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Computer Gateway User Manual 4-54 5/96
4.7
4.7 SINGLE-POINT DATA TRANSFERS
Single-point data transfers are used to get or store a value for a single parameter, for a
selected parameter within a parameter array, or for all elements of a parameter array.
(Parameter arrays are used with Custom Data Segments.)
Two methods are provided for single parameter gets and stores. The first allows you to
use the external name that requires the CG to make a separate external to internal point-
name conversion for each get/store call (requiring a search of other LCN devices to get this
information). The second method allows a program in the host processor to make a name
conversion call once (for each point) and then use the internal name with the (quicker)
get/store call.
The internal IDs returned to the user program by the single point convert call are the actual
LCN internal entity IDs. You need to take every possible precaution to protect this data
from corruption and you should not attempt to pass altered entity IDs to the CG for any
reason.
Single-point data transfers should not be used if large numbers of points are involved. In
those cases, use the table-driven requests instead.
4.7.1 Processing of Parameters and Parameter Arrays
The processing of parameters and parameter arrays deserves some special attention because
the single-point access to these values is not intuitively obvious.
The key is in two fields found in most single-point requests: "data type" and "parameter
array pointer/size."
To get or store all elements of a parameter array, use the appropriate array “data type”
with the value of “parameter array pointer/size” equal to array size.
To get only part of an array, use the appropriate array “data type” with the value of
“parameter array pointer/size” equal to the number of elements desired. If the “parameter
array pointer/size” is greater than the size of the array, the whole array will be returned.
Single-Point stores of “data type” 20 (ASCII array) may store part of an array, if the
“parameter array pointer/size” (size of array being stored) is smaller than the actual
array’s size.
Storing any other array “data type” with “parameter array pointer/size” not equal to the
actual array’s size will result in an error.
4.7.1.1 Array Length Restrictions
The maximum number of elements that can be transferred for each array data type is:
• Real Array 1024 • Internal Entity Identifier Array 1000
• Integer Array 1024 • External Entity Identifier Array 500
• Enumeration Array 1024 • LCN Internal Time Array 1024
• Ordinal Values Array 1024 • ASCII Array 152