Pakscan IIS Technical Manual Issue 1.0
Publication S130E 21
6 FUNCTIONS
6.1 Cable Fault Protection - Loopback system
Communication between the master station and the field control
units is via a 2-wire current loop. In normal operation, current flows
round the loop on one wire and returns within the master station
itself - the second wire is redundant in this case.
Each field control unit has the capability to short the 2 wires
together (loopback), 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 'arms'. If there are 2 loop
faults, those field units between the faults will, of course, be
isolated.
A loop fault may be an open-circuit, a short-circuit, or an earth
fault,
6.2 Current Loop Operations
The Pakscan IIS controls the current loop which communicates with
the attached field units. Loop communications divide into two
parts; configuration (when the master station decides whether the
loop is complete, and if not, where the faults are), and normal operation.
6.2.1 Loop Configuration
The loop configuration phase is entered after power-up, after a loop fault is detected, or after a loop configure
command is received.
Configuration consists of the following steps:-
1. Wait for all the field units to go into loopback. The time is dependent on the baud rate.
2. Issue a command from Port A to ask a field unit in loopback for its address. This finds the first field
unit.
3. Use that address to tell the field unit to remove its loopback.
4. Repeat steps 2 and 3 until no more field units are found.
5. Attempt to send a message right round the loop and receive it back at the sending end. This tests the
integrity of the complete loop.
6. If the loop is complete stop here. If the loop has a problem execute steps 7 to 10.
7. Repeat steps 2 to 4 from Port B.
8. Again wait for all field units to go into loopback.
9. Remove loopbacks one field unit at a time from Port A until only the last field unit is left in
loopback.
10. Repeat step 9 for Port B.
Loop configuration will identify and mark as out of service any field unit is found which has an address which
is zero, an address which is the same as one which has already been found, or which has an address which is
too high.
Tx
Rx
from
Port A
Normal Operation - Loopbacks Off
to
Port B
Tx
Rx
from
Port A
Fault Condition - Loopbacks On
fault
on
line
Tx
Rx
Fault Condition - Loopbacks On
fault
on
line
to
Port B
Fig. 9: Loopback system
