P44x/EN SC/Hb
P442, P444 (SC) 12-
1 SCADA COMMUNICATIONS
1.1 Introduction
This section outlines the remote communications interfaces of the MiCOM relay. The relay
supports a choice of one of five protocols via the rear communication interface, selected via
the model number when ordering. This is in addition to the front serial interface and 2
nd
rear
communications port, which supports the Courier protocol only.
The rear EIA(RS)485 interface is isolated and is suitable for permanent connection
whichever protocol is selected. The advantage of this type of connection is that up to 32
relays can be ‘daisy chained’ together using a simple twisted pair electrical connection.
The descriptions in this section do not detail the protocol itself. The relevant documentation
for the protocol should be referred to for this information. This section serves to describe the
specific implementation of the protocol in the relay.
1.2 Rear port information and connection advice – EIA(RS)485 protocols
1.2.1 Rear communication port EIA(RS)485 interface
The rear EIA(RS)485 communication port is provided by a 3-terminal screw connector
located on the back of the relay. See section P44x/EN IN for details of the connection
terminals. The rear port provides K-Bus/EIA(RS)485 serial data communication and is
intended for use with a permanently wired connection to a remote control centre. Of the
three connections, two are for the signal connection, and the other is for the earth shield of
the cable. When the K-Bus option is selected for the rear port, the two signal connections are
not polarity conscious, however for MODBUS, IEC 60870-5-103 and DNP3.0 care must be
taken to observe the correct polarity.
The protocol provided by the relay is indicated in the relay menu in the ‘Communications’
column. Using the keypad and LCD, firstly check that the ‘Comms. settings’ cell in the
‘Configuration’ column is set to ‘Visible’, then move to the ‘Communications’ column. The
first cell down the column shows the communication protocol being used by the rear port.
1.2.1.1 EIA(RS)485 bus
The EIA(RS)485 two-wire connection provides a half-duplex fully isolated serial connection
to the product. The connection is polarized and whilst the product’s connection diagrams
indicate the polarization of the connection terminals it should be borne in mind that there is
no agreed definition of which terminal is which. If the master is unable to communicate with
the product, and the communication parameters match, then it is possible that the two-wire
connection is reversed.
1.2.1.2 Bus termination
The EIA(RS)485 bus must have 120 Ω (Ohm) ½ Watt terminating resistors fitted at either
end across the signal wires – see Figure 1. Some devices may be able to provide the bus
terminating resistors by different connection or configuration arrangements, in which case
separate external components will not be required. However, this product does not provide
such a facility, so if it is located at the bus terminus then an external termination resistor will
be required.
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