User manual

Computer Gateway User Manual C-2 5/96
C.3
Get History performance has more variables; however, the essential thing to remember is
that the difference in time required for a larger set of points (within the limit of 5986 data
words) is relatively small.
Throughput may be enhanced by using multiple ACPs to request the acquisition of history
data because the CG can support up to four concurrent history requests.
C.3 INPUT DATA PRECOLLECTION
The input data precollection feature enables collection of multipoint data values very close
in time to a "trigger" event, and reduces the elapsed time required to complete a get data
request (measured from the time an ACP is triggered at the CG until the data has been
moved to the host processor).
Precollection requires that a Internal DDT (resident in the CG) is connected to an ACIDP-
ACP pair, and that one or more of its triggers (schedule, operator demand, and point
process special) are set ON. When a trigger occurs, data collection for those data points
specified by the DDT is started in parallel with the sending of an ACP turn on message to
the host processor, rather than waiting on a get data request to be initiated by the ACP.
Precollected data waits in CG memory for the first get data request from an attached ACP.
There is no indication in the get data response of whether the data being returned was
precollected or collected following the get data request.
For additional information see the following message types:
• Internal DDT Connect or Disconnect Request (paragraph 4.4.7)
• Store Internal DDT in CG Request (paragraph 4.5.3)
• ACIDP Execution State Change Request (paragraph 4.4.6)
The CG allows the same Internal DDT to be connected to more than one ACIDP-ACP pair;
however, this practice is not recommended because there is the possibility that one ACP
could get time dependent data intended for another.