On the Mars/MEI 9 pin connector in the Stern pinball machines, here is the wiring pinout for 120v ac acceptors:
Pin 4: AC120v "Hot" (black)
Pin 6: AC120v "Neutral" (white)
Pin 7: credit switch pulse
Pin 8: credit switch pulse
If you have a brand new machine and the bill acceptor AC power cable which is supposed to run from the backbox to the coin door was not included in the machine, contact your Stern Pinball distributor. They will send you one for free. It's Stern part number 036-5700-10.
If you need to use 12 volt DC bill acceptors, you will need to crimp/stuff new pins into the 9 pin connector at the appropriate locations. Tyco/Amp (TE Connectivity) # 770987-1 Pin (18-22 AWG) or Mouser Electronics # 571-7709871.
Pin 5: +12 volts DC power
Pin 7: credit switch pulse
Pin 8: credit switch pulse
Pin 9: power ground
12 volt bill acceptor pinout (resized).jpg