Echelon OpenLDV - Manual (Page 21)

OpenLDV Programmer’s Guide 13
The OpenLDV API does not include an application layer. However, the
OpenLDV Developer Example demonstrates how to integrate an application
layer (which dispatches incoming messages to an application-specific message
dispatcher) with the OpenLDV API. For more information about the OpenLDV
Developer Example and the message dispatcher it employs, see Chapter 4, The
OpenLDV Developer Example, on page 89.
Presentation Layer
The presentation layer is responsible for translating messages between the lower
layers, providing an easier-to-use presentation format used by the application
layer. For example, all incoming network variable update messages from a
network interface are recognized by this layer, updating the application’s
network variable values and notifying the application layer of the change.
The presentation layer can also manage network management messages and
diagnostics services, such as the responses to Query SI network management
commands (see Receiving Messages from the Network Interface on page 75).
The OpenLDV API does not include a presentation layer. However, the
OpenLDV Developer Example demonstrates how to integrate a presentation
layer with the OpenLDV API. The OpenLDV Developer Example also includes
code that handles several network management commands and diagnostics.
Overview of the OpenLDV API
The OpenLDV API is implemented by a 32-bit C DLL that is compatible with
both 64- and 32-bit versions of Windows. All data types defined for the API
assume a 32-bit compiler; for example, pointers reference 32-bit addresses, each
enum is a 32-bit type, size_t is an unsigned 32-bit type, and so on. The
OpenLDV header file, ldv32.h, includes the following type definitions for native
data types, rather than using compiler-dependent types:
typedef void* PVOID;
typedef short SHORT; /* signed 16-bit */
typedef long LONG; /* signed 32-bit */
typedef unsigned char BYTE; /* unsigned 8-bit */
typedef unsigned short WORD; /* unsigned 16-bit */
typedef unsigned long DWORD; /* unsigned 32-bit */
typedef char* LPSTR;
typedef const char* LPCSTR;
typedef void* HANDLE;
Each function of the OpenLDV API uses extern “C” and __stdcall calling
conventions.
An alternative to the OpenLDV API is the COpenLDVapi class provided with
the OpenLDV developer example described in Chapter 4. This class provides a
COM interface with thread-safe, synchronized, access to downlink messages
(ldv_write()). The same interface also implements a reader thread,
COpenLDVreader, which queries uplink messages (ldv_read()) and supplies
data to a protected queue. The COpenLDVapi::Read() function queries that
queue, thereby providing coordinated access to both uplink and downlink
messages.