Signal grounding should be used with all devices in RS-485 bus having an
isolated communication port. Grounding ensures that different RS-485
nodes have the same signal reference ground. Without grounding the
differential RS-485 signal might be superimposed between different
nodes with respect to the node's local ground. For signal grounding it is
recommended to connect to AGND pin in RS-485 connector an additional
ground wire which runs inside the shielded serial cable.
The optional communication modules include support for EIA-485 serial
communication (X5 connector). Depending on the configuration, the
communication modules can host either two 2-wire ports or one 4-wire port.
The two 2-wire ports are called COM1 and COM2. Alternatively, if there is only one
4-wire port configured, the port is called COM2. The fiber optic ST connection uses
the COM1 port.
Table 1071: EIA-485 connections for COM0001-COM0014
Pin 2-wire mode 4-wire mode
10 COM1 A/+ COM2 Rx/+
9 B/- Rx/-
8 COM2 A/+ Tx/+
7 B/- Tx/-
6 AGND (isolated ground)
5 IRIG-B +
4 IRIG-B -
3 -
2 GNDC (case via capacitor)
1 GND (case)
13.4.7.2 COM0023 jumper locations and connections
The optional communication module supports EIA-232/EIA-485 serial
communication (X6 connector), EIA-485 serial communication (X5 connector) and
optical ST serial communication (X12 connector).
Two independent communication ports are supported. The two 2-wire ports are
called COM1 and COM2. Alternatively, if only one 4-wire port is configured, the port
is called COM2. The fiber optic ST connection uses the COM1 port.
Table 1072: Configuration options of the two independent communication ports
COM1 connector X6 COM2 connector X5 or X12
EIA-232 Optical ST (X12)
EIA-485 2-wire EIA-485 2-wire (X5)
EIA-485 4-wire EIA-485 4-wire (X5)
1MRS757644 H IED physical connections
620 series
Technical Manual
1183
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