Endress+Hauser EngyVolt RV15, Руководство по эксплуатации

Страница: 38 / 64

Appendix EngyVolt RV15
38
The electrical interface is 2-wire RS485, via 3 screw terminals. Connection should be made using
twisted pair screened cable and the following electrical data:
Nominal inductivity 0.2 µH/ft
Nominal capacity line-line 24 pF/ft
Maximum operating voltage 300 V RMS
All "A" and "B" connections are daisy chained together. The screens should also be connected to the
“Gnd” terminal. To avoid the possibility of loop currents, an Earth connection should be made at
only one point on the network. Line topology may or may not require terminating loads depending
on the type and length of cable used. Loop (ring) topology does not require any termination load.
The impedance of the termination load should match the impedance of the cable and be at both
ends of the line. The cable should be terminated at each end with a 120
W
resistor. The load capacity
of the resistor should be 0.25 W. A total maximum length of 1 200 m (3 900 ft) is allowed for the
RS485 network. A maximum of 32 electrical nodes can be connected, including the controller. The
address of each EngyVolt device can be set to any value between 1 and 247. Broadcast mode (address
0) is not supported. The maximum latency time of an EngyVolt device is 60 ms, i.e. this is the amount
of time that can pass before the first response character is output. The supervisory program must
allow this period of time to elapse before assuming that the device is not going to respond.
The format for each byte in RTU mode is:
Coding system 8-bit per byte
Data format 4 bytes (2 registers) per parameter.
Floating point format (to IEEE 754)
Most significant register first (Default).
The default may be changed if required - See Holding Register "Register Order" .
Error check field 2 byte Cyclical Redundancy Check (CRC)
Framing 1 start bit
8 data bits, least significant bit sent first
1 bit for even/odd parity (or no parity)
1 stop bit if parity is used; 1 or 2 bits if no parity
Data coding
All data values in the EngyVolt device are transferred as 32 bit IEEE 754 floating point numbers,
(input and output) therefore each device value is transferred using two Modbus protocol registers.
All register read requests and data write requests must specify an even number of registers. Attempts
to read/write an odd number of registers prompt the device to return a Modbus protocol exception
message.
The multifunctional electrical energy meters can transfer a maximum of 40 values in a single
transaction, therefore the maximum number of registers requestable is 80. Exceeding this limit
prompts the device to generate an exception response.
Data transmission speed is selectable between 2400, 4800, 9600, 19200 and 38400 baud.
11.2.2 Input registers
Input registers are used to indicate the present values of the measured and calculated electrical
quantities. Each parameter is held in two consecutive 16 bit registers. The following table details
the 3X register address, and the values of the address bytes within the message. A tick (
 ) in the
column indicates that the parameter is valid for the particular wiring system. Any parameter with a
cross (
Ä ) will return the value Zero. Each parameter is held in the 3X registers. Modbus Protocol
Function Code 04 is used to access all parameters.
For example, to request Amps 1 (current Phase 1) Start address = 0006
No of registers = 0002