Re: RS-232 on VIC-20

From: Marko Mäkelä (
Date: 2004-06-21 20:12:24

On Mon, Jun 21, 2004 at 07:51:54PM +0200, Christian Johansson wrote:
> > Well, actually, the biggest problem is badlines - and they're 40 us
> > long even in 2 MHz mode. In 80 column mode with the VIC screen closed
> > it's possible to use 9600 bps though.
> I'm not very familiar with badlines. Does that problem exist even if I blank
> the screen in 40-column mode?

No.  In 2 MHz mode, there are no badlines, but the system will run at 1 MHz
during memory refresh (5 slow cycles on every rasterline), making the total
number of cycles per raster line equal 63*2-5 on PAL systems and 65*2-5 on
NTSC.  Maybe MagerValp was thinking the TED, which doesn't show garbage on
the screen, but slows down the processor to 1 MHz during graphics DMA.  The
TED also has two badlines for every line of text.

> What I want to know is if the Hug routine I posted in my
> previous email could perhaps be used to get at least 4800 bps by making some
> small modifications to it.

You may need to take the DRAM refresh cycles into consideration.  The bit time
for 4800 bps is 17734472/18/4800 = 205 slow cycles on the PAL C128 and
14318181/14/4800 = 213 slow cycles on the NTSC C128.  Okay, maybe you can
ignore the DRAM refresh even at 9600 bps.


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