The duration of each pulse is deļ¬ned by a timing module (Figure 3). Here, a 15-bit data is loaded to a down-counter at the beginning of each waveform step. The counter is driven by a time-base clock, the speed of which is selectable and may be 1 or 20 Mhz (crystal controlled), a slower, software-controlled clock (in 1 ms steps) or an externally supplied one (should be less than 20 MHz but can be very slow). The output of this counter produces a scanning clock, which, in turn, advances a memory-address counter.
The 16th data bit contains arbitrarily placed trigger information, used to synchronize an oscilloscope (or other device) with the desired step of the waveform.