Re: Plus/4 RS232 woes

On 9/5/2018 12:38 PM, smf wrote:
>
> 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.

a crystal input is a clock input.  It may be imprecise, but most 
datasheets use the same term for both input types.


>
>> 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)"

I'm not sure what you are arguing.  Even if the ACIA can handle 125Kbps, 
if the line drivers didn't support that speed, then Commodore and Apple 
would not propose it.  And, given Commodore's disdain for spending 
money, I would see them foregoing specifying the 125kbps compliant 
drivers in the design and then just not marketing 115200.  And, I 
suspect that the Aminet board drivers probably did not meet EIA 
standards for 115200 operation, but the devices connected probably 
handled the marginal signal OK.


> 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.
The 6551 came out years earlier.  And, Commodore and Apple knew about 
the speed.  Again, the drivers were no doubt the limiting factor.
>
>> 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."
You're misreading the DS.  There is no requirement that the /16 mode 
only works with an oscillator.  It will work fine with a crystal.
>
> 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.

I don't believe it does.


It says:

  * On chip rate generator, and you *only* need to use a crystal to take
    advantage of them
  * Or, if you want, you can also use a clock oscillator
  * And, you can create non standard bps rates, if you want (up to 125kbps)

One must remember, in the late 1970s when this device was developed, 
oscillators were more expensive and bulkier (in board space, at the very 
least) than a simple crystal.  Thus, the DS would play up the simple 
crystal use, but then needed to point out somewhere that an oscillator 
could be used as well, and non standard bps rates are supported.  
Someone tried to condense all 3 of those items into 2 bullets.


Still, I can confirm that /16 and all of the regular bps rates work with 
both oscillators and crystals, so the ACIA does not care.

JIm


>
>
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-- 
Jim Brain
brain@jbrain.com
www.jbrain.com