IBM PC CONVERTIBLE Technical Reference page 22

From
Part 3
Set pointer to
RS232_BASE
table (hex 400).
Set serial/parallel
adapter to
primary.
Turn on power to
adapter.
Delay 100 ms.
Write/read hex
AA and
55
to
work register at
hex 3FF.
To
Part 5
- - - - - - - { RS232 presence test
No (Internal
modem attached)
Move
RS23LBASE
table pointer to
next entry.
Write/read hex
AA and 55 to
work register at
hex 2FF.
To
Part 7
- - - - - - - { Test for primary
Figure 1-3 (Part 4 of 7). Printer/Communications Presence Test
Power-On Self-Test (POST) 1-15