External mode error with TMS320F28335

hello，When I use the simple model external run mode，I have the following problems：

Error occurred while executing External Mode MEX-file 'ext_comm':

Failed to connect to the target. A time-out occurred while waiting for the connection response from the target. Possible reasons for the time-out:

a) The target is not switched on.

b) The target is not connected to your host machine.

c) The application for the model is not running on the target. You might have clicked the Stop button. If the Run button is not dimmed, click it. Otherwise, click the Build button, which downloads and runs your application on the target.

I've made sure to change the GPIO configuration for the serial port，and try to us a different serial.I was confused. What should I do?

Help me please. Thank you!