User manual

Computer Gateway User Manual 2-3 5/96
2.2
2.17 Role of Data Definition Tables
The most efficient method of data exchange between the host processor and the LCN is by
the use of table-driven multipoint value transfers. This is the only method provided for
retrieval of history data (current LCN data can be read or written one point at a time as well
as by multipoint transfers). As many as four DDTs may get or store data at the same time.
Preparation of the Data Definition Tables (DDTs) is done at the host processor, starting
with the entry of lists containing external point-parameter names (organized by data type if
for current values). These external names are transferred to the CG for conversion into the
internal identifiers required by the LCN and then are stored by the host processor for use in
the actual data exchanges. Copies of up to 40 selected input data tables can be held by the
CG to save the otherwise repetitive table transfer from host to CG each time the table is
used. These CG-resident tables are required for input data precollection.
2.2 HOW THE CG HELPS
The following paragraphs introduce you to the features of the CG that help the host
processor accomplish its tasks.
2.2.1 Application Program Activation
The CG activates your application programs (ACPs) in the host processor (by use of a
data-link message) in response to the following event types:
• Time of day (Periodic Scheduling)
• Elapsed time since last activation (Cyclic Scheduling)
• Operator demand
• Process special event
The CG activates programs in the host processor on timed and/or demand basis as
determined by configuration entries. There are five configuration choices for CG
scheduling of host-processor ACPs: cyclic, periodic, demand, cyclic/demand, and
periodic/demand. Activation by the CG of host-processor programs can be inhibited by the
operator.
All CG scheduling of ACPs requires that the program to be scheduled is "attached" to an
ACIDP. See paragraph 2.2.5 for an explanation of the ACIDP and its uses.
ACP activation may be done by the host processor itself, but some restrictions apply. See
paragraph 2.3.1.