Fluctuating Voltage on an SLC
No matter what your hand-held voltmeter shows you, the voltage on an addressable, two wire, Signaling Line Circuit (SLC) is extremely well regulated and extremely stable. The signal is a computer-style data signal; basically, it's a squared off signal that switches rapidly between full voltage and almost zero voltage.
It's your eye perception and the un-imaginably slow reading digital voltmeter that just can't keep up; the extremely rapid turning-on and turning-off with the signal voltage only looks to be unstable.
In order for the panel to send information, or the devices to send data information, the stable voltage of the SLC has to be turned on and off very rapidly; it's turned on and off hundreds to thousands of times a second.
When taking a reading with a digital voltmeter, the voltmeter can't show a continuous voltage reading on its digital display. Your eye would only see a blur as the as the display changes so many times a second, making your readings useless.
To solve this "blur" problem, the voltmeter just takes it slow. Rather than trying to show everything at once, it takes a average level, or maybe an RMS (Root-Mean-Square) type of average reading over a half second time, and displays that reading. Basically, it says, "This is about the best I can do."
The RMS or average reading is somewhere between the minimum voltage (usually around 0.5 volts) and the full, normal SLC power supply voltage.
Fluctuating Voltage even with the SLC Disconnected
The fluctuating readings will be present when the SLC circuit is connected. The panel is rapidly turning on and off the voltage of the SLC to send messages to the devices, and each device is taking turns turning on and off the voltage of the SLC to send messages back to the panel.
When the SLC circuit is disconnected from the panel, the panel's SLC power supply is still sending power to the SLC's screw terminals, and the panel is still sending messages to the devices. When disconnected, though, the panel is still polling each on the circuit, asking each device in turn, "Device so-and-so, are you there?"
With the SLC circuit disconnected, the devices themselves won't be rapidly turning on and off the voltage, but even though the voltage won't fluctuate as much, the voltmeter will show some fluctuation. In other words, the fluctuating readings caused by the panel will still be there.
To answer your question, yes, there will still be fluctuating voltage on your voltmeter, even when the SLC circuit is disconnected.
Douglas Krantz
