Re: Plus/4 RS232 woes

Den Wed, 05 Sep 2018 18:51:51 +0200 skrev André Fachat <afachat@gmx.de>:
> 
> 
> Am 5. September 2018 4:39:09 PM schrieb Jim Brain <brain@jbrain.com>:
> 
> 
> > Most people did not make the connection that 1.8432/16 = 115200.
> > However, I think Commodore and Apple did understand.  At the time
> > the devices came out, 115200 was not a bps rate in general use, and
> > line drivers at the time would have struggled to keep up and adhere
> > to the standard. 
> 
> Indeed 115200 bps is more than 11k bytes per second, which leaves
> about 100 cycles per byte at 1MHz.
> 
> Without a working way to stop the other side from sending (see the
> previous RTS discussion here) there is not much left to do other
> things. .
> 
> You'd probably had best to use an interrupt to receive a byte to be
> able to do other things in parallel. That would take maybe 30 to 40
> cycles, taking almost half the CPU.

Another way to use those relative high speeds (as compared to the CPU
speed) on an UART is to use specially written software at both ends and
transfer data in burst, and do some software handshake to allow each
end to do other processing between the bursts. I assume this is
probably how the Amiga version of Twin Express does as it can run at at
least 230400 bps. Btw at those speed the hardware actually affects the
signal quality. From a distant memory even with rather short cables
(like about 50cm / two feet) the signal quality is not good enough to
support the actual highest speed the UART can achieve. I haven't looked
in to the possibility to add some external frequency response correction
hardware or similar though, that might help. I guess that it's not
really the 1488/1489 but rather the RF noise suppressor things close to
the actual port connectors that causes the signal to not work at max
baud rate.

-- 
(\_/) Copy the bunny to your mails to help
(O.o) him achieve world domination.
(> <) Come join the dark side.
/_|_\ We have cookies.