rotork p3 - Loop Checks

P3 - Pakscan Master Station Technical Manual
24 of 136 Publication PUB059-002-00 Issue 06/15
2.1 Loop Checks
The most common errors in installing the system occur on the field wiring.
Loop Continuity
With all the field units connected, but none of them powered up, check the continuity of the 2
cores of the Pakscan loop cable. Measure and record the resistance of each core. These
measurements will be useful as they can be compared with future measurements to determine
if cable resistance has changed significantly or not, this may help identify a cable fault. Cable
resistance (R) is the sum of the resistance of both cores.
Screen Continuity
Screen continuity must be continuous between each end of the loop. Ensure the screen is either
connected to a signal earth bar at only one point or to terminals on the loop driver plug; pin 3
connects directly to the enclosure earth and pin 6 connects to the enclosure earth via an
internal capacitor, preventing an earth loop. Both screens must be connected to the terminals
provided on the loop driver, so as to ensure the product meets the European Directive on EMC.
Cable Capacitance
The capacitance (C) between the cores of the cable is critical to system performance. Too high
a capacitance for the selected loop baud rate will result in poor communications, or even
communication failure. If a suitable meter is available, measure and record the capacitance
between the cable cores.
Maximum Loop Speed
The cable resistance must not exceed 500 Ohms (250 Ohms per core) and the total
capacitance must not exceed the maximum value for each communication speed. In addition to
cable capacitance, each field unit adds a small amount of capacitance to the loop which must
also be considered when determining maximum loop speed. Use the measured resistance and
capacitance values from the above tests to determine which loop speed should be used.
Baud Rate
R max (ohms)
C max (F)
110
500
4.5
300
500
2.1
600
500
1.54
1200
500
0.6
2400
500
0.3
Each field unit will add a capacitance of 2.2nF
The C max figure given is the maximum value for the network capacitance
including the field unit capacitance.
Test Equipment
A good quality multimeter with capacitance test facility is adequate for testing loop resistance
and capacitance. Under no circumstances should any high voltage test equipment be used
such as insulation Megger testers when any part of the cable loop is connected to either the
master station or actuators. The high test voltages generated by such equipment may damage
the Pakscan components.