Douglas Krantz - Technical Writer - Describing How It Works
Get the Book Make It Work - Convetional Fire Alarms
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How does an SLC Module get Power to Send Signals?

An addressable module receives all its power to operate from the Signaling Line Circuit. To send a signal, the addressable module performs a short series of electrical shorts on the circuit. In other words, to send computer style data, it interrupts power to everything else on the circuit.

How does an SLC Module get Power to Send Signals?


How does an SLC Module get Power to Send Signals?


Greetings Douglas,

I have read from How Does Multiplexing of the Signaling Line Circuit Work? the following: "To send data to the panel, the modules interrupt the power on the SLC."

So, I have a question: if the module interrupts the power, where is the energy source needed by the module for to send the data to the panel? I ask it because I understand the module has no internal battery, and the module's electronics needs an energy source to power it, doesn't it?

Thank you, J

The power being referred to as being interrupted is generated by the Signaling Line Circuit (SLC) power supply. This is a low current power supply and its output is designed to be regularly shorted out.

The panel, though, is looking at the SLC terminals.

A lot of the signal sending and receiving is dependent on what the panel is seeing. To see the signals, the Signaling Line Circuit terminals are constantly being checked by an internal voltmeter, inside the panel. When no signals are being sent, the panel's internal voltmeter is reading the voltage that the internal supervision power supply is putting out.

Electronically, though, with addressable fire alarm system modules, we have to think in terms of "a signal is sent" and "powering the electronics" as two separate issues. The electronics inside the modules deal with the two issues separately so we have to think of the two issues separately.

A Signal is Sent

To understand how a signal can be sent, do a mental exercise. Pretend that your have a single zone conventional fire alarm panel. On that single zone, there is a single pull station. The pull station, of course, is a switch that can short-out the wires, it has no battery.

Start the exercise at the panel. Use your voltmeter and measure the voltage at the zone terminals of the panel. Pretend that the voltage measurement is 20 volts, a common voltage for a normal supervised conventional zone.

Disconnect the battery so it won't mess up the following part of the exercise.

Remember, 20 volts was measured at the panel's zone-input terminals. Now walk over to the circuit breaker panel and interrupt the power to the panel. Go back to the panel and read the voltmeter's measurement on the panel's zone terminals. It should read zero volts because the circuit breaker interrupted the voltage.

Turn on the circuit breaker again. The panel's zone-input terminals should be 20 volts again.

Now walk over to the pull station and activate it. Keep in mind that all you have really done is used the switch to short-out the zone.

With the pull station activated, read the voltage on the panel's zone terminals. The voltmeter shows the exact same reading as when the circuit breaker interrupted the power. By shorting out the wires (activating the pull station), you have interrupted the power on the zone and "sent" an alarm signal. If there was a smoke detector on the same zone, the power would also be interrupted to the smoke detector.

Electronically, this is exactly what the addressable module does, it shorts-out the Signaling Line Circuit (SLC). In other words, to send a signal, the module interrupts power.

When a pull station is activated, though, the pull station latches so it stays on; when an addressable module sends a signal, the addressable module does not stay "on". Sending a short burst of data bits of information, the module shorts out the SLC quite a few times. However, because each bit of data lasts milliseconds, or less, the module shorts-out the SLC milliseconds, or less, with each data-bit of signal it sends.

Since the module only sends data to the panel every few seconds, most of the time the module is not affecting the SLC. Most of the time, when no module is sending data, the SLC is powering the devices.

Powering the Electronics

All power supplies have a resistor / capacitor (RC) network to filter the power. (This RC network is called a Decoupling Network and often has more than one resistor and capacitor.) All addressable modules have this RC filter inside them, also. The resistor(s) limits the current a little bit, and the capacitor(s) store up the current like a dam stores up water in a reservoir.
  • With a reservoir, the water can be let out at a metered rate; down-stream from the dam, even during a dry spell, water still goes down the river.
  • With a resistor / capacitor filter network, the electrical current can be let out at a metered rate; down-stream from the filter, even when the power is interrupted for a short time (milliseconds), power still goes to the electronics.

Because the RC network is used to filter the power, even though the modules keep interrupting the power to send data to the panel for milliseconds at a time, the electronics inside the addressable module have a constant source of power.

Douglas Krantz
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facpdoug@douglaskrantz.com
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