Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!osu-cis!sppy00!jmv
From: jmv@sppy00.UUCP (Jim Vickroy)
Newsgroups: comp.sys.ibm.pc
Subject: Re: com port status
Keywords: serial comm port status
Message-ID: <220@sppy00.UUCP>
Date: 27 Mar 89 20:35:57 GMT
References: <9864@polyslo.CalPoly.EDU>
Reply-To: jmv@sppy00.UUCP (Jim Vickroy)
Distribution: usa
Organization: Online Computer Library Center, Dublin, Ohio.
Lines: 62

In article <9864@polyslo.CalPoly.EDU> ddurbin@polyslo.CalPoly.EDU (Daniel A. Durbin) writes:
=>
=>        I am writing a BBS door program in TurboC and would like to know
=>        if there is a way to determine what baud, parity and number of
=>        bits a com port has previously been initialized to assuming there
=>        is a modem attached and it is online (carrier dectect high).
=>
=>        Also, could someone provide me with the addresses and interrupts
=>        assigned to com ports com3 through com8?  Sources in .asm or .c
=>        or just a description would be helpful.  Thanks in advance.
=>
=>        Daniel Durbin___________________________________________________
=>        SysOp: Cygnus X-1 BBS              | CIS: 73447,1744
=>        (805) 541-8505 (data)              | GEnie: D.DURBIN
=>        EL major at PolySlo                | ddurbin@polyslo.CalPoly.EDU 

The Parity, Data Bits, and Stop Bits can be retrieved from the Line Control
Register (LCR) of the appropriate 8250. The Baud Rate can be calculated from
information from the two Divisor Latches, also from the 8250.

The LCR is located at port 0x03FB for COM1, 0x02FB for COM2. It is a byte
register which maps as follows:

	Bit 7 - Divisor Latch Access Bit (DLAB)
	    6 - Set Break
	    5 - Stick Parity
	    4 - Even Parity Select (EPS)
	    3 - Parity Enable (PEN)
	    2 - Number of Stop Bits (STB)
	    1 - Word Length Select Bit (WLS1)
	    0 - Word Length Select Bit (WLS0)

Bits 1 and 0 map further as:

	Bit 1: 0	Bit 2: 0	- 5 data bits
	       0	       1	- 6 data bits
	       1               0	- 7 data bits
	       1               1	- 8 data bits

Calculating the baud rate is a little more compilcated. The information you need
is held in the two divisor latches. The Least Significant Divisor Latch (DLL) is
located at 0x03F8 for COM1 and 0x02F8 for COM2. The Most Significant Divisor 
Latch (DLM) is located at 0x03F9 for COM1 and 0x02F9 for COM2. Together these
two bytes make up the binary value for the Divisor Latch. On order for you to
retrieve the latches you will need to set the Divisor Latch Access Bit ON (=1)
in the LCR first. To calculate the baud rate I, sometime ago so I hope this is
accurate, penciled in this formula:

	BaudRate = 1/((DLM << 8) | DLL)*115200

Hope this helps........

jim
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