[Elecraft] K2 - KIO2 stop bit oddity

[Brian Lloyd]

>> The receiver's stop-bit setting needs to be greater than or equal  
>> to the stop bit setting of the transmitter. It is OK for the  
>> transmitter to
>
> You meant less than, not greater than, although, as you noted later,  
> receivers generally don't have a stop bits setting.

Yes.

>> send two stop bits and for the receiver to be set for one stop bit.  
>> It won't hurt a thing. Most UARTs use the stop bit setting to  
>> affect only the transmitter (RS-232 sending part of the device).  
>> The receiver will
>
> Delete RS232.  (In fact, historically, current loop was used for the  
> physical interface.)

True, once the mechanical teleprinters fell from grace, current loop  
fell by the wayside. (Unless you were connecting to DEC hardware. I  
have implemented more than one 20mA current loop to RS-232 converter  
in my life.)

>> handle anything that is at least one bit-time long for a stop bit.
>
> Modern UARTs accept stop bits that are just over half a signalling  
> unit in length (they sample in the nominal middle, but there is a  
> limited sampling clock resolution.  They need to accept ones that  
> are strictly shorter than the transmitted ones, because, as we are  
> talking about asynchronous signalling, they need to be able to cope  
> with recovering from false start bits and cope with clock rate  
> differences (more common on mechanical devices, but some electronic  
> devices rely on these to allow working with convenient crystals.
>
> (When sending asynchronous data over 1200 bps synchronous modems,  
> sometimes no stop bits could  be sent over the wire, if the source  
> clock was fast, as, being synchronous, there was no option to  
> shorten the stop bit.  Stop bits were re-inserted before creating  
> the baseband output; I believe they ran the output clock fast to  
> ensure that this worked.)

You are, of course, correct. I should make sure that I am rigidly  
correct when I write and I was being sloppy. Having implemented UARTs  
in both software and hardware, and then the protocols to run over them  
(I am one of the authors of PPP and the architect for MLPPP) I do have  
a modicum of understanding.

>> Longer stop bits just reduce the maximum rate (characters per  
>> second) that you can send data.
>
> And give better recovery from false start bits - not a problem you  
> should have on a short piece of wire.
>
> Incidentally, 4800 baud is normally sent with one stop bit.  As  
> noted elsewhere, it is only really for mechanical devices that one  
> needed longer ones, so it tends to be 110 and below (maybe 300) that  
> uses 2, or for, 5 unit, Baudot, 1.5.
>
> -- 
> David Woolley
> "The Elecraft list is a forum for the discussion of topics related  
> to Elecraft products and more general topics related ham radio"
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--

73 de Brian, WB6RQN
Brian Lloyd - brian HYPHEN wb6rqn AT lloyd DOT com