Using SetCommand(_S,1,500) in Closed-Loop Speed mode

I'm now using the controller SDC2130 from your company. What I want to do is just to let one motor run at a constant speed in Closed-Loop Speed mode. In the script, when I used the command SetCommand(_G,1,500), the motor ran. However, when I used command SetCommand(_S,1,500) to set the velocity in RPM, the motor didn't run. I did connect the encoder to the motor and controller correctly. Do I need to add some special code?

Use the G command. S will not work in this mode.

Then which mode does S command work for ?
Also, when I tried to use the command GetValue(_EPPR,1) to get the PPR value of encoder, the controller always got lost? Do you know why this happened?

The S command works in the "position velocity" mode. It is a new mode that is not described in the manual and that lets you go to a given count value at a set speed and acceleration.
You need to use the getconfig() function