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PC com ports

To: <ct-user@contesting.com>
Subject: PC com ports
From: reisert@wrksys.enet.dec.com (Jim Reisert AD1C 09-Aug-1994 1246)
Date: Tue, 9 Aug 1994 12:42:43 -0400
Steve KO0U wrote:

>I'd always thought the bus mouse defaulted to COM1, and so had always set all 
>other COM ports as COM2 on up. Last night however, I loaded a program I 
>got from the Tradewinds BBS which tests all serial comm devices for their 
>types (whether 8250, 16450 or 16550) and shows their addresses and IRQs; 
>it showed that COM1 is, indeed, vacant, just as I'd set it up in the 
>belief that the bus mouse defaulted to COM1!

COM ports have two communication channels, if you will - one for output, the
other for input.  There is an address in memory (I/O space, really) to which
a program writes a character to the UART (8250, etc.).  The character will
be sent OUT the COM port to the modem or whatever is connected to it.  These
address are something like 3F8 for COM1 and 2F8 for COM2.

There is also an IRQ (interrupt request) that the UART raises (asserts)
whenever the receives a character from the modemo or whatever.  The CPU will
recognize that an interrupt has occurred, and the program will branch off
and grab the received character from the UART.

Mice only generate interrupts to the processor.  Therefore, they only take
up an IRQ line, but don't consume one of the COM addresses in memory (I/O

73 - Jim AD1C

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