This circuit allows an operator to watch the light ( ) and know whether
the switch is open or closed. The switch may indicate that a motor is running or stopped, or
whether a door is open or closed, or even whether there has been a fault or the equipment is
A simple animated example showing a tank being filled, and a pump starting and emptying it
is available here. (PLC Tutor site) This example
does not involve telemetry as the pump is close to the tank.
So far there is nothing special about this. But now imagine that the switch and the lamp were
100 kilometres apart. Obviously we couldn't have an electrical circuit this large, and it would
now be a problem involving communications equipment.
Now complicate the problem a bit further. Imagine we had 2000 such circuits. We could not afford
2000 communications circuits. However someone found that we could use one communications circuit by
sharing it. First we send the status (open/closed or 0/1) of the first circuit. Then we send the status
of the second circuit, and so on. We need to indicate which circuit the status applies to when we send
The poor operator at the other end still has a problem. He has to watch all 2000 circuits. To simplify
his task, we could use a computer. The computer would monitor all circuits, and tell the operator when
he needs to look at a particular circuit. The computer will be told what the normal state of the circuit is and what state is an "alarm".
It monitors all circuits, and informs the operator when any circuit goes into alarm.
Some circuits may contain "analog" data, ie they represent a number eg level of water in a tank. In
these cases the computer will be told a top and bottom level value that are to be considered normal. When the value goes outside
this range, the computer will consider this an alarm, and the operator will be informed.
Here is a page that describes some of the early
attempts at long distance communications by wire
(PLC Tutor site)