Configuration Aspects for Programmers Appendix B
B-18 ChartScan User’s Manual
Acquisition Buffer, A Description
Once scans are read from the buffer by the controlling computer, they are erased
from ChartScan’s buffer. The acquisition buffer is a FIFO (First-In First-Out)
buffer. In other words, the oldest data to be written into the buffer is the first data
to be read from the buffer when a read operation is performed. The buffer may be
broken up into one or more “trigger blocks.” A trigger block is comprised of pre-
trigger scans (optional), a trigger point, post-trigger scans, a stop event, and post-
stop scans (optional). These components constitute a trigger block in the internal
memory of the unit. Each trigger block defines an acquisition and each may be of
variable length.
Trigger blocks are allocated sequentially and the scans within the Trigger Blocks
are written and read sequentially. In other words, it is not possible to randomly
access a Trigger Block or a scan within a trigger block. When a read operation
is performed, the scan that is read is the oldest scan in the oldest trigger block
currently defined. The figure represents a sample acquisition buffer with 5 trigger
blocks, each having different amounts of scan data.
Trigger Block Organization
Each Trigger Block in the buffer has an associated trigger block descriptor used by internal processes for
Acquisition Buffer maintenance. Since the trigger block descriptor does take up memory it will have an indirect
affect on some user defined processes. For instance, the 75% limit condition and the buffer overrun condition
will occur on a fewer number of scans as the number of trigger blocks increases.
A trigger block becomes defined when the configured trigger start event occurs. Once the start event occurs the
data within the trigger block (including any pre-trigger dat a) is available to be read, even though the trigger
block may not yet be complete. Scan data is read and written to the trigger block in a FIFO fashion. As the
controller requests buffered scan data, ChartScan supplies the oldest available scan. Once the scan is supplied it
is no longer available. There is no mechanism which allows jumping around in the buffer or going back to a
scan that had already been read by the controller.
Reading the oldest scan automatically advances the read pointer in the current trigger block so that the next scan,
if acquired, is immediately available.
Since scans are written into the trigger block sequentially, the trigger scan may appear at any location within the
trigger block. ChartScan will automatically convert these physical scan locations to logical scan locations where
the trigger scan is always oriented at location 0. The placement of the trigger scan at the origin facilitates the
quick interpretation of the read pointer and gives a frame of reference for other locations within the trigger
block. All trigger block pointers are relative to the trigger scan. From this point on text discussions will assume
references to logical trigger blocks only.
The U6 command may be used to query the current read trigger block. For example, the U6 response for the
previous figure (assuming it was the only trigger block) would be as follo ws:
PRINT#1,”OUTPUT 07; U6X” ‘ Get status of current read trigger block
PRINT#1,”ENTER 07” ‘ Get the response
INPUT A$ ‘ Screen shows…
0000001,0001234,-0000076,12:34:54.200, 03/23/97,00000767,12:54:12.900, 03/24/97,00001156,01