Re: Plus/4 RS232 woes

On 05/09/2018 15:35, Jim Brain wrote:
> On 9/5/2018 2:54 AM, smf wrote:
>> On 05/09/2018 01:26, Jim Brain wrote:
>>> It works as designed.  /16 mode gives 115200. The T232 used this to 
>>> get 115200 and 230400 (by using the 6551A and double clocking it)
>>>
>> Whoever wrote the data sheet seemed to think the /16 mode is only for 
>> working with synchronous TxC (XTAL1) and not an asynchronous with an 
>> external crystal (XTAL1 + XTAL2).
>>
>> "On-chip baud rate generator: 15 programmable baud rates from a 
>> standard 1.8432 Mhz external crystal (50 to 19,200 baud)."
>>
>> "External 16x clock input for non-standard baud rates (up to 125Kbaud)"
>>
> Not sure how you arrived there from that text, but I see no 
> connection.  The ACIA supports 15 standard baud rates and 1 non 
> standard one.

Right, but it says that the 15 standard baud rates use a crystal 
connected to xtal1 & xtal2, but for the non standard one you should hook 
up a clock to xtal1 & xtal2 should float.

> 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)"

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.

> 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.