Talk:Programmable logic controller
From Academic Kids
I removed the following from an article written at "Programmable Logic Controller". The extra information should be merged in:
A Programmable Logic Controller (PLC or programmable controller) is a programmable computer dedicated to industrial applications. The main difference with a computer is the connection to the process through the use of inputs and outputs linked to sensors and actuators. It is used to control and monitor machines. Everything from limit switches, simple level devices and temperature indicators to complex positioning systems or even machine vision can be detected from a PLC. On the actuator side, pneumatic or hydraulic cylinders or diaphragms, magnetic relays or solenoids as well as any kind of electric motors can driven.
The earliest PLCs expressed all decision making logic in simple [Ladder Logic] inspired from the electrical connection diagrams. The electricicians were quite able to trace out circuit problems with schematic diagrams using ladder logic. This was chosen mainly to reduce the apprehension of the existing technicians.
Recently, inspired from Grafcet, the PLC have integrated the Sequencial Function Charts : a new graphical language which allows now to directly program the sequencial nature of processes.
Today, the line between a programmable computer and a PLC is thinning. With the [IEC-1138] standard, it is now possible to program these devices using structured programming languages (such as C), and logic elementary operations.
Programming PLC
There is some info missing: PLC can be programmed not only in ladder languages or C, but there is a galore of possibilities:
- ladder logic (mentioned),
- structural languages - mostly C (mentioned),
- low-level languages- assembler, basic,
- block languages, like FBD, PD, FUP...,
- sequential function charts.
However, I don't feel wise enough to write more :D
RE: History
There seems to be some confusion about the history of the PLC. The present article seems to indicate that GM was the driving force behind the development of the PLC. However, in this article - http://www.barn.org/FILES/historyofplc.html, Dick Morley, its inventor, seemed to have had in mind the machine tool industry when he first envisioned it. Can anyone clarify this issue? Which came first - GM's proposal for a PLC or the PLC its self that neatly fit into GM's needs?
