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From: fred@cbmvax.UUCP (Fred Bowen)
Newsgroups: comp.sys.cbm
Subject: Re: modem woes
Message-ID: <2442@cbmvax.UUCP>
Date: Thu, 1-Oct-87 14:33:28 EDT
Article-I.D.: cbmvax.2442
Posted: Thu Oct  1 14:33:28 1987
Date-Received: Mon, 5-Oct-87 08:31:41 EDT
References: <14897@topaz.rutgers.edu>
Organization: Commodore Technology, West Chester, PA
Lines: 44

> I think that my C64 clock frequency may be slightly off.  I have also heard
> that the 1200 baud rate that modems expect is not the same as what the 1200
> baud rate is that Commodore programmed into the C64 "bit-busting" routine.
> Can somebody from Commodore shed some light on this?

I doubt your clock frequency is off.  a baud rate of 1200 is easy enough to
handle on a C64, and a rate of 2400 is possible on a C128 in FAST (2MHz) mode.
When you consider it's a software emulation, it's really not bad.  As you had
indicated, there is substantial differences in performance among the many
terminal packages- the C64/128 is rather constant if programmed correctly.

Few people realize there is a difference, too, between direct connections
and communicating with a modem.  A modem typically communicates at the high
end of the spec-ed range of any data rate.  For example, at a baud rate of
1200, a modem communicates with the terminal at something like 1210.  This
is why you have to "tweak" the C64/128's data rate- it's dead-nuts-on 1200,
and the +/- range it can accomodate is small.  Hence you see alot of garbage.
If the terminal package allows you to adjust the rate for communicating with
a modem you should, especially for the more critical faster rates.

Earlier C64's did not take into account the different clock rates in Europe.
This was corrected a long time ago, and unless you have a very old (pre-1983)
C64, you need not concern yourself with this.  There was no error in the
baud rate tables other than this, and I can find not mention of any such
bug in the book someone mentioned, "Mapping the Commodore 64".

Perhaps the greatest limitations of the software UART are the small buffer
sizes- 256 bytes each for input and output, and no built-in flow control.
When you consider a single 40x25 line screen is 1000 characters, them little
buggers are really humping!  Admittedly, a screenful is usually something
less than full, but that is not the point- for communications like editing or
reading email, it works pretty well.  For dumping to the screen a hugh file
without utilizing flow control, yer gonna lose.  This where the particular
terminal package you choose plays a big part.  Same holds true for whatever
you use to up/download stuff.  But the software UART has been flexible enough
to grow beyond that which for it was originally intended.

--
-- 
Fred Bowen			uucp:	{ihnp4|rutgers|caip}!cbmvax!fred
				arpa:	beats me
				tele:	215 431-9100

Commodore Electronics, Ltd.,  1200 Wilson Drive,  West Chester,  PA,  19380