Functions
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Publication S177E V2.0 Issue 03/05
5 FUNCTIONS
5.1 Cable Fault Protection - Loopback system
Communication between the master station and the field control units is via a twisted pair 2-wire cable
carrying a 20 mA current in a loop. In normal operation the current flows round the loop on one wire
and returns via the master station on the second wire. If high-speed operation with ‘doubling on’ is
selected the current returns inside the master station and the second wire is not used.
Each field control unit has the capability to connect the 2 wires together (apply a loopback circuit), and
this mechanism is used to divert the current in the event of a loop fault. With field units each side of a
loop fault 'in loopback', the system can still communicate with all field units by operating the system as
2 segments. If there are 2 loop faults, those field units between the faults will be isolated.
A loop cable fault may be an open circuit, a short circuit, or an earth fault.
5.2 Current Loop Start Up Operations
The Pakscan IIE controls the current on the loop to communicate with the attached field units. Loop
communication divides into two parts: configuration (when the master station decides whether the loop
is complete, and if not, where the faults are), and normal operation.
Fig 4: Loopback system
Master Station
Port B
Port A
Add 2
Add 4
Add 3
Add 1
In
Out
Out
In
Master Station
Port B
Port A
Add 2
Add 4
Add 3
Add 1
In
Out
Out
In
Normal operation
Broken loop operation
Cable
Fault
LB on
LB on
