onTime = floor(duty*100);
This will always be 0. So your code is going to set PWM_OUT and 9 HIGH for only a tiny fraction of time before setting them LOW again.It also doesn't make sense that you have pinMode and digitalWrite statements controlling both PWM_OUT and pin 9, since PWM_OUT is defined as 9.You need to provide a detail explanation of what you're trying to accomplish.
Now my question -- on the right side edge of the board, is a 4-pin, a 2-pin, and a 3-pin. Those will also go to the top board. Do you know what they control? As was mentioned, we're doing an awful lot of guessing here.
3-pins is for incline and 2-pins connected to speed sensor and 4-pin not connected to any thing
Ok, good to know. That means speed control is going through the 6 pin connector. Now the 6 pin has VCC (power), S/W (kill switch), and NC (not connected). That hopefully means speed is controlled through some combination of RX, TX, and Ground. Try to confirm this. Without Arduino, on the 6 pin going to lower board, jump together VCC and S/W. Then jump together RX, TX, and GND of the lower 6 pin connector and the upper 6 pin connector. See my pic. If all goes well with that setup, you should still be able to run the treadmill. If you're able to speed up and slow down the treadmill with that combination, try it with just RX and Ground, or just TX and Ground. I'm now thinking that you'll need to use all three to control the speed.
about rxd signal i measure it with gnd with multi meter with frequency option i found when the treadmill is in low speed the frequency 5Hz and the duty cycle 99% to 100% and when i raise the speed the frequency goes up and the duty cycle is staying at 99% to 100% and i try this arduino code with pin rxd and gnd and nothing happened