User manual

Computer Gateway User Manual 4-11 5/96
4.1.5
PARAMETER INDEX—Applies only to certain parameter types (see Computer Gateway
Parameter Reference Dictionary and Hiway Gateway Parameter Reference Dictionary);
when not applicable must equal zero.
INTERNAL DDT TABLE NAME—The Internal DDT Table name must be from one-to-nine
printing ASCII characters long and contain no leading or embedded blanks.
ACP NAME—The ACP name as stored in the ACIDP parameter ACPROG when the ACP is
attached. It must be from one-to-twelve printing ASCII characters long and contain no
leading or embedded blanks.
4.1.5 Data Definition Tables
The most efficient method for acquiring data from, or sending data to, other LCN nodes is
by means of prebuilt Data Definition Tables (DDTs) that contain all necessary point specific
data (including the internal data point addresses).
These tables are used for three types of data transfers: input or output of current values and
input of history. The allowed data types and maximum point counts for each type are
shown in Table 4-5, below.
Table 4-5 — Data Definition Table Types
TABLE TYPES ALLOWED DATA TYPES PT COUNT MAX
Input Real 300
Integer 300
ASCII (24- or 40-Character) 300
Enumeration 300
Internal Entity id 300
External Entity id 300
LCN Internal Time 300
Output Real 300
Integer 300
ASCII (24- or 40-Character) 300
Enumeration 300
Internal Entity id 300
LCN Internal Time 300
History See heading 4.8 24
Two forms of data tables are required. The External (source) DDT consists of a list of
external data-point names (point name and parameter), and is sent by the host processor to
the CG. The Internal DDT contains internal point addresses used for LCN data access, and
is returned by the CG to the host processor. For current values (non-history) input only, a
copy of the internal table can be held by the CG as a way to reduce data-link overhead
when processing data-input requests.
Note that these Internal DDTs need to be rebuilt any time that any referenced point in the
LCN database is deleted and/or reloaded.