On a device like a speaker, the quick answer is that most of the time, it doesn't matter which of the Plus Terminals and which of the Minus Terminals are to be considered the input connections (from the control panel), and which ones are considered output connections (going to the end of line resistor).
But then again, once in a while, it does matter. The hard part is knowing which device is internally wired so the inputs and outputs don't matter, and which device is internally wired so the inputs and outputs do matter.
The Installation sheets that come with the device will show the exact wiring for fire alarm devices, ranging from smoke detectors to speakers, and everything in between. On these sheets, somewhere, is a stamp from a testing laboratory like UL, ULC, CE, CCC, FM, etc.
The schematic drawing on the sheet has been tested - exactly - by the third-party testing laboratory, and that schematic has been listed by the laboratory as a working wiring diagram. You can trust that exact schematic to work.
Common Sense Works - Most of the Time
The issue is that most of the time, people like you and me could change the wiring a little without affecting how the fire alarm system works. We could even use our ohmmeter to make sure the terminals are internally connected together.
With fire alarm systems, there are a few times where this kind of common sense shouldn't be used.
For instance, if there's a relay
inside the circuitry, an ohmmeter may show that the two Plus Terminals are shorted together; falsely showing that it doesn't matter which is the input or output. Yes, I've found where someone else had done this in-and-out-terminal reversal, and because the manufacturer had a hidden relay, I had to correct the wiring.
One manufacturer of addressable systems
shows the input and output terminals on all the Signaling Line Circuit (SLC) devices clearly on their installation sheets. With most of their fire alarm control panels, however, reversing the wiring between the input and output terminals will not affect anything. But, on a few of their control panels, reversing the ins and outs on the devices will prevent the system from fully functioning. The ohmmeter shows almost a dead short between the in and out terminals of the device, but because of a very low value resistor, the system doesn't always work if it's wired in reverse.
Conventional smoke detectors made by another manufacturer have a "Dirty Smoke Detector
" signal that can be sent to the control panel. The "Dirty Smoke Detector" signal only works when the smoke detector has its input and output terminals wired according to the wiring diagram on the installation sheet.
Common Sense and Life Safety
People depend on fire alarm systems
to detect fires, and warning them of the danger. If somebody isn't warned when there is a real fire, their life may be in danger.
Most of the time, the input terminals and the output terminals of the devices, like smoke detectors, pull stations, horns, strobes, and speakers can be reversed. Yes, just to see how they're wired, I've taken apart some of the devices after they failed and have been replaced. Many of the devices can be wired so their input and output terminals are reversed; a few of the devices have to be wired according their installation diagram.
My problem is with most devices; most devices I have not personally taken apart. On most devices, I don't know for sure which ones can have their input and output terminals reversed.
Yes, sometimes it does matter that a device like a speaker is wired exactly as shown on the installation sheet. Because people depend on the fire alarm system to always work, I always read and follow the published installation directions from the manufacturer. That's a policy I can trust.
Bottom line, if I always read and follow the published installation directions, I will never accidently reverse the wires on some terminals that should not be reversed.