User manual

Computer Gateway User Manual 2-1 5/96

FUNCTIONAL OVERVIEW

Section 2

This section shows how the CG works with the host processor to enable your application

programs to interact with the various modules on the TDC 3000

Local Control Network (LCN).

2.1 WHAT THE HOST PROCESSOR CAN DO

The following paragraphs introduce you to the types of work the host computer can do by

working through the CG to access data from, and send data to, devices on the TDC 3000

Local Control Network. Additional detail on specifics of these facilities is found in later

sections.

NOTE

This manual uses the abbreviation ACP to stand for any application program at the host

processor that communicates with the CG by means of the various data-link messages.

2.1.1 TDC 3000

Data Access

Before starting an explanation of how the host processor can get and give LCN data, a

short explanation of the TDC 3000

database organization and data access is appropriate.

Because the TDC 3000

is a distributed system, its data is distributed among the various

nodes. Each piece of data is assigned to a data owner (program) in the node where it

resides. When data is addressed by its "external address"—a character-string name in the

form point_name.parameter—a request is broadcast to all nodes on the LCN. The

data owner responds with a numeric "internal address" that can be used for direct access to

the data.

Each time an ACP requests a piece of data by its "external address," the "internal address"

first must be obtained. Time and LCN loading is saved by having the ACP obtain, and

save for reuse, the "internal addresses" for those data points that are referenced regularly.

This is automatically done during the building of the Data Definition Tables used in

multipoint data accesses.

Current process values are obtained from the CG, the Applications Module, the Hiway

Gateway (data from Hiway boxes), or the Network Interface Manager (data from UCN

boxes). Historized process values are obtained from the History Module.