i. A analogue signal is a electrical signal thats varies in frequency or amplitude and can used to measure curtain things such as heat, pressure or weight, with a high degree of accuracy, this is because analogue signals can be adjusted more finely, compared to digital in such respects, in the above application the as the temperature varies the analogue signal will also vary, and using preset values for the amount heat or lack of it, the temperature can be accurately read.
ii. Digital signals on the other hand are either fully on or fully off, and can be used in programming because you can represent these as 1’s or 0’s, this is useful in lighting or communications, in respects to the LED display stated above the signal you could program each number to be shown by using a combination of the LEDs on the display, or instance for a 8, all the LED’s could be turned on or a 7 it would be A, B and C on the diagram below. The PLC would assign each one of the LED’s a output and when a curtain number is needed it could use these outputs and a digital signal to turn a combination of them on.
iii. Wireless signals can be used to communicate with the need for hard wires and actual physical connections, the most famous and commonly used is radio, but in respects to PLC’s wireless signals differ to radio signals, PLC’s can communicate with each other using devices such as the Omron WD30 which use connects a master PLC to slave PLC’S, below are the specifications of the WD30 showing the wave lengths and frequencies ect.
This wireless system would work by using a PC to program a master PLC, and using that PLC to communicate to other slave PLC’s which could be used to operate machinery, basically using a master plc to tell other ones what to do and control output from one source, without wires, as shown below in the diagram, this is also of the omron FAQ section.