GS20(X) VFD: PLC Quick Start Part 1 - Preliminaries

Every GS20 Variable Frequency Drive has a built in PLC that's perfect for those times when you need just a little extra logic. For example, suppose you have an irrigation system and you just want to run the pump a couple times each day. All you need to do that is a timer and a little bit of logic. The PLC that’s built into the GS20 drive is perfect for that. Or, suppose you have a conveyor that you need to stop automatically after a certain number of objects is counted? All you need for that is a couple inputs and a little bit of logic. Again, the built in PLC is perfect for these kinds of applications. Since you no longer need an external controller, power supply, wiring or enclosure, you get your system up and running quicker, have lower maintenance costs, save on control system foot print, save on reduced spares inventory, and will have an all-around less expensive and more reliable system. BUT, because you have both a PLC and a Drive in the same box, there‘s are a couple thigs you need to be aware of before programming the PLC. While the drive does come with a built-in PLC, it comes from the factory with the PLC disabled. And for all practical purposes, it’s not there until you enable it. That means you can’t even talk to it or set it up or program it until you enable the PLC. How do you enable it? You have three options. Option 1: From the key pad go to the PLC menu. PLC 0 is Disable, PLC1 is RUN and PLC 2 is STOP. So either PLC 1 or 2 will enable the PLC. Option 2: You can use the Digital Inputs to control the PLC. Assign any two digital inputs as PLC mode control bits. You then control the PLC by setting the switches according to this table. Option 3: Use the optional GS4 Keypad. This is my favorite because I simply drop into the menu, and go to the PLC. Now I just select the mode I want. Regardless of which method you use, just remember that you won’t be able to work with the PLC if it is disabled. It has to be enabled in STOP or RUN mode to work with it. Once the PLC is not disabled you now have something new to keep in mind. The GS20 Drive has built in analog and digital I/O. When the PLC is enabled you now have one set of I/O that both the DRIVE and the PLC have access to. You can’t have two devices controlling the same I/O, so which one gets control? The answer is: The PLC AWLAYS wins IF the PLC is not disabled AND the PLC uses the I/O in its ladder code. So, for example, if the PLC uses Ouput3 in the ladder code – then the drive’s corresponding digital output won’t function – even if you set it up in the drive to do something. If you disable the PLC, then the drive will regain control of that digital output until you re-enable the PLC. Keep an eye on the various mask parameters – they will show you which I/O are owned by the PLC. For example, this says bit 3 is being used by the PLC. That’s easier to see on the optional GS4 keypad because it actually gives you a bit by bit display. This is showing us graphically that same bit 3 is in use by the PLC. Again, the PLC always wins. That includes both digital and analog I/O. And finally, when trying to remotely communicate with the two devices, remember that the Drive and the PLC have different ModBus addresses you can change. So be careful not to set them to the same address. They come from the factory as address 1 for the drive and 2 for the PLC. Why do they have different addresses? Because they both use the SAME RS485 serial port for communications! External devices can talk to the PLC and or the Drive via the modular RJ45 Jack or the pins on the terminal strip. These are connected in parallel inside the drive so you can easily Daisey chain devices with different connector types. Of course, you can only have one master device on a 485 network and since GSoft2 and GSLogic are both masters – that means you can only have one connected to the drive at any time. That is, you can only program the PLC OR Configure the drive – you can’t do both at the same time because they will try to talk over each other and mess up everybody’s communications. OK, that’s enough background stuff to get us started. Join us in Part II of this Quick Start where we’ll do a live demo of how to get the drive’s built-in PLC up and running quickly. Click here to see all of the GS20 variable frequency drive video tutorials in this series. Click here to subscribe to AutomationDirect’s YouTube channel so you will be notified when we publish new videos like this and click here to learn about AutomationDirect’s free award-winning support options.