x/EN SC/Na7
-4 MiCOM P74
Allowed:
Poll Class 1 (read spontaneous events)
Poll Class 2 (read measurands)
General Interrogation (GI) sequence
Transmission of Disturbance Records sequence
Time Synchronisation (ASDU6)
INF23 activate characteristic 1 (ASDU20)
INF24 activate characteristic 2 (ASDU20)
INF25 activate characteristic 3 (ASDU20)
INF26 activate characteristic 4 (ASDU20)
(2) Courier protocol
Blocked:
All setting changes
Reset Indication (Trip LED) command
Operate Control Input commands
CB operation commands
Auto-reclose operation commands
Reset demands / thermal etc... command
Clear event / fault / maintenance / disturbance record commands
Test LEDs & contacts commands
Allowed:
Read settings, statuses, measurands
Read records (event, fault, disturbance)
Time Synchronization command
Change active setting group command
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 P74x/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, IEC60870-5-103 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 120Ω (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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