Application Of The Vfd; 2-Wire Rs485 - Kruger Ecowatt Drive K120 Series Operation Manual

Ecowatt ® Drive K120 series

7.2 Application of the VFD

The VFD uses the Modbus RTU mode and the physical layer is 2-wire RS485.

7.2.1 2-wire RS485

2-wire RS485 interfaces works in half-duplex mode and send data signals in the
differential transmission way, which is also referred to as balanced transmission. An
RS485 interface uses a twisted pair, in which one wire is defined as A (+), and the other
B (-). Generally, if the positive electrical level between the transmission drives A and B
ranges from +2 V to +6 V, the logic is "1"; and if it ranges from -2 V to -6 V, the logic is
"0".
On the VFD terminal block, the 485+ terminal corresponds to A, and 485- corresponds to
B.
The communication baud rate (P14.01) indicates the number of bits sent in a second,
and the unit is bit/s (bps). A higher baud rate indicates faster transmission and poorer
anti-interference capability. When a twisted pair of 0.56mm (24 AWG) is used, the
maximum transmission distance varies according to the baud rate, as described in the
following table.
Max.
Baud
transmission
rate
distance
(bps)
(meter)
2400BPS 1800m
When RS485 interfaces are used for long-distance communication, it is recommended
that you use shielded cables, and use the shielding layer as the ground wires.
When there are fewer devices and the transmission distance is short, the whole
network works well without terminal load resistors. The performance, however,
degrades as the distance increases. Therefore, it is recommended that you use a 120Ω
terminal resistor when the transmission distance is long.
7.2.1.1 When one VFD is used
Figure 7-1 is the Modbus wiring diagram for the network with one VFD and PC.
Generally, PCs do not provide RS485 interfaces, and therefore you need to convert an
RS232 or USB interface of a PC to an RS485 interface through a converter. Then, connect
end A of the RS485 interface to the 485+ port on the terminal block of the VFD, and
connect end B to the 485- port. It is recommended that you use shielded twisted pairs.
When an RS232-RS485 converter is used, the cable used to connect the RS232 interface
of the PC and the converter cannot be longer than 15 m. Use a short cable when possible.
It is recommended that you insert the converter directly into the PC. Similarly, when a
Max.
Baud
transmission
rate
distance
(bps)
(meter)
4800BPS 1200m
Max.
Baud
transmission
rate
distance
(bps)
(meter)
9600BPS 800m
-125-
Communication protocol
Max.
Baud transmission
rate (bps) distance
(meter)
19200BPS 600m