Technical information

NewBIOS Reference Serial Solutions
The time-out period, after which a time-out error is
returned, is fixed by a software loop. This time period will
therefore vary from machine to machine. For example in the
original 4.77MHz 8088 PC it is about 1.3 seconds, and on a
20MHz 80286 AT it is about 0.43 seconds.
Other Points.____________
The character specified in AL is written unchanged to the
16450, which automatically truncates it to the correct length (5,
6, 7 or 8 bits) and adds the correct parity bit if parity has been
set. The user need not worry about these.
RTS and DTR are left asserted, and as a side effect loop,
out1 and out2 (the other bits in the modem control register) are
left reset. This is important because PC serial cards use out2 to
gate interrupts off and on. Resetting out2 prevents interrupts
from the 16450 reaching the PC slot. This is fine for the BIOS
routines which do not use interrupts, but NewCOM, BASIC and
any sensible serial communications software, do. Use of this
BIOS service alongside these is therefore not recommended.
Chapter 5 Page 79