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Publication S130E
Issue 1.0
Pakscan IIS Technical Manual
10.2 Checking the Loop Itself
The most common problem in commissioning is that the field wiring is incorrectly made. If the cable is not
connected correctly between the Output side of one field unit and the Input side of the next then the loop will
fail to operate. As field units which are powered 'off' divert the signal directly from input to output a simple
resistance check will not reveal this cabling error. Similarly if the cable is 'twisted' so that the common wire is
not connected to all the common terminals this will also not be revealed by a simple resistance check.
It is extremely important to check all the wiring to ensure it is correctly connected throughout the loop.
Hint:
The loop wiring can often be checked by using the Paktester on the end of the cable. Power ‘on’ all the
actuators and successively power them ‘off’ starting with the one nearest the master station. [Page 4 of the
Paktester manual (R5161-016) has a picture showing how to connect the Paktester.]
This is only possible if the total loop resistance is
less than 200 ohms
.
Disconnect the loop wiring from the master station, connect the Paktester to the wires that were connected to
Port A of the master station. Ensure that all the actuators are switched on. The Paktester should be able to
establish communication with the nearest field unit to the master station. Once communication is verified,
switch off the nearest field unit, and repeat the communication test with the next field unit. Successively power
‘off’ each field unit ‘forwards’ round the loop until communication has been verified with all the field units.
Make a note of the order of the addresses of the field units on the loop (i.e. a Map).
Establishing communication in this way verifies the loop cabling, except for the final connection from the last
field unit back to Port B of the master station. To check this final part of the wiring connect the Paktester to
Port B wires (ensuring the correct polarity) and establish that you can communicate with the field unit nearest
Port B.
Checking the Loop with the Master Station
Initially set the Loop Driver circuits to deliver maximum loop voltage. Do this by turning the two
potentiometers RV181 and RV182 fully anti-clockwise.
Connect Port A wiring only to the master station, leave Port B disconnected. This time make sure that all the
field units except the one furthest away are powered off. Select the Master Station Status screen, go to the
Master Station Commands screen, and Reset the Loop by pressing '1'. Return to the Status screen (by pressing
'Menu').
The status screen will show 'Waiting for Loopbacks 1', and be followed by 'Finding FCU A’, ‘Finding FCU
B’, ‘Waiting for Loopbacks 2’, ‘Loopbacks Off A’, ‘Loopbacks Off B’ and finally ‘Loopback On’. Now press
'1' to select the master station commands, followed by '4' to select the diagnostic screen. This is illustrated in
section 8.4.3. If all is well on the loop it should show that the reason for the last loop configuration is ‘Reset
Command’ and that there is a loopback at the last field unit.
