For older style RadioTrack cards only.

This function decreases the volume on the old-style RadioTrack cards with a speaker output jack. This card has 16 discrete volume levels. You must keep track of the volume setting in your own program. To establish a known level at initial program start, you can decrease the volume 16 times and then increase it to a preset level. The format of the function is:

return_data = RT1VolDn( rt_address )

The rt_address is the decimal value of the card's I/O location 780 (30C hex) or 524 (20C hex).

return_data returns the following values:

0 No errors.
1 Volume can't be set.
10 TESTCFG.SYS could not be opened.