User`s guide

E-Prime User’s Guide
Appendix A: Timing Test Results
Page A-3
Figure 2. E-Prime Refresh Clock Test display after 50 runs.
The E-Prime Refresh Clock Test program provides a good diagnostic of a computer’s suitability
for data collection, and assesses several critical characteristics about the computer. The most
important test is the Clock Test. The Clock Test monitors the clock for a period of 10000
milliseconds and checks to see if continuous readings of the clock ever failed to identify
sequential clock ticks. E-Prime cannot completely stop the operating system from suspending an
executing experiment. However, E-Prime can determine if the reading of the clock was halted
long enough to skip a clock tick (e.g., if the values 2001, 2002, 2005, 2006 were returned on 4
successive clock reads, E-Prime sees the 3ms tick between 2002 and 2005), indicating that the
operating system would, in a similar situation, have delayed the experiment. E-Prime uses a
microsecond precision clock for all internal time assessment. If E-Prime is unable to read the
crystal clock when it needs to (e.g., to timestamp the onset of a stimulus), the precision of the
timing is compromised (see Chapter 3-Critical Timing).
The second test is the Refresh Test. This test has three functions. The first is to examine the
display card hardware to see if it is providing a vertical blanking signal (i.e., the period of time
when the raster scan is moving from the bottom to the top of the screen between refreshes). The
second function is to determine the rate of missed refreshes of the hardware. Some display
cards provide only a very short (e.g., 10 microseconds) signal of the presence of a vertical
blanking interval. This signal can be so short that it cannot reliably be detected by E-Prime.
However, to maintain stimulus synchronization to the display card it is important to detect vertical
blank events. Hence, to stay synchronized, the rate of missed refresh detections
(RefreshMissedPercent) should be less than 0.10% without using the vertical blank simulation