snap ethernet-based i/o units protocols and programming guide

CHAPTER 3: USING THE SNAP ETHERNET I/O DRIVER TOOLKIT
Structures
Since some ActiveX containers, such as Visual Basic, support predefined structures rather than
just the basic data types, several methods also support structures. These structures can be easier
to use. The names of these methods all end with Ex, and they are similar to other methods.
For example, in the table on page 65, compare GetDigPtReadArea to GetDigPtReadAreaEx. The
former takes individual parameters, whereas the latter takes the following structure:
struct SIOMM_DigPointReadArea
{
long nState; // bool
long nOnLatch; // bool
long nOffLatch; // bool
long nCounterState; // bool
long nCounts; // unsigned int
};
See the file OptoENET-IO\DriverToolkit\Source\C++\O22STRCT.h for structure definitions. These
definitions are included in the ActiveX type library.
[In] and [Out] Parameter Types
Parameters marked as [in] are not changed by the method. Parameters marked as [out] can be
changed by the method. The exact parameter mechanism depends upon the programming
environment being used. For instance, in C++, all [out] parameters use pointers.
Variant Methods
A variant is a special data type that can represent values of many different types, such as
integers, floats, Booleans, strings, or pointers. A variant represents only one type at a time. Some
languages, such as Visual Basic, make extensive use of the variant data type. VBScript supports
only variants.
The OptoSnapIoMemMapX interface includes two versions of the most commonly used methods.
The regular versions have parameters and return values that are strongly typed, with types such
as long integer and float. The variant versions have parameters and return values of the variant
type only, because some languages, such as VBScript, support only the variant data type. All
variant-only methods start with the letter V.
We recommend you use the non-variant methods whenever possible to provide the strong typing.
Even though a parameter is a variant, the variant type must match that of the corresponding
regular method. For example, the method VSetStatusOperation (VARIANT nOpCode) takes one
parameter, nOpCode. The variable nOpCode must be a long integer at the time when
VSetStatusOperation is called. In VBScript, use CLng() to force a variant to be a long integer.
Example:
gBrain2.VSetStatusOperation(CLng(OpCode))
60 SNAP Ethernet-Based I/O Units Protocols and Programming Guide