Standards
Once apon a time in the 'good old days' there was a standard to ease interfacing of different equipment together.
It was called 'RS-232' for simple and effective serial interfacing.
The voltage was a given always a -12 to +12 volt swing, a safe voltage with reasonable noise immunity.
Then one fine day in order to reduce parts count it was implemented as 0 to +12 volt.
Still almost as good and cheaper.
As terminals became PC's the parts count was reduced so the readily available 0 to +5 volt
was used as a power source.
Not quite as good but even less expensive.
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