OK, Hoffman, here's a basic rundown on signals in and out of a simple AC drive system.
You will see on the control terminal wiring diagrams some inputs that may be called digital inputs or contact inputs. These are drive control inputs that are intended to see an on/off kind of signal such as from a relay contact or switch. Your relay contact outputs on your PLC are like that. A drive usually expects this kind of signal to control Run/Stop, Forward/Reverse, etc. Sometimes these inputs are used to form external Run Enable loops (closed to run, open to stop) or external Fault loops (closed to run, open to force a drive to fault). Also, these kinds of inputs can be used to force a drive to run at a predetermined fixed speed, select between two different sets of accel/decel ramps, and a host of other possible functions. Three things to watch out for on these inputs--(1)the inputs expect to see a particular voltage, 24VDC, 12VDC, or 120VAC are common. Be sure you've got the right one (2)be sure to understand what happens when the signal is present and not present. The logical sense of the signal is important and has driven many-a tech to sleepless nights (3)Since all of the digital inputs have a single common return, be careful when mixing signals from different sources. Conflicts in commons require isolation relays or optic isolators to eliminate.
You will also see analog inputs often identified as AI1, AI2, etc. These are inputs that are looking for a DC signal that sweeps from one point to another. 0-10V, 4-20ma, -10V-0-+10V are common. There are a number of uses for these but, for basic applications, the only purpose is to tell the drive how fast to run. Using 0-10V for an example, in the drive parameters you will have to decide what speed or frequency is represented by the 0V (Min Speed) and by the 10V (Max Speed). Often these are set for Min Speed = 0hz and Max Speed = 60hz. In that case the drive would run the motor at 30hz (5V) and at 48hz (8V) etc.
Most drives also have one or more relay outputs, sometimes called digital outputs. These can be used to confirm the status of the drive, the most common being as a Fault relay (operating only if the drive is faulted) or as a Run/Stop relay confirming that the drive is or is not running the motor. Note on this one that it does not indicate how fast the motor is running, only that the drive has the motor energized at some speed. You can use these relay contacts to control lights, horns, or signal back to your PLC the status of the drive.
Finally, there are usually one or more analog outputs. Again, these are indicators of drive status, the most common being an analog speed output and an analog current output. These again may be 0-10V, 4-20ma, or other types of analog signals. If you are sending these to your PLC you will need an analot input card to receive and interpret the signals. At least in the case of a 0-10V signal, you can send that to a 10VDC meter and label the meter face for whatever the signal means.
I judge from your earlier posts that you might have a PLC with no analog input or output capabilities at all. That would normally keep you from generating any kind of variable speed input to tell the drive how fast to run. Some drives have a clever way of getting around this problem. In the software, they have the option of setting up two of the digital inputs as speed controls, one as a reduce speed input and the other as an increase speed input. This feature is usually called MOP speed control or sometimes Floating Point speed control. With this arrangement, using two PLC relay outputs, you can shift the running speed of the drive up or down by closing either of the digital inputs you have set up for that purpose.
There are lots of other things that can be done with drives but these are the basics. Good luck and have some fun learning about these things. Twenty years ago, I was asking those same questions and, given enough time, they will become second nature to you.
