Before you go into Run Mode, make sure that the E-stop button works properly. I have not included wiring diagrams in this book, but most codes state that all control power is removed if the E-Stop button is pressed.
NFPA 79 Electrical Standard for Industrial Machinery states the following:
“9.2.2* Stop Functions. The three categories of stop functions shall be as follows:
(1) Category 0 is an uncontrolled stop by immediately removing power to the machine actuators . . .
9.2.5.4.1 Emergency Stop. Emergency stop functions provided in accordance with 9.2.5.3 shall be designed to be initiated by a single human action.
9.2.5.4.1.1 In addition to the requirements for stop, the emergency stop shall have the following requirements:
(1) It shall override all other functions and operations in all modes.
(2) Power to the machine actuators, which causes a hazardous condition(s), shall be removed as quickly as possible
without creating other hazards . . .
(3) Reset of an emergency stop circuit shall not initiate a restart.”
(National Fire Protection Association, 79-22, 79-23)
Make sure you follow your company’s start-up procedures as you begin testing the system.
The next step is to put the processor in Run Mode. Do not enter this step lightly; you MUST keep the safety of your co-workers in mind. Look around the machine and make sure everyone is safely located.
Find the mode drop-down menu in the upper left. It currently says “PROGRAM”. This means that the PLC is not currently running the program.
The key switch in the PLC has 3 positions: RUN (run mode), REM (remote run mode, meaning that the mode can be selected from RSLogix) and PROG (program mode).
Carefully switch to REM. This puts the processor in remote run mode. Notice what the dropdown menu indicates.
Click on the dropdown and choose “Test Continuous”. This is a special mode that allows the program to run, but disables all outputs.
Scroll through the program using the scrollbar to the right in the LAD 2 – MAIN window.
Looking at Rung 0003, for example, you can see that the System Enable bit B3:0/0 XIC is true, as indicated by the green highlight on the instruction.
Any XIC instruction tells the PLC to look at a bit, and if the bit is ON, then the instruction is true.
The system is not batching as shown by the XIO instruction used for B3:0/2. An XIO instruction tells the PLC to look at a bit, and if the bit is OFF, then the instruction is true. The XIO instruction in Rung 0003 is true because bit B3:0/2 is off.
Because all of the preceding instructions before the B3:0/4 System Ready bit are true, the OTE is true. Again, it is highlighted.
Sometimes it is necessary to cycle power on your computer.
Making the first connection to a PLC can be tricky. Prepare yourself by having the phone number for your Rockwell rep handy.
Think of the equivalent hardwired relay circuit that would generate the same function as Rung 0003. Imagine that we have 3 relays, labeled B3:0/0, B3:0/2 and B3:0/4. We have wired a normally open contact of B3:0/0 in series to a normally closed contact of B3:0/2 and to the coil of B3:0/4. We have also wired a pilot light in parallel with the coil of our imaginary relay B3:0/4.
The coil for relay B3:0/0 is energized, so the normally open contact is closed. The coil for relay B3:0/2 is not energized, so the normally closed contact of B3:0/2 remains closed. The coil of B3:0/4 is now energized. The pilot light is illuminated.
Getting back to our PLC rung, you can see that the branch that contains the pilot light output shows that OTE is true also. However, since we are in TESTCONT mode, the output on the module is not electrically energized. You can look at the card and see that the LED for this output is off.
Look at Rung 0007. The first two instructions look right, as the system is not batching and we are not in Batch Step 2. Neither of these is highlighted. However, the LEQ (less than or equal to) is true; Source A is 0, which is certainly less than the value of 1665 in Source B.
RSLogix does not highlight some instructions, such as an LEQ. You have to mentally compare the numbers yourself to see if the instruction is true.
In Allen-Bradley PLC
vernacular, upload means get the program from the PLC and load it in RSLogix on your computer.
Download means send the program from RSLogix on your computer to the PLC.
We need to check our analog inputs. Double-click on “LAD 3 – ANALOG” in the Project tree.
There is no liquid in the tank, so our input reads 0. The SCP instruction puts a value of 0 in the output register N7:0.
Rung 0001 of the analog ladder also reads correctly. Click on the “MAIN” tab at the bottom of the ladder window to return to the main ladder.
After you are convinced that all of your logic is working properly, click on the mode dropdown and select “Run”. A screen will appear asking you to verify the mode change to Run. If you choose “Yes”, the outputs on the PLC will be energized, so be ready with the E-Stop switch.
The dropdown menu will now display “REMOTE RUN”.
You can look at the LED on the PLC for output O:5.0/0 and see that it is on. This output is for the System Ready pilot light.
Scroll to Rung 0004. Press the Start Batch pushbutton and you should see the XIC instruction for I:4/0 highlighted. This makes the rung true, and B3:0/10 is turned on. Release the
pushbutton and the rung will look like this.
Notice that even though the rung is no longer true, B3:0/10 is on. This is because it is a latch instruction and will remain on until the corresponding unlatch instruction is true.
Scroll to Rung 0005. You can see that system is in Batch Step 1 and that the City Water valve is open. The pilot light is on. The liquid weight in the tank is rising as shown in the LEQ instruction.
The sequence continues as the specified weight for each ingredient is reached.
Test your E-Stop circuit thoroughly, even if you are working on a system that has been running for months or years. Be ready to hit the E-Stop button if you see a hazardous situation developing.