The MAME Windows output system sends commands to control the hardware on the original arcade cabinet that was controlled by commands from the original game PCB.
Examples of this include the Q*bert knocker, the flashing start button LED after adding a coin in Dig Dug (0:55-1:00), and lighting up the selected mission on the Lunar Lander Mission Control Panel.
This should not be confused with using LEDBlinky and "controls.ini" to light RGB LED buttons. That is an external process not controlled by MAME because the original cabinets did not have RGB LED buttons.
five important questions that need to be answered:
1. Which version of MAME broke the old output system? 0.170
(so people with earlier versions know that they can disregard this post)
2. Which version of MAME did the old output system start working again? 0.176 (?)
(so people know that they need to upgrade to that version or newer to use the old output system)
3. How do I use the old output system with MAME 0.176 and later?
Just set the following in mame.ini:
# OSD OUTPUT OPTIONS
4. Which version of MAME added the new Windows output system? Headkaze's patch was added in 0.175.
5. Which version of MAME has the new Windows output system working? To be determined.
(As mentioned here
there is still testing to do and further patches to implement and
software developers will need to rewrite their .DLL library files to work with the new system)
EDIT: Added probable version #s.
EDIT 2: Added more questions and examples.