Netlinx_send_level

Depending on how you want to do this, you can send the level from the panel itself and use a LEVEL_EVENT in programming to send it to your device.
If you make buttons for 'volume up' and 'volume down' on the touchpanel, in the properties of these buttons you can set the 'level control type' to 'relative' and the 'level control value' to what you want, +5 for the 'up' button and -5 for the 'down' button, for instance. If you also make a bargraph (display only) which displays the same level that will show the current level.

I usually don't change the 'standard' AMX level (0..255) to something else, but you can do that. I prefer to use some math, like below.

the bVolumeUp en bVolumeDown var's are used because when the panel goes 'offline', the levels get reset to zero. Doing it this way makes sure it won't send level changes to your device unless you actually push a button.

The programming would look something like this: (for an AMX DVX)

DEFINE_VARIABLE
//volume
DEVLEV dlMainVolume  = {dvTP,1}   //main volume

PERSISTENT INTEGER nMainVolume              //holds mainvolume level
VOLATILE  SINTEGER sMainVolume              //holds mainvolume converted to dvx xpoint level
VOLATILE   INTEGER bVolumeUp                //true when volume up button is pushed
VOLATILE   INTEGER bVolumeDown              //true when volume down button is pushed

LEVEL_EVENT [dlMainVolume] //volume
{
    IF (bVolumeUp OR bVolumeDown) //only if button is pushed
    {
    //volume max:0; min:-100
    nMainVolume = LEVEL.VALUE //store 'amx' level (0..255)
    sMainvolume = ((LEVEL.VALUE * 100) / 255) - 100 //calculate level for dvx
    SEND_COMMAND dvDVXAudioOut1,"FORMAT('XPOINT-%d',sMainVolume),',1,',ITOA(LS_OUT)"
    }
}

BUTTON_EVENT [dvTP,nMainVolumeButtons]
{
    PUSH:
    {
    SWITCH (GET_LAST(nMainVolumeButtons))
    {
        CASE 1: //volume up
        {
        TO[bVolumeUp]
        }
        CASE 2: //volume down
        {
        TO[bVolumeDown]
        }
    }
    }
}

As part of the routine that executes when the touch panel comes online, the level that is stored on the controller (in nMainVolume) gets send to the bargraph display:

DATA_EVENT [dvTP]
{
    ONLINE:
    {
    WAIT 15
    {
        SEND_LEVEL dlMainVolume, nMainVolume //send volume level to panel
        SEND_COMMAND dvTP,'ADBEEP' //confirm online status
    }
    }
}

If you use something like this in your feedback for the volumebuttons:

[dvTP,41] = bVolumeUp   AND nMainVolume < 255   //volume up
[dvTP,42] = bVolumeDown AND nMainVolume > 0     //volume down

The buttons will display the 'on' state as long as the are pushed, until the level reaches max (or min)