You could use analogue and/or digital I/O from the PLC, or depending on your drive and PLC you could use RemoteIO (RIO), DeviceNet, ControlNet, Ethernet,
Profibus, or others. The choices are many, with various pros and cons.
Most VFDs have digital, analog and communication interfaces you can use to exchange with an external controller. Digital signals will include for example Start, Stop, Rotation CW, Rotation CCW, Jog, Set Speed 1, Set Speed 2 (these Set Speeds are from the drives parameter set), drive status, alarms... Analog signals will include Speed Reference, Speed Feedback, motor current, voltage, DC bus voltage, RTD feedback...
Communication fieldbuses will give you both digital and analog signals, through Modbus, Profibus-DP, Ethernet/IP, ControlNet, and many others, depending on the manufacturer. And snce they allow multiple connections, you will save a lot on cabling, one cable from PLC to many drive nodes.
Many VFDs are available with communication cards, so you may wish to communicate over Profibus, DeviceNet, etc. Without knowing more about your system and the type of PLC/VFD it would be difficult to provide a detailed solution.
This is really a broad question. It depends if your integrating into an existing system or if it is a stand alone project. The selection also depends on your choice of technology. Certainly what I call the old way of hardwiring I/O still works, although it's not real practical with the new technology available to us.
Speaking from the direction of Allen Bradley (what I normally use) you can use Ethernet/IP, ControlNet, and DeviceNet. I don't believe they have sercos on VFDs, I believe this is a servo protocol only. My choice is Ethernet/IP. As I do not know what "brand" your using, I will say the Logix Platform (PLC) and PowerFlex 4, 70, and 700 series are wonderful. Using Ethernet/IP (22 or 20 comm E module) and ControlLogix is very cool. It's as easy as adding an ENBT card in your I/O tree and adding the drive under that. You're done. Even your tags are created for you. All you have to do now is right the logic and use the tags that were created for you. Can't get much easier than that. ControlNet and DeviceNet are fairly easy as well, those two are just not choice of protocols for drives. I have been using Ethernet/IP for years now and it works wonderfully.
1. Install 1 'pilot' relay in the drive enclosure. This is your 'RUN' relay. Drive the relay with a discreet output from your PLC.
2. The drive should have a dry contact output that indicates that it is 'RUNNING.' Wire this to a discreet input to your PLC.
3. If you need a speed reference or setpoint. The preferred way would be a 4-20 mA isolated analog output to the drive.
Of course, depending on the features of the drive, there may some digital interfaces available such as Allen-Bradley Remote I/O, DeviceNet or FieldBus. Depending on the application, you may still want an analog speed reference while using digital start/stop/status. For safety you will also want to use some sort of dry-contact Emergency stop.
The interface could be as simple as one wire for run, with a single preset speed in the drive... to analog signals for speed and/or torque control... to a comms bus as previously mentioned. If you want to get the best answer to your question you need to give some application details as to what you want to accomplish.
Be cautious of someone recommending everything AB, I don't put much stake in what they have to say. AB drives stink, a lot of good drive companies make drives that interface to AB comms (So you can have all the CLX [ControlLogix] in the world, but buy a real drive). Don't buy into the integrated architecture line! Do some research on drives before you buy...
If You Want only on/off control from PLC, then connect the drive to digital output module. and if you want to vary the frequency through PLC, then connect PLC analog output module.
