viReadAsync
Purpose
Reads data from device or interface asynchronously.
C Syntax
ViStatus viReadAsync(ViSession vi, ViPBuf buf, ViUInt32 count, ViPJobId jobId)
Visual Basic Syntax
N/A
Resource Classes
GPIB INSTR, GPIB INTFC, GPIB-VXI INSTR, Serial INSTR, TCPIP INSTR, TCPIP SOCKET, USB INSTR, USB RAW, VXI INSTR, VXI SERVANT
Parameters
| Name | Direction | Description |
|---|---|---|
|
vi |
IN |
Unique logical identifier to a session. |
|
buf |
OUT |
Location of a buffer to receive data from device. |
|
count |
IN |
Number of bytes to be read. |
|
jobId |
OUT |
Job ID of this asynchronous read operation. |
Return Values
| Completion Codes | Description |
|---|---|
|
VI_SUCCESS |
Asynchronous read operation successfully queued. |
|
VI_SUCCESS_SYNC |
Read operation performed synchronously. |
| Error Codes | Description |
|---|---|
|
VI_ERROR_INV_OBJECT |
The given session reference is invalid. |
|
VI_ERROR_RSRC_LOCKED |
Specified operation could not be performed because the resource identified by vi has been locked for this kind of access. |
|
VI_ERROR_QUEUE_ERROR |
Unable to queue read operation (usually due to the I/O completion event not being enabled or insufficient space in the session's queue). |
|
VI_ERROR_IN_PROGRESS |
Unable to queue the asynchronous operation because there is already an operation in progress. |
Description
The viReadAsync() operation asynchronously transfers data. The data read is to be stored in the buffer represented by buf. This operation normally returns before the transfer terminates.
Before calling this operation, you should enable the session for receiving I/O completion events. After the transfer has completed, an I/O completion event is posted.
The operation returns jobId, which you can use with either viTerminate() to abort the operation, or with an I/O completion event to identify which asynchronous read operation completed. VISA will never return VI_NULL for a valid jobID.
 |
Note If you have enabled VI_EVENT_IO_COMPLETION for queueing (VI_QUEUE), for each successful call to viReadAsync(), you must call viWaitOnEvent() to retrieve the I/O completion event. This is true even if the I/O is done synchronously (that is, if the operation returns VI_SUCCESS_SYNC). If you are using LabVIEW, this is done for you automatically. |
Related Topics