GS4 Variable Frequency Drive (VFD) Modbus RTU Quick Start - Part 1

Connecting to a GS4 Drive via Modbus RTU is really pretty easy. Just make sure parameter 9.86 is set to Modbus RTU – not BACnet. The Modbus is what your controller needs it to be and that the com settings match what your controller expects. That’s for reading and writing any parameter to the drive. If you want to CONTROL the drive via Modbus RTU then you need to also set the local or remote source of operation to one of the RS485 options and the source of frequency control to one of the RS485 options. And of course, don’t forget to switch to remote mode on the key pad! Let’s do a quick example so we can see how all of that fits together. For this demo, instead of using a PLC as our Modbus RTU master, we’ll use this Modbus Poll application which I got from here. It has a free trial that runs for 10 minutes if you want to try it yourself. This isn’t an AutomationDirect product, so please don’t call AutomationDirect’s support team about it – they won’t be able to help you with it. To use Modbus Poll, you just go to Connection, Connect. I’m using this USB to serial converter from AutomationDirect, so I need to specify the serial port by selecting that adapter which is this Silicon Labs one . These are the default com settings for the drive so we’ll set Modbus Poll to use those. Your controller will probably already have these set to something else, so you will want to change the drives default settings to match those using these parameters. Also make sure RTU is checked since that is what we are using to talk to the drive. Uh-oh … what happened? Well, you can’t have two apps using the same USB to seral port adapter and I have a copy of GSLOGIC running with Monitoring on so I could see the drives reading PLC Analog input 1 and writing it to Drive parameter 820 – which is a generic buffer register for the PLC to use. But that monitoring function uses the serial port. So, let’s turn off Monitoring, then try to connect to the drive via Modbus Poll. Ahh, much better. Which brings up another point: You can’t use GSLOGIC and your controller at the same time on the same Modbus network – there can only be one RTU master on the bus at a time, right? So, be sure to keep that in mind. So, since the drive’s PLC is writing to Parameter 820, let’s see if Modbus Poll can read that. I see in the GS4 user manual that Parameter 820 corresponds to a Modbus address of 42069 decimal. This is important: notice that the GS4 drive uses the hex address plus 1 Modbus convention for the decimal address, so in Modbus Poll we go to the Display menu and set that option here – otherwise all of our addressing in Modbus poll will be off by 1. Make sure your controller also takes that plus one convention into account when talking to the GS4 drive. So, under setup, read write definition, we just enter a 2069 as the starting address – the 40,000 is assumed. Let’s look a block of 20 addresses, and update them once a second. So, we told Modbus Poll to start at address 2069, but Modbus poll actually starts on ten word boundaries, so it starts at address 2060. Which means the address 2069 is down here at the ninth offset. Again, that’s parameter 8.20 which I’ll add in this alias column just to remind us. If I reach over and change the potentiometer that I have connected to analog input one, sure enough Modbus Poll sees the value the built in PLC put in parameter 8.20 and updates it at the 1 second rate we specified. If I go to parameter 8.21 on the keypad and write to it from Modbus Poll, sure enough, we see the result on the drive. Cool! We can read and write from a remote Modbus RTU Master – the Modbus Poll app in this example - which is all we wanted to do. Perfect. So, this Modbus Poll application is a great way to independently verify that Modbus RTU communications are working with your GS4 Drive. Once you have that working, it should be easy to get your controller up and running over Modbus RTU with the GS4 Drive. And all we did to read and write parameters to the drive was: Make sure we are in RS485 Mode – Not BACnet, Setup the Serial parameters, and the Modbus address to be compatible with our Modbus RTU Master. We also made sure we only had one RTU master. Given that we just looked up the address in the manual for the item we wanted to read or write to and made sure that our controller was using the Modbus Base 1 decimal addressing scheme. What if we want to run or stop the drive or change drive’s frequency? To do that we need to tell the drive to accept remote commands. We’ll do that in Part II. Click here to learn more about the GS4 Drive. Click here to learn about AutomationDirect’s free support options. Click here to subscribe to AutomationDirect’s YouTube channel so you will be notified when we post new videos.