GETTING STARTED WITH PLCS: PART 2

摘要

Last month, Part 1 provided a basic introduction to PLC programming, and this month, additional information and PLC programming features will be discussed, including a more complicated circuit — the roll-up door opener controller. The first article also left us with a start/stop circuit that did not have a hold-in feature; you had to hold the start button in to keep the motor running, which is inconvenient. To add a hold-in feature to the original start/stop circuit from Part 1, you must first go offline if you are currently online with the PLC. To do this, change to offline in the remote-run pull-down box and then modify the ladder diagram by adding a branch parallel to the start button with an XIC contact with the address O:0/0 (Figure 1). You may notice a series of the letter "e" appearing to the left side of the rung after you make modifications to it. This indicates that the rung has been changed or "edit- ed" and has not been veri-fied again after the changes when in the REMOTE RUN mode with the PLC, You must go offline and then download the change back into the PLC before you can test them.