Locking Sample Code

This example uses a shared lock because two sessions are opened for performing trigger operations. The first session receives triggers and the second session sources triggers. A shared lock is needed because an exclusive lock would prohibit the other session from accessing the same resource. If viWaitOnEvent() fails, this example performs a viClose() on the resource manager without unlocking or closing the sessions. When the resource manager session closes, all sessions that were opened using it automatically close as well. Likewise, remember that closing a session that has any lock results in automatically releasing its lock(s).

#include "visa.h"
#define MAX_COUNT 128
int main(void)
{
   ViStatus     status;                    /* For checking errors */
   ViSession    defaultRM;                 /* Communication channels */
   ViSession    instrIN,                   /* Communication channels */
   ViChar       instrOUT;                  /* Access key for lock */
   ViByte       accKey[VI_FIND_BUFLEN];    /* To store device data */
   ViEventType  buf[MAX_COUNT];            /* To identify event */
   ViEvent      etype;                     /* To hold event info */
   ViUInt32     event;                     /* To hold byte count */ 
                retCount;
    /* Begin by initializing the system */
    status = viOpenDefaultRM(&defaultRM);
    if (status < VI_SUCCESS) {

        /* Error Initializing VISA...exiting */
        return -1;
    }
    /* Open communications with VXI Device at Logical Addr 16 */
    status = viOpen(defaultRM, "VXI0::16::INSTR", VI_NULL, VI_NULL, &instrIN);
    status = viOpen(defaultRM, "VXI0::16::INSTR", VI_NULL, VI_NULL, &instrOUT);

    /* We open two sessions to the same device */
    /* One session is used to assert triggers on TTL channel 4 */
    /* The second is used to receive triggers on TTL channel 5 */

    /* Lock first session as shared, have VISA generate the key */
    /* Then lock the second session with the same access key */

    status = viLock(instrIN, VI_SHARED_LOCK, 5000, VI_NULL, accKey);
    status = viLock(instrOUT, VI_SHARED_LOCK, VI_TMO_IMMEDIATE, accKey, accKey);

    /* Set trigger channel for sessions */
    status = viSetAttribute(instrIN, VI_ATTR_TRIG_ID,VI_TRIG_TTL5);
    status = viSetAttribute(instrOUT,VI_ATTR_TRIG_ID,VI_TRIG_TTL4);

    /* Enable input session for trigger events */
    status = viEnableEvent(instrIN, VI_EVENT_TRIG, VI_QUEUE, VI_NULL);

    /* Assert trigger to tell device to start sampling */
    status = viAssertTrigger(instrOUT, VI_TRIG_PROT_DEFAULT);

    /* Device will respond with a trigger when data is ready */
    if ((status = viWaitOnEvent(instrIN, VI_EVENT_TRIG, 20000, &etype, &event)) < VI_SUCCESS) {
        viClose(defaultRM);
        return -1;
    }
    /* Close the event */
    status = viClose(event);

    /* Read data from the device */
    status = viRead(instrIN, buf, MAX_COUNT, &retCount);

    /* Your code should process the data */

    /* Unlock the sessions */
    status = viUnlock(instrIN);
    status = viUnlock(instrOUT);

    /* Close down the system */
    status = viClose(instrIN);
    status = viClose(instrOUT);
    status = viClose(defaultRM);
    return 0;
}

Private Sub vbMain()
Const MAX_COUNT = 128

Dim stat      As ViStatus                    'For checking errors
Dim dfltRM    As ViSession                   'Communication channels
Dim sesnIN    As ViSession                   'Communication channels
Dim sesnOUT   As ViSession                   'Communication channels
Dim aKey      As String * VI_FIND_BUFLEN     'Access key for lock
Dim buf       As String * MAX_COUNT          'To store device data
Dim etype     As ViEventType                 'To identify event
Dim event     As ViEvent                     'To hold event info
Dim retCount  As Long                        'To hold byte count

   Rem Begin by initializing the system
   stat = viOpenDefaultRM(dfltRM)
   If (stat < VI_SUCCESS) Then

    Rem Error initializing VISA...exiting
    Exit Sub

   End If

   Rem Open communications with VXI Device at Logical Addr 16
   stat = viOpen(dfltRM, "VXI0::16::INSTR", VI_NULL, VI_NULL, sesnIN)
   stat = viOpen(dfltRM, "VXI0::16::INSTR", VI_NULL, VI_NULL, sesnOUT)

   Rem We open two sessions to the same device
   Rem One session is used to assert triggers on TTL channel 4
   Rem The second is used to receive triggers on TTL channel 5

   Rem Lock first session as shared, have VISA generate the key
   Rem Then lock the second session with the same access key
   stat = viLock(sesnIN, VI_SHARED_LOCK, 5000, "", aKey)
   stat = viLock(sesnOUT, VI_SHARED_LOCK, VI_TMO_IMMEDIATE, aKey, aKey)

   Rem Set trigger channel for sessions
   stat = viSetAttribute(sesnIN, VI_ATTR_TRIG_ID, VI_TRIG_TTL5)
   stat = viSetAttribute(sesnOUT, VI_ATTR_TRIG_ID, VI_TRIG_TTL4)

   Rem Enable input session for trigger events
   stat = viEnableEvent(sesnIN, VI_EVENT_TRIG, VI_QUEUE, VI_NULL)

   Rem Assert trigger to tell device to start sampling
   stat = viAssertTrigger(sesnOUT, VI_TRIG_PROT_DEFAULT)

   Rem Device will respond with a trigger when data is ready
   stat = viWaitOnEvent(sesnIN, VI_EVENT_TRIG, 20000, etype, event)
   If (stat < VI_SUCCESS) Then

    stat = viClose (dfltRM)
    Exit Sub

   End If

   Rem Close the event
   stat = viClose(event)

   Rem Read data from the device
   stat = viRead(sesnIN, buf, MAX_COUNT, retCount)

   Rem Your code should process the data

   Rem Unlock the sessions
   stat = viUnlock(sesnIN)
   stat = viUnlock(sesnOUT)

   Rem Close down the system
   stat = viClose(sesnIN)
   stat = viClose(sesnOUT)
   stat = viClose(dfltRM)

End Sub