of floating point numbers. Additionally, the Modbus protocol does not define
how floating point values are packed (stored) into the internal memory addresses or
“Registers” of the flow meter. If you are integrating our Modbus-capable flow meters, be
aware that these meters store and process floating point numbers in the exact same
format as the Modicon Compact 984 PLC.
All current models of Modicon PLCs, including the Compact 984, pack two bytes of data
into each register. This alone presents no problems. Unsigned two-byte (16-bit) integer
values in the range of 0 to 65535 can be stored and retrieved from these registers without
any problems or complications. The complications arise when the stored value is a
floating point value, which by IEEE definition, require 4 bytes (32 bits). The IEEE
standard for floating point values states in part that the 8 most significant bits represent
the exponent and the remaining 23 bits (plus one assumed bit) represent the mantissa
and the sign of the value.
Since a data “word” consists of two bytes, a floating point value is represented by two
data words. Because a single Modicon register consists of one word (or 2 bytes), two
consecutive Modicon registers are needed to store one floating point value.
The representation of a floating point value can be broken down into a “High Order” and a
“Low Order” word. Additionally, each word can be broken down into a high order byte and
a low order byte.
Table 47 and Table 48 depict how a IEEE floating point value is usually represented and
how the Modicon stores a floating-point value.
Pin Description Pin Description Pin Description
Pin 1 Data Carrier Detect (DCD)
1
Pin 4 Data Terminal Ready (DTR) Pin 7 Request to Send (RTS)*
Pin 2 Received Data (RD) Pin 5 Signal Ground (SG) Pin 8 Clear to Send (CTS)
Pin 3 Transmitted Data (TD) Pin 6 Data Set Ready (DSR) Pin 9 Ring Indicator*
1
Not used.