IBM PC CONVERTIBLE Technical Reference page 17

Yes (Other adapter present)
Turn on power to
internal modem.
Set loop counter
t05.
Delay
100
ms.
Write/read hex
AA and 55 to
work register at
hex 3FF.
To
Part 3
Figure 1-2 (Part 2 of 3). Internal Modem Presence Test
1-10 Power-On Self-Test (POST)
Set first entry in
RS232_BASE
table to hex 3F8.