Measurement Computing Data Acquisition Systems rev.10.4 - Manual (Page 19)

Monitoring and Receiving Driver Buffer Model Data
The Driver Buffer Model allows the flexibility of the User Buffer Model in Circular mode without
the complication of having to manage the circular buffer at the application level. In fact, the
Buffer Model is simply a special case of the circular buffer mode in that the driver handles the
details of managing the circular buffer and “hands off” scan data to the application only when
the application requests it. The scan data is handed off to the application in easy to use and
manipulate linear buffers. Once handed off the data is removed from the Driver Buffer
permanently. In other words requesting data from the Driver Buffer is a destructive read
operation.
Upon request the Driver Buffer Model allows the application to request data from the internal
Driver Buffer as follows:
• Return Scan Data Available Returns any unread scan data that is available in the
Driver Buffer up to the requested amount. The application must ensure that it has
enough space in the User Request Buffer to store any amount of unread scan data that
may be present in the Driver Buffer up to the amount requested.
• Wait Until the Requested Amount is Available Waits until the requested amount
is available in the Driver Buffer. When the amount of scan data available is greater
than or equal to the amount requested the Driver Buffer will return the amount
requested.
• Do Not Wait Until the Requested Amount is Available Upon receipt of the request
if the amount of scan data available in the Driver Buffer is not greater than or equal to
the amount requested then the Driver Buffer will return no scan data. If the amount
requested is available at the time of the request the driver buffer will return the
amount requested.
Models
Initialization and Error Handling
This section demonstrates how to initialize the Daq
device and use various methods of error handling. Most
of the example programs use similar coding as detailed in
the following VB example. Functions used include:
•
VbdaqOpen&(daqName$)
•
VbdaqSetErrorHandler&(errHandler&)
•
VbdaqClose&(handle&)
All Visual Basic programs should include the DaqX.bas
file into their project. The DaqX.bas file provides the
necessary definitions and function prototyping for the
DAQX driver DLL.
handle& = VBdaqOpen&(“DaqBook0”)
ret& = VBdaqClose&(handle&)
The Daq device is opened and initialized with the
daqOpen function. daqOpen takes one parameter—
the name of the device to be opened. The device name can be accessed and changed via the Daq*
Configuration utility located in the operating system’s Control Panel (see following figure).
Programmer’s Manual 988594 API Programming, General Models 2-9