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MagTek Device Drivers for Windows
84
‘set up error handling
On Error Resume Next
‘submit echo command
Put #1, , "/echo Hello" + Chr$(10)
‘declare an input buffer
a$ = String(2000, Chr$(0))
‘read echo response from device
Get #1, , a$
If Err.Number <> 0 Then
<<process error>>
End If
Note
File I/O interaction with the device is synchronous; the read operation
will block until a response is received from the device or is returned by
the driver (as in a property request). This means that a read command
cannot be canceled because the computer will not accept any new
commands while one is pending. The only exception to this is when the
development environment provides access to the Win32 API, giving the
application the ability to use overlapped file I/O.
Releasing access to the device
Releasing access to the device is very simple. If using MSComm, close the device by setting its
PortOpen property to False:
‘close the port
mscomm1.PortOpen = FALSE
If opened as a file, close it as in the following:
‘close the port
Close #1
ERRORS AND ERROR PROCESSING
A command’s execution status is returned to an application in the command’s response, if it has
one. The status value is a two digit numeric field located at positions 23 and 24 of the response
(refer to Appendix C. Status Codes for a description of all error conditions) .
Errors are processed differently for property manipulation. If an error occurs while getting a
property, the response will be returned with an empty property value. No status is returned when
setting a property because the
set command has no response defined for it.
If a command returns a non-zero status, indicating an error, an application can typically respond
in the following manner: