F McCaffrey,
why do we on the European continent (and others) drive on the right hand side of the road, and the English (and a mere few others) drive on the left hand side? Why do people write names starting with capitals, which is considered polite, and why is the use of capitals on the internet considered as shouting, and thus being rude?
It all has to do with standards and/or agreements. Some of them are simply choices made sometimes in the past, others are based on solutions for certain problems.
It might help you to know that the oldest standards for analog signals were 0-10V and 0-20mA. Later on the standards -10V to +10V and 4-20mA were introduced, which are now commonly used. Ask yourself what could be the reason for this. Consider the distance between the transducer and the PLC. What to do with bidirectional signals? Motor do tend to turn both ways! What if you had to measure their speed? Ask yourself this kind of questions and you'll find the answers yourself.
Kind regards,
Jean Pierre Vandecandelaere
Instructor PLC - SCADA - Basic ICT
VDAB Bruges
Belgium