Pakscan IIE System Manual
122 of 132
Publication S177E V2.0 Issue 03/05
16.3 GP Field Unit
Environmental Spec
Operating Temperature -30 to +70
o
C
Storage
Temperature
-40
to
+85
o
C
Power Supply
110V ac +/-20%, 240V ac +/-10%, 50/60Hz +/- 1Hz
Output Relays
2A, 130V, 60W, 125VA (exclusive maxima)
16.4 2-Wire Loop
Cables
The recommended cables to use for the loop wiring should be a twisted pair with an overall screen.
The construction will usually be copper conductors and PVC insulation. There may be additional outer
protection in the form of armour. This type of cable has a reasonably constant performance
characteristic and the following is typical of the cable parameter data found from suppliers such as
Beldon.
Cross Section (mm
2
)
Resistance
Ω/
km @ 20
o
C
Capacitance pF/m
0.5 solid
36.8
Ω
/km
115 pF/m
0.5 flexible
39.7
Ω
/km
115 pF/m
1.0 solid
18.4
Ω
/km
115 pF/m
1.5 stranded
12.3
Ω
/km
115 pF/m
Maximum Loop Distances
The cable resistance and capacitance must not exceed the permitted maximum values for each
communication speed. The limiting values of C and R for each speed will depend on the number of
field units actually connected. The following tables show the maximum values excluding the
contribution from the field units themselves.
Maximum Resistance (ohms) of the Cable excluding field units
Number of field units
Baud Rate
0 to 240
110 - 2400
500
Note: 1 field unit has 1.14 ohms resistance
Maximum Capacitance (mF) of the Cable excluding field units
Number of field units
Baud Rate
0 60 120 180 240
110
4.5 4.36 4.24 4.10 3.97
300
2.1 1.98 1.84 1.70 1.57
600
1.54 1.41 1.27 1.14 1.02
1200
0.6 0.47 0.34 0.20 0.07
2400
0.3 0.17 0.04 N/A N/A
Note: 1 field unit has 2.2nF capacitance
