Section 12 – Power Requirements12-7Determining the Voltage Drops along the RS-485 BusesThe input voltage to any gateway or operating panel must be at least 8.5VDC or 9.7VDC if a gatewayhas horn/strobes. If the voltage is too low, either position the gateway or operating panel closer to thecontrol unit or add a supplementary power supply. To determine the voltage drops along the RS-485buses, perform the following steps:Step Action1 Use the graph in Figure 12-3 to determine the input voltage available to each gatewayand operating panel. This graph shows distance in feet along the horizontal axis, andvoltage drop across the bus along the vertical axis.Refering to the Load Diagram you prepared, find the cable length of the first section ofthe RS-485 bus (distance in feet between the control unit and the electrically first deviceon the RS-485 bus).2 Locate this distance on the horizontal axis of Figure 12-3. Then, use a straight edge toproject a line straight upward until it hits the diagonal current line that is equal to, orgreater 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 verticalVoltage Drop axis. Record the voltage that is indicated, rounding it off to the next highernumber 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 thevoltage drop determined in the previous step from the supply voltage of 10.2 volts. Forexample, if the voltage drop is 0.3 volts, then the input voltage to the first device is 9.9volts (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 busas 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.05A00.10.20.30.40.50.60.70.80.911.11.21.31.41.51.61.71.80 200 400 600 800 1000 1200 1400Length of RS-485 Bus Section (feet)Voltage Drop (volts)Figure 12-3: Determining the Voltage Drop on the RS-485 Bus
