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Old December 15th, 2002, 09:59 PM   #1
sandhu
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Communication between Ladder Diagram and PLC

Hi,
I am doing a project on sensors. I am also using a serial communication (RS232) for my application but i have realized that i can't run 2 applications at the same time. I have got a problem in communications between the ladder diagram and the PLC. How do i design my Ladder Diagram in such a way that when a sensor is detected the signal is sent from the I/O port input to the serial cable back to the Ladder Diagram (showing sensor has detected something).Thanx

Sandhya
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Old December 15th, 2002, 10:54 PM   #2
chavak
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Hi Sandhya,

It is not very clear what are you trying to do. Is it a PLC application, if so, what is the stuff i/o port input -> serial cable
-> ladder diagram ??. In normal PLC stuff, the sensors are connected to the input cards of PLC and the ladder reside in the PLC memory. During program run, you can go online and monitor the status of the inputs, outputs or bit status and so on using the application software.

Tell us more of what you have, in terms of hardware, software and setup of your system, most of us here can help thus
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Old December 15th, 2002, 11:21 PM   #3
sandhu
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Hi,
I am using a Ladder Builder to control the way how the PLC works. I have got a sensor which is attached to the pin1 of the I/O Connection (KV-300) of the PLC. I am also using a serial connection to coomunicate between the PLC and the PC. When the sensor senses an object, a signal will be seen at the builder. I would like to know how do I build the Ladder diagram to control the way I want the PLC to work. Is it confusing.. Pls let me know. Thanx.

Sandhya
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Old December 15th, 2002, 11:33 PM   #4
Allen Nelson
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I suggest you start by going through the tutorial on this site and buying Phil's book.

But to get you started....

The PLC hardware takes care of reading the input signal. The hardware first reads all the input points. If the voltage is high, the hardware writes a '1' in a memory address that corresponds to that input point. If the voltage is low, the hardware writes a '0'.

Then the hardware runs the ladder logic, a rung at a time. The ladder logic reads that memeory address, usually with a -| |- (="does the memory address = 1") or a -|/|- (= "does the memory address = 0").

Then the hardware examines the memory addresses that correspond to the output points. If there is a '1' there, the output is energized. If it's a '0', the output is de-energize.

Then the hardware does the inputs again, followed by the ladder logic, followed by the outputs. (This series is called a "scan")
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Old December 15th, 2002, 11:42 PM   #5
sandhu
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Since the hardware takes care of reading the input signal how will it show in the diagram that it has sensed an object and i believe i need to draw the diagram. How do i do this..
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Old December 15th, 2002, 11:56 PM   #6
rogerhollingsworth
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sandhu

As Allen has suggested you start by going through the tutorial on this site and buying Phil's book.

I know it sounds like an easy out but there are too many things to be learned before you start drawing a ladder diagram, it would be really difficult to cover it in this forum.
Go through the tutorial then if you have questions come back.

Good luck
Roger
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