ADT Commercial Fire & Burglary Alarm System FOCUS 200 PLUS, Installation And Setup Manual

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Section 12 – Power Requirements
12-7
Determining the Voltage Drops along the RS-485 Buses
The input voltage to any gateway or operating panel must be at least 8.5VDC or 9.7VDC if a gateway
has horn/strobes. If the voltage is too low, either position the gateway or operating panel closer to the
control unit or add a supplementary power supply. To determine the voltage drops along the RS-485
buses, perform the following steps:
Step Action
1 Use the graph in Figure 12-3 to determine the input voltage available to each gateway
and operating panel. This graph shows distance in feet along the horizontal axis, and
voltage drop across the bus along the vertical axis.
Refering to the Load Diagram you prepared, find the cable length of the first section of
the RS-485 bus (distance in feet between the control unit and the electrically first device
on the RS-485 bus).
2 Locate this distance on the horizontal axis of Figure 12-3. Then, use a straight edge to
project a line straight upward until it hits the diagonal current line that is equal to, or
greater than, the total alarm current load obtained in Step 4 previously.
3 Use the straight edge to project a horizontal line to the left until it hits the vertical
Voltage Drop axis. Record the voltage that is indicated, rounding it off to the next higher
number if necessary. This is the voltage drop across the first section of the bus.
4 Calculate the input voltage to the first device on the RS-485 bus by subtracting the
voltage drop determined in the previous step from the supply voltage of 10.2 volts. For
example, if the voltage drop is 0.3 volts, then the input voltage to the first device is 9.9
volts (10.2 volts minus 0.3 volts).
5 Repeat for each device on the bus. Use the input voltage to the previous device on the bus
as the supply voltage in determining each subsequent voltage drop.
6 If you are using additional bus runs, repeat all the above steps for each bus run.
1.0A .75A .50A .40A .30A .20A
.15A
.10A
.05A
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
0 200 400 600 800 1000 1200 1400
Length of RS-485 Bus Section (feet)
Voltage Drop (volts)
Figure 12-3: Determining the Voltage Drop on the RS-485 Bus