User manual

Computer Gateway User Manual 2-5 5/96
2.2.2
2.2.2 Process Data Exchanges
The CG receives data-exchange requests (get or store) from the host processor, performs
any necessary reformatting and external-to-internal identification translation, transmits the
request over the LCN, then returns the data and/or status information to the host processor.
Two types of data requests are supported, single-point and table-driven. Typically, single-
point requests are used when only a few point values are required; while table-driven
requests are used when larger volumes of data are required.
NOTE
Other nodes on the LCN cannot directly access data in the host processor; however, they can
read data points stored in the CG database.
2.2.3 History Data Acquisition
The CG receives history-gathering requests from the host processor, performs any
necessary reformatting, transmits the request over the LCN, then returns the history data
and status information to the host processor. All history-data acquisition calls are table-
driven.
2.2.4 Message Transfers
The CG receives and stores ASCII-string messages from other nodes on the LCN. It also
is involved in the routing of host-processor messages to process operators at Universal
Stations.
2.2.5 CG Database Components
The CG database can contain three types of information used by the host processor:
• Advanced Control Interface Data Points (ACIDP)
• Calculated Results Data Points (CRDP)
• Data Definition Table Internal Data Tables (Internal DDT)
Each ACIDP can have three purposes: It holds information necessary to CG scheduling of
an associated ACP in the host processor; it can hold messages in transit to the ACP from
other LCN nodes; it can hold calculated data values (in Custom Data Segments, see the
Control Language Reference Manual for details) stored by the host processor (or other
LCN nodes). A CG can hold no more than 250 ACIDPs.
Each CRDP holds calculated data values (in Custom Data Segments) stored by the host
processor (or other LCN nodes). There is no specified connection between a CRDP and
any ACP. A CG can hold no more than 500 CRDPs.
Each Internal DDT holds a list of point internal identifiers to be used by table-driven get-
data requests. A CG can hold up-to-40 Internal DDTs.
See paragraph 6.2 for additional information on CG memory requirements.