Douglas Krantz - Technical Writer - Describing How It Works

Is an IDC the Same as an SLC?

By Douglas Krantz | Descriptions

Is an IDC the Same as an SLC?

Is an IDC the Same as an SLC?

Greetings Douglas,

I have a quick question, are the terms Initiating Device Circuit (IDC) and Signaling Line Circuit (SLC) used interchangeably?

If not, what sets the two terms apart from one another?

Thank You, JL

A Signaling Line Circuit, or SLC, is a circuit that is used by the panel to communicate with both input and output devices, while an Initiating Device Circuit, or IDC, is an input circuit used by the panel to receive alarms from devices that detect fire or initiate alarms.

Since the Notification Appliance Circuit (NAC) is a very common circuit that is used with fire alarm systems, I'll add the circuit description to the descriptions for an Initiating Device Circuit (IDC) and a Signaling Line Circuit (SLC).

All fire alarm systems are designed to have one major goal: detect fire, and to sound the alarm if there is a fire.

A secondary goal for fire alarm systems is to self-supervise itself, and to supervise other systems. If there's a fault, the fire alarm system turns on it's local-to-the-control-panel trouble alarm or supervisory alarm:
  • It turns on a trouble light if a detection device can't send in its alarm

  • It turns on a trouble light if a notification device can't sound the alarm

  • It turns on a supervisory light if a non-fire detection device has detected something

  • It turns on a trouble light if anything else fails or is adjusted improperly

When the fire alarm system detects anything that is not correct or not normal, the system turns on a yellow or amber light, and sounds its buzzer to let the building owner know that something needs to be corrected or fixed. That's what supervision is about.

The Basic Circuit

Outside waterflow alarm system for fire department - lets them know there is a waterflow - uses the waterflow switch to turn on the bell or hornstrobe

The above drawing shows the entire wiring scheme for a very basic fire alarm system. This type of wiring system is used, right now, in many buildings to let the arriving firefighters know that water is flowing in the building's sprinkler system. Presumably, the flowing of the water is suppressing a real fire.

Also, the alarm sound from this system is used to tell the arriving fire fighters where to connect their hoses to a special Fire Department Connection on the outside of the building.

The waterflow switch shown here is just a turn-on/turn-off switch. The switch is automatically turned on like a light switch on the wall when water is moving into the sprinkler system to suppress a fire.

The older basic fire alarm systems only used a fire alarm bell, the newer basic fire alarm systems use a horn and strobe.

The basic system shown here is not supervised. If the circuit breaker that supplies power to the system is turned off, or else a wire breaks or comes loose, no one will know about the fault. Actually, the only time anyone will know the system fails is if the system fails during the regular testing of the system, or if there is a fire, whichever comes first.

The two wires going into the waterflow switch can be considered to be an unsupervised Initiating Device Circuit (IDC) because it goes to the device that initiates the alarm: the waterflow switch.

The two wires going to the bell can be considered to be considered to be an unsupervised Notification Appliance Circuit (NAC) because it carries power to the device (Appliance) that Notifies people (firefighters) that the water flowing.

Two Wire Circuit

The IDC circuit, the NAC circuit, and most SLC circuits are two-wire circuits. The IDC and NAC circuits are a very old design, and as such, they haven't been changed much for nearly a century. The SLC circuit is a rather new invention (if you call new as being less than 50 or 60 years old) because it relies on solid state circuitry.

An Initiating Device Circuit (IDC) is a two-wire input circuit that carries signals from the devices that initiate the alarm, like smoke detectors, pull stations (sometimes known as MCPs or Manual Call Points), waterflow switches, etc. to the fire alarm control panel.

The initiating devices send in their alarms by shoring the IDC, or partially shorting the IDC. All devices that use the IDC are too dumb to do anything else.

A Notification Appliance Circuit (NAC) is a two-wire output circuit that carries power from the panel to the notification appliances, like horns, strobes, speakers, chimes, bells, etc. that sound the alarm.

Normally, the panel is just checking the continuity of the wires of the NAC. When there's a fire, the panel reverses the voltage polarity on the NAC, and then supplies enough power to cause the horns to sound and the strobes to flash.

A Signaling Line Circuit (SLC) is commonly a two-wire input and output circuit that carries digital information between all the addressable input modules, all the addressable detectors, all the addressable output modules, and the panel.

The SLC carries power from the panel to the addressable portion of the modules in the field. It also carries digital information between the panel and all the modules.

Included in this digital information is the address of the device being communicated with, alarms and troubles from the input devices, and commands and status information between the panel and the output devices in the field.

Are the Devices on the IDC and the SLC Interchangeable?

The answer is no. The devices on the IDC and the SLC are not interchangeable. If an alarm is sent by an interchanged device, unless the device is compatible with the panel, bad things happen:
  • A Conventional Device from the IDC shorts out the SLC. If a conventional device connected to the SLC goes into alarm, the conventional device automatically shorts out the wires. It's supposed to. If the wires are an SLC, the device shorts out the SLC. In that case, no addressable devices on the SLC are able to send or receive anything, and the entire fire alarm system itself effectively fails.

  • An Addressable Device from the SLC sends and receives digital signals. If an addressable device connected to an IDC goes into alarm, it sends digital data. The panel won't understand the digital signal, and during the reset process, the panel won't send digital commands to the addressable device. Possibly the addressable device won't prevent any other device from sending in their alarms, but I wouldn't count on the IDC to function properly.


Compatibility is extremely important. Compatible means "It will work." When considering connecting a device to the panel, always know whether or not the device is compatible with the panel.

Look up what devices are compatible to use with the panel in the panel's installation manual. If the installation manual is lost, most fire alarm installation manuals can be found on the web.

Another source to find out about compatibility is the technical service team for the fire alarm system's manufacturer. These people will be glad to let you know about what's compatible with their system. Remember, the technical support team is there for you, the installer or technician.

All fire alarm systems are different from each other, and all fire alarm systems have different requirements from each other.

Example: Sometimes, even though a smoke detector looks to be exactly the same as another smoke detector, only one model of the smoke detectors can be used, while the other model will fail to work.

When trying to interchange anything in a fire alarm system, always make sure that the new device is compatible with the system. If it's not compatible, "IT WON'T WORK."

Douglas Krantz
Life Safety
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