BigDumbDinosaur wrote:
Prior to the introduction of electronics to railroad signalling (which was a slow process), a device called a polar relay was used at the receiving end to control different parts of the signal system according to input signal polarity. A polar relay has three states: off, and two on states that open/close different contacts depending on the polarity of the applied voltage. Aside from adding a measure of reliability to the system, a polar relay could produce three possible states from a single pair of wires, a not-inconsequential consideration when you think of the many miles of wiring needed on a long railroad line.
Polar relay circuits are still in use on many railways. Although in the U.K. the more modern version is normally a polar circuit made with two identical relays, each of which is made polarity sensitive (by magnetic means) wired back to back, with opposite polarities. So you get one of three states: both de-energised, one energised, or the other energised. Used to save the cost of long cables with lots of cable cores.
Note that the standard has changed name over the years, TIA-232 is often known by its older name of RS-232. The Wikipedia page is
here.
TIA-232 has been used at 1200 baud to cover distances of greater than 400 yards when using good quality twisted pair telecom cables.
With short range connections (say three metres or less) TIA-232 will normally work fine at +/-5V.
Some of these line driver chips run off the +5V rail, and use +10V/-10V because they use a capacitor based voltage doubler and inverter. So as they run off a single +5V supply, double is approximately +10V, and when inverted becomes nominally -10V. But when under load, the voltages will dip slightly.
Mark
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