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Phone Line Polarity Checker
It might not seem that telephone line polarity is important, since the ring and voice signals sent over telephone lines are AC. However the lines are actually biased to 48V DC (less when in use) and so the polarity can matter. The main problem with incorrect polarity is that some DSL (Digital Subscriber Line) modems and routers can perform poorly in this circumstance. Since telephone wires are color-coded, it should be possible to simply check that the sockets are wired correctly. Unfortunately, there are multiple wiring colour schemes and they have changed over time. Believe it or not, the old color scheme (from around 15 years ago) is identical to the current colour scheme except that the polarity of both lines is reversed! This is why so many homes have this problem and yours may well be one of them.
The common telephone line is simply a copper pair, ie, two wires. As mentioned, there is usually a 48V DC bias across the pair which drops to around 8V when a telephone is “off-hook”. The ring voltage (around 90V AC) and the audio signal voltage (also AC) are overlaid on this DC bias. The DC power is “rectified” by each telephone on that line to run its own circuitry. Note, though, that this does not include cordless phones which usually use a plugpack, as their power requirements are far in excess of what the telephone line can deliver. (As an aside, that is the reason it is important to keep a line-powered telephone in your home so you can still make and receive calls if the mains power goes out.
Telephone exchanges can usually supply power from their backup batteries for up to some days, even if they are blacked out). Usually, telephone lines are run with 4-core cable. This allows up to two lines on the one cable. The first line is on the inner pair (pins 2 and 3) and the second line, if present, is on the outer pair (pins 1 and 4). Modern telephones use modular plugs, specifically RJ11 (6P2C, one line), RJ14 (6P4C, one or two lines) or RJ25 (6P6C, 1-3 lines). By the way, 6P4C stands for “six pins, four connectors”. Incidentally, “RJ12” connectors are physically compatible – and commonly available – so that is what we have used in this project.
Because modern phones rectify the DC voltage from the telephone lines before regulating it and because the ring and voice signals are AC, for voice communications the polarity doesn’t really matter.