Lamp Matrix Basics
Pinball machines use a lamp matrix to drive the lamps on the playfield, in the cabinet and in the backbox which are controlled (turned on and off) by the pinball software. Using a matrix significantly reduces the wiring between CPU and the other pinball components, mainly the playfield.
The lamp matrix is controlled by multiplexing the signals. This is achieved by enabling the columns one after the other in a very fast way. As both the originally used incascendent bulbs and the human eye are slower than this fast switching this multiplexing is not visible.
Source: Wikipedia
WPC machines update the lamp matrix every 16 milliseconds, i.e. with a frequency of 62.5Hz. For an 8x8 matrix this means that the signal for an individual column is turned on for 2ms and then becomes inactive for the following 14ms. To account for the fact the the lamp is driven only 1/8th of the time the 6.3V bulbs are powered with 18V from the power driver board.
The following image shows columns 1 and 2 as it is driven from the WPC Power Driver Board. The example comes from an Whitewater pinball machine.
If we look closely at the column transition we can see that there is a gap in between where no column is activated for around 30us:
Note that these graphs were produced with a Whitewater Home ROM LH6 revision. This ROM already contains the improvements for anti-ghosting (see below).