Re: Plus/4 RS232 woes

Den Wed, 5 Sep 2018 18:38:23 +0100 skrev smf <smf@null.net>:
> On 05/09/2018 15:35, Jim Brain wrote:
> > 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.  I also seem to recall that EIA relaxed the serial
> > standard at some point to allow the higher speeds to comply with
> > the signalling standard.
> 
> I doubt the line drivers was the reason as the supported baud rate is 
> higher "External 16x clock input for non-standard baud rates (up to 
> 125Kbaud)"

Back in the days you could roll your own drivers and receivers, and
that was about the only way to do it with 20mA current loop.

Specifically for Plus/4 Commodore might had got angry customers if some
of the existing interfaces didn't work at a speed specified by Plus/4.
Who knows what interfaces already existed. We know for sure that
Commodore wasn't the only one producing RS232 level converters (and
complete modems) for the user port.

> The intel 8250 came out in 1981 and supported 115200, commodore and 
> apple had plenty of time to realise. My guess is didn't because the 
> datasheet is written in a way that indicates it wouldn't be supported.

Even though PC used 8250, it took some time for PC to actually dominate
the market enough to make it's own de-facto standard when considering
baud rates over RS232.

> > As you can see, connecting an oscillator to XTAL1 would drive the 
> > inverter, but nothing will be connected to XTAL2.  The circuit will 
> > operate as designed, though.
> >
> My point is that when you have a crystal it will be connected to
> XTAL1 & XTAL2
> 
> The /16 mode is for externally generated clocks and it says 
> "Alternatively, an externally generated clock may be used to drive
> the XTAL1 pin, in which case the XTAL2 pin must float. XTAL1 is the
> input pin for the transmit clock."
> 
> So the question is what happens when the XTAL2 isn't floating, i.e.
> does it cause some internal short circuiting that we can't see. If
> it's safe then why does the datasheet make it so clear the 15 baud
> rates are only for a crystal connected to xtal1 & xtal2 and the /16
> mode is only for an external clock connected to xtal1 with xtal2
> floating.

Pasi has kind of already answered that.

The data sheet probably refers to external clock v.s. built in crystal
oscillator circuits due to that you probably would use the built in
crystal oscillator for standard frequencies but feed in a clock signal
if you wanted some non-standard frequency.

Consider it a typo. There is no reason for the hardware to treat the
xtal 1/2 pins different in different baud rate modes.


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