A 4‑wire data link as an alternative to a 2-wire communications link is also
possible. A cable with two symmetrical twisted pair wires is required for a 4-wire
data link. A 2-wire data link is shown in Fig. 5-13, (p. 5-19), and a 4-wire data
link is shown in Fig. 5-14, (p. 5-20) as an example for channel 2 on the
communication module. The same is valid if channel 1 on the communication
module is available as a RS 485 interface.
2-wire data link:
The transmitter must be bridged with the receiver on all devices equipped
electrically with a full duplex communication interface, e.g. the P634. The two
devices situated at either far end must have a 200 to 220 Ω resistor installed to
terminate the data transmission conductor. In devices from the Easergy MiCOM
30 family, and also in the P634, a 220 Ω resistor is integrated into the RS 485
interface hardware and can be connected with a wire jumper. An external
resistor is therefore not necessary.
4-wire data link:
Transmitter and receiver must be bridged in the device situated on one far end
of the data transmission conductor. The receivers of slave devices, that have an
electrically full-duplex communication interface as part of their electrical system,
e.g. the P634, are connected to the transmitter of the communication master
device, and the transmitters of slave devices are connected to the receiver of the
master device. Devices equipped electrically with only a half duplex RS 485
communication interface are connected to the transmitter of the communication
master device. The last device in line (master or slave device) on the data
transmission conductor must have the transmitter and receiver terminated with
a 200 to 220 Ω resistor each. In devices from the Easergy MiCOM 30 family, and
also in the P634, a 220 Ω resistor is integrated into the RS 485 interface
hardware and can be connected with a wire jumper. An external resistor is
therefore not necessary. The second resistor must be connected externally to the
device (resistor order number see Chapter “Accessories and Spare Parts”).
P634 5 Installation and Connection
5-18 P634/EN M/R-42-A // P634‑311‑653
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