From: utzoo!decvax!harpo!npoiv!npois!ucbvax!C70:info-cpm
Newsgroups: fa.info-cpm
Title: Re: IBM PC question
Article-I.D.: ucb.1611
Posted: Mon Jul 26 23:34:34 1982
Received: Sat Jul 31 07:50:26 1982

>From COMSAT.SoftArts@Mit-Multics Mon Jul 26 23:34:23 1982
Local:  William "Chops" Westfield ,info-cpm at BRL
Original-date:  26 JUL 1982 08:33:28
Via:  Mit-Multics; 26 Jul 82 23:05-EDT
Via:  Brl; 26 Jul 82 23:15-EDT
Via:  Brl-Bmd; 26 Jul 82 23:32-EDT

IBM failed to fully document everything that you need in order
to get the RS232 to interrupt.  The "user-defined" pin OUT2 on
the communications chip MUST be asserted if you wish to ever see
an interrupt.  If you do the following, your program will work
correctly:

1. Set DLAB = 1 via LCR port
2. Output the baud rate divisor
3. Output the line control bits via LCR port
4. Set the modem control register (use the following as a guide)

     mov       al,$0b                    ; DTR, RTS, OUT2 (UNDOCUMENTED)
     mov       dx,rs232_mcr_port         ; modem control
     out       al,dx

5. Set the interrupt mode via IER port
6. Enable the i8259a interrupt for the rs232

Hope this helps...
-- Ray