OTT Hydrolab DS5, Руководство пользователя

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External Communications
B.3 RS485 Interface
Series 5 Multiprobes are compatible with RS485 interfaces. RS485 is a standard that
specifies a particular method to transmit and receive digital signals. This standard is
maintained by the Electronic Industries Association in a document titled "Standard for
Electrical Characteristics of Generators and Receivers for Use in Balanced Digital
Multipoint Systems."
RS485 involves sending an inverted or out-of-phase copy of the signal simultaneously on
a second wire. This is called a balanced transmission. Any outside electrical noise adds
coherently to both signal copies. The receiver electrically subtracts the two signals to
reproduce the original signal. The advantage in the subtraction is that only the intended
signal gets reproduced since they are out-of-phase. The in-phase noise on the two wires
are also subtracted from each other to produce a net zero noise component in the
reproduced signal. This noise immunity allows the RS485 interface to transmit digital
signals at faster rates over longer distances than the RS232/SDI-12 interface. The
RS232/SDI-12 interface does not use balanced transmission and is therefore susceptible
to noise interference which limits the transmission distance and speed.
Connections
RS485 can use two wires to both transmit and receive data. A common software protocol
must be shared between devices to prevent data collisions on the wires. RS485 also
allows for multiple transmitters and receivers to be easily connected together.
Be sure to connect the signal grounds of all devices on the network together. The
connection can be made using a conductor in the transmission cable or each device can
be connected to a good earth ground. This connection keeps the common mode voltage
(the voltage which the signal must overcome to be reproduced) low. The network devices
may operate without the signal ground connection, but may not be reliable.
Some RS485 applications require impedance termination because of fast data rates or
long cables. The most popular termination involves installing a ½ watt resistor across the
receiver at each end of the network. See the RS485 interface user manual for termination
requirements on the PC being used.
Alternatively, the preferred network may be AC terminated by placing a 0.01 µF capacitor
in series with the terminating resistor. The capacitor appears as a short circuit during
signal transitions but appears as an open circuit to any DC loop current. This will reduce
the power supply current required to operate the network and still provide the proper
terminating impedance.
Do not add a terminating resistor to every receiver in the network. For networks with more
than about four nodes, the transmitters will be unable to drive the cable. Only terminate
both ends of the main cable.
B.4 Modbus Interface
The multiprobe is configured to respond to Modbus function 3 (Read Holding Registers) in
RTU mode through the RS232 or RS485 communications port at 19200 baud, 8 data bits,
Even Parity, and 1 stop bit (19200:8:E:1).
All data values are returned in IEEE Floating Point Format (4 bytes). Each data value is
returned in two 16 bit words with the low word being transmitted first. Within each word,
the high byte is transmitted first. Each byte is transmitted Most Significant Bit first. For
example, the floating point value 1.56 = 0x3FC7AE14 would be transmitted as
0xAE 0x14 0x3F 0xC7.