IBM PC CONVERTIBLE Technical Reference page 18

From
Part 2
Set modem
installed flag in
equipment word.
Set first entry in
RS232-BASE
table to hex 3F8.
No
Subtract 1 from
loop counter.
Turn off modem
power.
Return to POST.
To
Part 2
Figure 1-2 (Part 3 of 3). Internal Modem Presence Test
Power-On Self-Test (POST) 1-11