Fig-1 shows the relationship between the programmable controller (PC) and its peripheral devices.
It is a PC that enables a microcomputer to take place of the signal processing part (logical operation / control) in the logical circuit (hard-wired logic) which consists of the relays, timers and counters. The PC is activated by the input from the push button switches and sensors (proximity / limit switches) and control the output loads such as motors and pilot lamps.The control contents for the output loads are determined by the programs given to the PC.
Fig-2 shows how a human being is involved with the PC.
When the human being inputs the program through the programming console, the content is stored into the memory. Once "RUN" command is given by the human being to CPU, CPU executes the program to control the inputs and outputs.
Signal Flow
Fig-3 shows the signal flow in the case of that a switch turns a lamp ON. The signal flow in a relay circuit is shown in the left figure while that in a PC is in the right figure.
In case of PC, the input relay "X000" must be primary turned ON by an external signal such as a switch. This signal activates PC.
Secondary, PC activates the internal circuit through the input relay contact. This part is called an internal sequence circuit or a ladder circuit which is actually created by the program.
Finally, PC activates the external load such as a pilot lamp through the output relay.
Even though it seems easier to use a relay circuit for such a simple control, it becomes very hard to work for wiring when the control becomes complicated.
As a merit of PC, it is still easy to create the internal circuit no matter how the control is complicated since it is just a matter of encoding through the programming console.
Even though it seems easier to use a relay circuit for such a simple control, it becomes very hard to work for wiring when the control becomes complicated.
As a merit of PC, it is still easy to create the internal circuit no matter how the control is complicated since it is just a matter of encoding through the programming console.
(C) 2004 ADWIN Corp.