National Instruments 321645c-01

Chapter 2 Function Reference — Config_Alarm_Dea dband

National Instruments Corporation 2-65 NI-DAQ FRM for PC Compatibles

alarmOnMessage and alarmOffMessage are messages you define. When the alarm-on

condition occurs, NI-DAQ passes alarmOnMessage back to you. Similarly, when the

alarm-off condition occurs, NI-DAQ passes alarmOffMessage back to you. The messages

can be any value.

In Windows, you can set the message to a value including any Windows predefined messages

such as WM_PAINT. However, to define your own message, you can use any value ranging

from WM_USER (0x400) to 0x7fff. This range is reserved by Microsoft for messages you

define.

callbackAddr is the address of the user callback function. NI-DAQ calls this function when

DAQEvent occurs. See Config_DAQ_Event_Message for restrictions on this parameter.

Using This Function

To meet the high alarm-on condition, the input signal must first go below

(trigLevel-deadbandW idth/2) volts and then go above (trigLevel+deadbandWidth/2)

volts. On the other hand, to meet the high alarm-off condition, the input signal must first go

above (trigLevel + deadbandWidth/2) voltsand then go below

(trigLevel–deadbandW idth/2) volts. See Figure2-1 for an illustration of the high alarm

condition.

Figure 2-1. High Alarm Deadband

The low alarm deadband trigger condition is the opposite of the high alarm deadband

trigger condition. To meet the low alarm-on condition, the input signal must first go above

(trigLevel+deadbandWidt h/2) and then go below (trigLevel-deadbandWidth/2). On

the other hand, to meet the low alarm-off condition, the input signal must first go below

(trigLevel – deadbandWidth/2) and then go above (trigLevel + deadbandWidth/2).

See Figure2-2 for an illustration of the lowalarm condition.

off

trigLevel +

deadbandWidth/2

trigLevel -

deadbandWidth/2

trigLevel

On: high alarm on

Off: high alarm off

Time