Measurement-computing TempBook rev.3.0 Manual de usuario descargar pdf (Pagina 99)

TempBook User’s Manual

Enhanced

API Programming Models (TempBook) 10-3

Initialization and Error Handling

This section demonstrates how to initialize the Daq* and use

various methods of error handling. Most of the example

programs use similar coding as detailed here. 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&(“TempBook0”)

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 information can be accessed or changed via the

Daq* Configuration utility located in the operating system’s Control Panel. The

daqOpen

call, if

successful, will return a handle to the opened device. This handle may then be used by other functions to

configure or perform other operations on the device. When operations with the device are complete, the

device may then be closed using the

daqClose

function. If the device could not be found or opened,

daqOpen

will return

-1

The DAQX library has a default error handler defined upon loading. However; if it is desirable to change

the error handler or to disable error handling, then the

daqSetErrorHandler

function may be used to

setup an error handler for the driver. In the following example the error handler is set to

(no handler

defined) which disables error handling.

ret& = VBdaqSetErrorHandler&(0&)

If there is a Daq* error, the program will continue. The function’s return value (an error number or

if no

error) can help you debug a program.

If (VBdaqOpen&(“TempBook0”) < 0) Then

“Cannot open “TempBook0”

Daq* functions return

daqErrno&

Print “daqErrno& : ”; HEX$(daqErrno&)

End If

The next statement defines an error handling routine that frees us from checking the return value of every

Daq* function call. Although not necessary, this sample program transfers program control to a user-

defined routine when an error is detected. Without a Daq* error handler, Visual Basic will receive and

handle the error, post it on the screen and terminate the program. Visual Basic provides an integer variable

(ERR) that contains the most recent error code. This variable can be used to detect the error source and

take the appropriate action. The function

daqSetErrorHandler

tells Visual Basic to assign ERR to a

specific value when a Daq*error is encountered. The following line tells Visual Basic to set ERR to 100

when a Daq*error is encountered. (Other languages work similarly; refer to specific language

documentation as needed.)

handle& = VBdaqOpen&(“TempBook0”)

ret& = VBdaqSetErrorHandler&(handle&, 100)

On Error GoTo ErrorHandler

The

On Error GoTo

command in Visual Basic allows a user-defined error handler to be provided, rather

than the standard error handler that Visual Basic uses automatically. The program uses

On Error GoTo

to transfer program control to the label ErrorHandler if an error is encountered.

Daq* errors will send the program into the error handling routine. This is the error handler. Program

control is sent here on error.

ErrorHandler:

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