MX-7000 Installation and operation manual
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Section 9 Appendix A
Multiplex copper loop power distribution calculations
The MX-7000 can supply the power for its connected FPS2-2M processors. However, supplying the
power to the distributed field units from a central location causes a voltage drop in the power cable due to
the distances involved in a perimeter protection system. Although the system can compensate for the
voltage drop over distances, voltage dropping resistors may be required at some processors on a long
multiplex loop.
Each FPS2-2M field processor supply voltage must be 12-16 VDC. If the end of line processor supply is
below 12 VDC, the MX-7000 copper loop card output voltage at TB1, pins 4, 5 (see Figure 22) can be
increased by trimming R40 on the loop card. If the end of line processor voltage cannot be brought up to
12 V without exceeding 16 V at the first in line processor then voltage dropping resistors must be added.
This section includes procedures to assist you in calculating the voltage drop, and for selecting
appropriate voltage dropping resistors, if required.
The following calculations use engineering approximations to determine:
•
resistor values to be installed at specific processors (one of three resistor values)
•
the output voltage for the MX-7000 multiplex loop card
•
the values of the calculated loop voltage at each processor
Rc = the resistance of the power cable between two processors
Dc = the distance between two processors in thousands of feet
Vc = the voltage drop between two processors
Ic = the current flow through a power cable segment (Ic1 is the farthest from the MX-7000, Ic2 is the
second farthest; Ic3 is the third farthest, etc)
(30 mA is the typical current draw of an FPS2-2M processor)
Rc of 14 is the linear resistance factor used for calculations. This includes the source and return wire
resistance based on 7 Ohms per 1000 ft. for 18 AWG wire.
V1 = the voltage at the transponder that is the farthest cable distance away from the MX-7000
V2 = the voltage at the second farthest transponder
V3 = the voltage at the third farthest transponder (etc.)
Rd is the voltage drop resistor installed at processors to limit the voltage to a value between 12 and 16
volts
The wire resistance (Rc) between two processors (XPO) depends upon the distance (Dc) between them
(in thousands of feet) and the linear resistance of the power wires as follows:
For 18 AWG wire Rc = (14) x (Dc)
For example, if the XPO locations are 2,000 feet apart, then Rc = 14 x 2, therefore, Rc = 28 ohms.
Procedure
Assume that the voltage at the XPO which is the farthest cable distance away from the MX-7000, is at the
minimum operating voltage of 12 VDC.
