Receive and Send instructions will allow you to send and receive serial data to an external device. The communication method that you setup can be ASCII or Modbus. ASCII (American Standard Communication for Information Interchange) can be used to send to devices such as a printer. Receiving ASCII can be used for connecting barcode scanners to the PLC. The barcode will be read as an ASCII string in the PLC. Modbus serial communication (Modbus RTU) is a standard protocol used in many automation devices. We will demonstrate the Send and Receive instruction by communicating Modbus to a Solo Temperature Controller. Our example will read the current process (PV) value and write the set point value (SV) in the controller. Keep on Reading!
Drum instructions are great tools when you have a simple sequence of events that need to occur at a set time interval or as a result of an event. They mimic an electromechanically drum sequencer. The Click PLC has a drum in the instruction set. We will discuss the drum instruction and look at an example of controlling traffic lights. Keep on Reading!
Allot of times when programming a PLC you need to track what has previously happened. Shift registers allow you to do just that. The Click PLC has a shift register in the instruction set. We will discuss the shift register and look at an example. Keep on Reading!
Continuing our series, we will now look at the numbering systems and addressing used in the Click PLC. Previously we have discussed:
Click PLC System Hardware
Click PLC Installing the Software
Click PLC Establish Communication
The programming software and manuals can be downloaded from the Automation Direct website free of charge. These are being used exclusively in our Click PLC series. Keep on Reading!
Many people ask me what I do when looking at a new PLC model or system. My approach is very straight forward and we will view this in action with this Click PLC series. This series will go from examining the hardware to programming and communicating to the PLC in several ways. If you have questions along the way, please let me know. Keep on Reading!
We will look at a PLC basic tutorial of a paint spraying station. Following the 5 steps to program development this PLC programming example should fully explain the procedure for developing the PLC program logic. Ladder will be our PLC programming language.
We will be using the Do-more Designer software which comes with a simulator. This fully functional program is offered free of charge at automation direct.
Define the task:
Paint spraying system where boxes are fed by gravity through a feeder magazine one at a time onto a moving conveyor belt. Upon the start signal, boxes are pushed towards the conveyor by valve 1. This is a cylinder which extends and retracts which operates switches S1 and S2 respectfully. A spraying nozzle paints each box as it passes under the paint spray controlled by valve 2. A sensor (S3) counts each box being sprayed. When 6 boxes have been painted the valve 2 shuts off (paint spray) and valve 1 (cylinder) stops moving boxes onto the conveyor. Three seconds later the conveyor stops moving and the hopper with its load moves forward (valve 3) where it is emptied. Ten seconds later the hopper returns to the original position. The cycle is then complete and waits for a start signal again.
Define the Inputs and Outputs:
Start Switch – On/Off (Normally Open) – NO
Stop Switch – On/Off (Normally Closed) – NC
S1 – Valve 1 (cylinder retract) On/Off – NO
S2 – Valve 1 (cylinder extend) On/Off – NO
S3 – Box Detected- On/Off – NO
Motor – On/Off (Conveyor Run)
Valve 1- Cylinder to feed boxes – On/Off
Valve 2- Paint Spray – On/Off
Valve 3- Cylinder to move hopper – On/Off
Develop a logical sequence of operation:
Fully understanding the logic before starting to program can save you time and frustration.
Sequence Table: The following is a sequence table for our paint spraying application.
1 – Input / Ouput ON
0 – Input / Output OFF
x – Input / Output Does not Matter
When power goes off and comes on the sequence will continue. This means that we must use memory retentive areas of the PLC. The stop pushbutton will stop the sequence. The start will resume until the end.
Develop the PLC program:
The best way to see the development of the programmable logic controller program is to follow the sequence table along with the following program. You will see the direct correlation between the two and get a good understanding of the process.
Control of the Motor (Conveyor) and the paint spray is done with the V0:0 contact in front of the actual PLC output. The conveyor and paint spray will stop when the timer 0 is done. This is the delay after the last box is detected to allow the box to be painted and loaded onto the hopper.
Test the program:
Watch on YouTube : PLC Programming Example – Paint Spraying
If you have any questions or need further information please contact me.
If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.
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