Technical information

NewBIOS Reference Serial Solutions
interrupt signal from the 16450 to reach the system bus and
interrupt the host microprocessor. When the out2 bit is false the
interrupt line is disconnected from the system bus. To maintain
full PC compatibility all serial cards perform this gating function
on out2, and newer serial chips such as the 16552 include within
themselves the logic to do this, so that out2 does not appear
outside the chip.
Outputs from the 16450 use negative logic, that is setting a
bit true in the chip causes the relevant output line to go false.
This can be largely ignored from the software’s point of view,
because the serial card will re-invert the lines when converting
them to the +/- 12 volt levels required by RS232 or the +/- 5
volt differential lines of RS422 and RS485.
Chapter 5 Page 89