Measurement Computing USB-1602HS-2AO - Manual (Page 21)

USB-1602HS-2AO User's Guide Functional Details
21
Counter/Totalizer
The acquisition is triggered with the counter/totalize inputs. You can program triggering to occur on a
frequency, or on total counts that are equal, not equal, above, below, or below a value, or that are within/outside
of a window rising/falling edge. Latency is one sample period, maximum. You can select of the included
counter channels as the trigger source.
Software-based triggering usually results in a long period of inactivity between the trigger condition being
detected and the data being acquired. However, the USB-1602HS-2AO avoids this situation by using pre-trigger
data. When software-based-triggering is used, and the PC detects the trigger condition — which may be
thousands of readings after the actual occurrence of the signal — the USB-1602HS-2AO driver automatically
looks back to the location in memory where the actual trigger-causing measurement occurred, and presents the
acquired data that begins at the point where the trigger-causing measurement occurs. The maximum inactive
period in this mode equals one scan period.
Set the trigger value > 0 when using a counter input as the trigger source
When using a counter for a trigger source, set the trigger value to 1 or greater. Since all counters start at zero
with the first scan, there is no valid reference in regard to rising or falling edge. Setting the trigger value to 1 or
greater ensures that a valid reference value is present, and that the first trigger will be legitimate.
Stop trigger modes
You can use any of the software trigger modes explained previously to stop an acquisition. For example, you
can program an acquisition to begin on one event, such as a voltage level, and then stop on another event, such
as a digital pattern.
Pre-triggering and post-triggering modes
The USB-1602HS-2AO supports four modes of pre-triggering and post-triggering. When using pre-trigger, you
must use software-based triggering to initiate an acquisition.
No pre-trigger, post-trigger stop event
In this mode, data acquisition starts when the trigger is received, and the acquisition stops when the stop-trigger
event is received.
Fixed pre-trigger with post-trigger stop event
In this mode, you set the number of pre-trigger readings to acquire. The acquisition continues until a stop-
trigger event occurs.
No pre-trigger, infinite post-trigger
In this mode, no pre-trigger data is acquired. Instead, data is acquired beginning with the trigger event, and is
terminated when you issue a command to halt the acquisition.
Fixed pre-trigger with infinite post-trigger
You set the amount of pre-trigger data to acquire. Then, the system continues to acquire data until the program
issues a command to halt acquisition.
Counter inputs
The USB-1602HS-2AO has four counters that can accept frequency inputs up to 20 MHz. The counter channels
can be read as 16 or 32-bit counters. A counter can also be configured to be gated by one of the other counter
inputs mapped as a gate input (refer to Mapped channels, on page 22). The input type is TTL, rising-edge
triggered.
The counters can concurrently monitor time periods, frequencies, pulses, and other event driven incremental
occurrences directly from pulse-generators, limit switches, proximity switches, and magnetic pick-ups.
Counter inputs can be read asynchronously under program control, or synchronously as part of an analog or
digital scan group.