Installation and Connection
29 of 132
Publication S177E V2.0 Issue 12/04
7.3.1 Screen
Connections
The screen terminal 14 is connected internally to the supply earth pin, and the chassis. Terminal 17 is
connected to earth via a capacitor, (to meet the CE specification for EMC emission standard), or
directly to earth if link LK1 for master station A or hot standby systems, or LK2 for master station B, is
fitted. If no alternative clean earth bar is provided then the screen will have to connect to the supply or
safety earth.
If you choose to use the internal earth point on terminal 14 or 17 you must ensure that a safety earth is
connected to the master station. Connect the screen of the cable on one side only, say Port A using
terminal 14. DO NOT CONNECT THE SCREEN ON BOTH ENDS OF THE CABLE IF LK1 (LK2 for
station B) IS FITTED. If you connected the loop directly to earth at two or more points, an earth loop
will result that will probably degrade the loop communications.
If your system includes a Signal Earth bar then connect the screen of one of the loop cables to this bar
only. Do not link the bar to the master station earth via the cable screen and terminal 14.
7.4 Loop
Checks
Once the loop is fully wired there are a number of checks that should be made before power is applied
to any of the connected field units, actuators, or the master station. These simple tests will always help
in the setting to work of the system.
7.4.1 Loop
Continuity.
With all the field units connected, but none of them powered up, check the continuity of the 2 cores of
the cable using a simple resistance (ohm) meter. [The most common errors in installing the system
occur on the field wiring.]
1. Disconnect all the loop wires and the screen from the master station.
12
13
14
15
16
17
Port A Out
Port A In
Port B Out
Port B In
LK1
12
13
14
15
16
17
Port A Out
Port A In
Port B Out
Port B In
LK1
Safety Earth
Safety Earth
Signal Earth
Safety Earth
Safety Earth
Fig 10: Typical Screen Connection
Note: The screen must only be connected directly to earth at one point, and at one end of
the cable.
