Thanks Pete for your response.
I enabled logging..now i'm receiving
*** EVENT: Cntrl= 65860 (0x00010144), Param= 0 (0x00000000) AUTO_THROTTLE_ARM
*** EVENT: Cntrl= 65891 (0x00010163), Param= 1 (0x00000001) AP_PANEL_SPEED_HOLD
*** EVENT: Cntrl= 65918 (0x0001017e), Param= 0 (0x00000000) AP_MACH_HOLD
as you can see in logged, first i turn on AUTO_THROTTLE_ARM, in 2nd line i press speed hold switch AP_PANEL_SPEED_HOLD, in 3rd i press C/O button. All I did in virtual cockpit by mouse. or without joystick. also C/O button working fine it's changing airspeed display value into decimal point.
when i assign AP_MACH_HOLD to a joystick's button it does not change airspeed values into decimal point. I don't know why?
I'm not using PMDG add-on.
Thanks.