Re: Plus/4 RS232 woes

czw., 6 wrz 2018 o 21:00 Mia Magnusson <mia@plea.se> napisał(a):
>
> Den Wed, 5 Sep 2018 23:11:28 +0100 skrev smf <smf@null.net>:
> >
> > On 05/09/2018 19:40, Jim Brain wrote:
> > >
> > > a crystal input is a clock input.  It may be imprecise, but most
> > > datasheets use the same term for both input types.

Hint 1 (for Smf): how do you build an oscillator using a crystal and a
(e.g. TTL) inverter ?  Have you ever tried this ?

> > It's not clear from the datasheet whether the circuit is bypassed
> > when you select the /16 external clock mode, or whether an external
> > clock will go through the internal clock circuit.
> >
> > I'd like to see a decap of the chip to know what clock circuit it
> > uses and what it does when you select / 16 external clock mode.

Hint 2 (for Smf): what does the fig. 5 in the 6551 CSG/MOS datasheet suggest ?

> > > The 6551 came out years earlier.  And, Commodore and Apple knew
> > > about the speed.  Again, the drivers were no doubt the limiting
> > > factor.
> >
> > Except it seems to work reasonably reliably & commodore repeatedly
> > ran chips outside spec where they barely work. The A2232 board came
> > out in 1990, way late enough that 115200 would have been useful.
> >
> > The people who wrote that driver obviously didn't know that it could
> > do it, or they would have used it.
>
> There is a more reasonable explanations:
> [....]
> Modems at the time were at best 9600bps (V32) and 19200 were good
> enough to communicate with those modems even with MNP5 and/or V42"
> compression.
>
> They of course did choose what IC's Commodore made in-house as they
> seemed fit for purpose, an 8-bit CPU and UART's that were specified to
> run at up to 19200 baud.

In theory 3.58 MHz 65ce02 (yes, it is _CE_ - thus you guys with A2232
treat them well, when the 65ce02 breaks I bet it would be hard to find
a replacement) should be able to handle 7 6551-s @ 115200 at the same
time... or maybe it was too much for it (when we think about all the
possible overhead). I am wondering if anyone ever looked into the code
that runs on the card's CPU.

> Anyone with some knowledge of electronics hardware can attest that
> there is no reason for some special hardware selecting different mode
> of operation for the XTAL 1/2 pins.

Jim already explained this.