Path: utzoo!attcan!uunet!mcvax!tuvie!tugiig!plipp
From: plipp@tugiig (Lipp Peter)
Newsgroups: comp.os.minix
Subject: Re: AN HISTORIC MOMENT!
Message-ID: <20@tugiig>
Date: 15 Jul 88 09:37:01 GMT
References: <841@ast.cs.vu.nl| <4926@husc6.harvard.edu> <845@ast.cs.vu.nl>
Organization: none
Lines: 34

In article <845@ast.cs.vu.nl>, ast@cs.vu.nl (Andy Tanenbaum) writes:
> If Dan has tested it, I believe him. Nevertheless, let us all get out our
> IBM PC-XT Technical Reference Manuals and read together starting in the
> middle of page 1-200 where it says:
> 
> \fBNote:\fR The maximum operating frequency of the baud generator is 3.1 MHz.
> In no case should the data rate be greater than 9600 baud.
> 

Right, but lets look at the motivation behind these sentences (at least
what I think the motivation might be):

a) maximum frequency: default frequency is 1.8432 MHz - and I guess
	all machines stick to this. If two machines communicate using
	different frequencies, errors with high speeds are possible.

b)	9600 baud: This limitation might be because of the quite slow
	IBM-PC, which might get into trobles at dealing with interrupts
	if higher speeds are used.

I just tested sending data between PC`s at a rate of 115200 bps, without 
troubles. They were 10 Mhz- ATs and communication done without interrupts.
I have not tested interrupt-driven I/O at this speed.

This is not in contrary to the Technical Reference, which says (poor translation,
because I have a poor german translation only):

The INS8250 contains a programmable baudgenerator ..... and the division
by a divisor between 1 and (2 to the power of 16)-1.

A divisor of 1 meens 115200 baud (at 1.8432 MHz).
I would say: 9600 is a recommendation, not a barrier

Peter Lipp (plipp@tugiig.uucp)