Douglas Krantz - Technical Writer - Describing How It Works
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How does an SLC Carry Both Signals and Power?

The key is in the rapid shorting-out of the SLC by the addressable modules, and the decoupling circuit inside each module. To understand the method, the circuit has to be looked at from many directions at once.

signals are sent both ways on an SLC and at the same time there's power being sent from the FACP to the modules.

Greetings Douglas,

First of all I have to thank you for your useful website and information you share for readers, then, I have a question and if there is no problem I would be really happy to know your answer.

I wonder what kind of technique addressable fire alarm systems utilize to send both power and data on just two wires. Is that a kind of SLC or any special protocol? How can I access more detailed information about this topic?

Thank you, AK

Question

First, I have a question for you to think about.

When a person activates a conventional pull station (MCP or Manual Call Point), all that they actually do is to turn on a switch. This turning on of a switch "sends" an alarm signal to the fire alarm control panel. The switch doesn't have a battery; the switch only connects two wires together.

How does the conventional pull station switch "send" an alarm without "sending a signal"?

Point of View

To understand how it works, a simple circuit has to be seen from many directions at once:
  1. The wires are being supervised by the control panel
  2. The control panel is providing voltage on the screw terminals of the panel
  3. The control panel is using voltage to force a small electrical current through the wires, and also through the end of line resistor
  4. The control panel is checking the continuity of the wires to make sure they are connected
  5. The end of line resistor has voltage across it because the power supply is using voltage to drive the current (Ohm's Law "E = I x R" or "Voltage equals Current times Resistance")
  6. The voltage is the same on the terminals of the control panel as it is on the end of line resistor, and the voltage is the same on every device on the circuit
  7. The circuitry inside the control panel is a very weak power supply
  8. The power supply can be shorted out easily because it is so weak
  9. The switch connects the wires together when the pull station is activated
  10. The switch shorts out the power supply
  11. The switch increases the current from the power supply by increasing the current through the wires
  12. The voltage at the terminals of the control panel drops to zero
  13. All devices on the circuit, including the terminals of the control panel, have their voltage shorted out to zero
  14. All devices on the circuit, including the terminals of the control panel, have their voltage turned off (this is a different way of saying that the voltage is shorted to zero)
  15. The panel is monitoring the voltage on its screw terminals
  16. The panel sees this zero-voltage (the voltage has been turned off) as an alarm that was sent
  17. The conventional pull station switch "sends" an alarm without "sending a signal"

How does an Addressable Device Send a Signal?

Over the SLC (Signaling Line Circuit), the devices send their signals as strings of data. The data signals can be looked at as a rapid string of "0s" and "1s", or another way of looking at it is the data is a rapid string of voltage being turned off and on.

Like a pull station, the addressable devices leave on, or they turn off the voltage by electronically shorting the wires; unlike a pull station, the addressable devices turn off the voltage repeatedly and very rapidly. Another way of looking at it is the addressable devices send data by leaving on, or rapidly turning off, the voltage.

All of the addressable devices, and the panel, monitor the data signals on the SLC; the addressable devices, and the panel, receive data from the SLC.

By Douglas Krantz Douglas Krantz Check It Out

Wait a Second!

If the voltage is rapidly being turned off, where do the addressable modules and detectors get their constant power. . . from the circuit that has its voltage rapidly turned off?

Yes.

Each addressable module, and each addressable detector has a "decoupling circuit". The decoupling circuit has a few capacitors, resistors, and maybe a diode or two arranged to provide a clean internal power supply. The decoupling power supply inside the module gives constant voltage inside the module circuitry even though the incoming voltage is being rapidly turned on and off.

The Protocol is Proprietary

Each manufacturer has a different circuit layout, data speed, and format to send and receive their data. There is no standard protocol in use throughout the fire alarm industry because of the fear of liability; the exact protocol is always secret.

Douglas Krantz Douglas Krantz
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer
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facpdoug@douglaskrantz.com
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