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From: hamilton@uiucuxc.Uiuc.ARPA
Newsgroups: net.micro.pc
Subject: Re: 19.2K bps from std serial card?
Message-ID: <21900081@uiucuxc>
Date: Tue, 10-Sep-85 00:49:00 EDT
Article-I.D.: uiucuxc.21900081
Posted: Tue Sep 10 00:49:00 1985
Date-Received: Thu, 12-Sep-85 10:41:47 EDT
References: <322@sesame.UUCP>
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Nf-ID: #R:sesame.UUCP:-32200:uiucuxc:21900081:000:1278
Nf-From: uiucuxc.Uiuc.ARPA!hamilton    Sep  9 23:49:00 1985


> I was in touch with a company who sells a communication program
> for the IBM-PC and has a capability of communicating at 19.2K bps.
> Most serial cards for the IBM-PC that I know of are only pro-
> grammable to 9600 bps (according to documentation).  The company
> mentioned that the standard (IBM) serial port could be configured
> to rates of 19.2K bps, and even as far as 38.4K bps.
>      
> Anybody know if this is possible?  Is it a hardware modification
> on the port, or undocumented software change?

the hardware doesn't limit you to 9600; the ROM BIOS does.  if you
check out the BIOS serial port services (INT 14), you'll see that you
specify the desired baud rate, parity, and framing in a single byte;
the baud rate gets 3 of the 8 bits.  the actual baud rate the hardware
uses comes from dividing the clock frequency (1.19Mhz?) by a 16-bit
number.  the BIOS has a table of 8 of these divisor values, and sets
the serial port baud rate by loading one of them into the UART's
divisor latch.  the BIOS writers chose to supply divisors for 110,
150(!), 300, 600(?), 1200, 2400, 4800, and 9600 baud.  anyhow, it's
a simple matter to load your own divisor with a few i/o instructions.
as i recall, the divisor for 9600 is 12, 19.2K would be 6, and 38.4K
would be 3.