Does Anyone Make a Generator/Tester for SLC Circuits?

Greetings Douglas,

I'm new to fire alarms. But mostly I deal with high voltage and PLCs [Programmable Logic Controllers], so fire alarm systems are new to me.

When working on the controls, I have and use a signal generator / tester that does all the common PLC signals. The signal generator makes trouble shooting a breeze. It even gives me distances to ground faults and devices.

When I looked for one that does SLC signals and other fire alarm signals, that device doesn't exist.

Don't get me wrong, the NAC [Notification Appliance Circuit] signals are easy enough to simulate. But making an SLC circuit and zone circuit generator is beyond my skill. But it would be very handy in installing and troubleshooting fire alarms.

If you know of one or would be interested in building one I would definitely be interested in buying one.

Thank you, KR

The following is based on 50 years of experience working with, designing, and repairing electronic equipment, and 20 of those years were directly involved in the fire alarm industry. I know the test instrument that you're looking for, but that test instrument does not exist on the market.

Why There are No Test Instruments on the Market

There are no actual test generators designed for fire alarm system Signaling Line Circuits (SLCs). The real reason has a root cause in attitude. All manufacturers are extremely proprietary. The attitude is because they are afraid that if anyone can connect another manufacturer's equipment to their Type Accepted Fire Alarm System, and something bad happens, they can and will be held liable for the damage caused by someone else.

As an emphasis to that, you will find that no one in the fire alarm industry is ever willing to say "The Fire Alarm System Works". They will only say "The Fire Alarm System is Normal". That is because something might have been missed in the original installation, and if they are hauled into court, they can say on the stand, "I didn't say the Fire Alarm System Works".

Unlike PLC communications, because of the fear of litigation, there isn't any standard voltage, baud rate, or even protocol for SLC communications. That is why all SLCs world-wide are different from each other, and no test instrument manufacturer can possibly keep up with the huge variety of systems.

I have, however, been able to troubleshoot fire alarm system circuits using a cheap multimeter, a low voltage insulation tester, and on extremely rare occasions to find outside interference, an oscilloscope.

Signaling Line Circuit (SLC)

All is not lost, though. An understanding of the SLC can be used for troubleshooting. Even though each manufacturer's voltage, baud rate, and protocol is different from every other, some of the ideas are similar enough to use for troubleshooting almost all SLCs.

Power Supply

From the point of view of troubleshooting, the SLC is a power supply. This is where all the modules and addressable devices get their power to operate.

The power supply is very limited in current. That is because in order to send signals back to the panel, but the modules don't inject a signal on the SLC, they short out the SLC. Very much like using a telegraph key rapidly turning on and off to send Morse Code, the SLC is shorted out rapidly using solid state electronics in the module. If the power supply was capable of sending out a lot of current, the shorting-electronics in the modules would burn out when trying to short out the excess current.

To prevent the component burn-out, the power supply voltage can be easily pulled down to zero. What we would consider a high resistance short can affect the power supply. Water on the loop, for instance, is capable of causing problems.

The power supply voltage being used on the SLC varies from manufacturer to manufacturer, and often even from model to model produced by a single manufacturer. Unless you have an oscilloscope, it's hard to get a good reading of an exact voltage for the SLC's power supply. A standard voltmeter won't give a steady reading because of the large amount of data going back and forth on the SLC. Although, if you watch the meter, eventually you might be able to make a guess on the voltage.

The voltage on the power supply for the SLC is usually somewhere between 16 volts and 35 volts, with most fire alarm systems using an SLC power supply voltage of 18, 20, 22, or 24 volts.

Data - Baud Rate and Protocol

The actual baud rate (bits-per-second) and the protocol (the order and quantity of bits) being used usually isn't information you and I need for most troubleshooting. Even though higher baud rates might be an issue with wire type, length, and outside interference, and getting this kind of information can usually be found by calling the technical support for the equipment.

Getting information on protocol is a different matter. I know of no manufacturer that gives this information out to anyone.

Test Generator

While it is possible to make a test generator (which is really a power supply) for testing SLCs, easier to use is the fire alarm panel as the power source, with its already provided SLC output. No, unless you have the proper equipment, it's too much to hope for to be able to measure wire propagation distance to ground faults, opens, and shorts. But at least some of the information can be indirectly determined from what the panel says, and what the voltmeter, ohmmeter, and insulation tester say.



Douglas Krantz

Mr. Krantz

Thank you very much for the info. Like I said very new to fire alarms. But I found it very interesting on how it works. I personally think we need to add supervision to plc instruments. So we would know when a sensor goes bad or isn't working.

I do have a meter I use for finding the distance to faults it's a cable fault meter PCE-CLT 10 by PCE. I'm not sure how it calculates the distance. But I wasn't sure if it would be safe to use on SLC. But based on the info you have given it should work.

Thank you, KR