There is no means to "internally" connect the digital outputs and digital inputs. You can always use the input signal to the Digital Output component elsewhere in the PLECS model, but this approach isn't suitable for PWM Output and PWM Capture since it bypasses the RT Box FPGA fabric.
You will have to loopback the signal through a physical connection. You can use a male-to-female DSUB-37 cable to loopback all digital I/O signals, the RT Box breakout boards along with a jumper across the digital I/O pins, or some other kind of wiring approach.
