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From: cs4344af@evax.arl.utexas.edu (Fuzzy Fox)
Newsgroups: comp.sys.cbm
Subject: Re: SwiftLink 232 + other carts
Message-ID: <1991Mar15.214308.3848@evax.arl.utexas.edu>
Date: 15 Mar 91 21:43:08 GMT
References: <1991Mar13.131655.26195@news.nd.edu> <91073.110357U15803@uicvm.uic.edu> <1991Mar14.232719.24621@iitmax.iit.edu>
Distribution: usa
Organization: Computer Science Engineering Univ. of Texas at Arlington
Lines: 33

In article <1991Mar14.232719.24621@iitmax.iit.edu> technews@iitmax.iit.edu (Tech News Account) writes:
>I have a RS-232 adapter (like a cartridge) which connects to the USER port,
>like a modem would, and provides RS-232 standard output...
>
>is there any reason this wouldn't work? $25 and it's yours.

Some people may not know the difference between these, so perhaps I
should explain.

A modem adapter that plugs into the User Port is a simple device, which
merely takes output signals from the port and translates them to the
proper voltages for the RS-232 standard.  However, it is still the
responsibility of the CPU to shift every little bit out through this
port, at the proper time, in order to send or receive at any particular
baud rate.  The 1 MHz C64 processor becomes sorely taxed at high baud
rates such as 9600 or so, and can do very little else when trying to run
at this speed.

An adapter that plugs into the Cartridge Port is a more complex device,
namely a UART.  This works differently, in that the UART will accept
whole bytes of input at a time, and shift the bits out all by itself,
thus taking ten times less overhead away from the processor.  So, at
9600 baud, the processor does not have to sevice 9600 interrupts per
second (as with the User Port), but only 960 interrupts per second.
Thus UART communications are much more reliable.  The price of this is
that most terminal programs do not support the UART, so you are tied to
specific programs in order to reap the benefits.

-- 
David DeSimone, aka "Fuzzy Fox" on some networks.          /!/!
INET:    an207@cleveland.freenet.edu                      /  ..
Q-Link:  Fuzzy Fox                                        /   --*
Quote:   "Foxes are people too!  And vice versa."         /  ---