Re: Beginner's question on RS232

On Wed, 08 Mar 2006 05:27:41 -0800, Steven P wrote:

> maybe a stupid question.

The only stupid question is the one you don't ask. :-)

But I've set followups to sci.electronics.basics, since this is a basics
question.

> I know rs232 data flow normally consists of 1 Start bit, 8 Data bits, 1
> Stop Bits and 1 Check bits for a Byte data.
>
> Can someone tell me whether this Start Bit is high or low, and stop
> bit? If this depends on the hardware implementation, then please give
> me the answer under the condition of using PC and windows.

Maybe some of these links could be helpful:
http://www.google.com/search?q=rs-232+specification
http://www.google.com/search?q=rs232+specification

Cheers!
Rich

This is my favourite website.
It have a complete RS232 information, and is easy to read.
http://www.beyondlogic.org/serial/serial.htm

Enjoy.


Best Regards,
Lim Siong Boon

(E-Mail Removed)
www.siongboon.com



"Rich Grise" <(E-Mail Removed)> wrote in message
news(E-Mail Removed)...
> On Wed, 08 Mar 2006 05:27:41 -0800, Steven P wrote:
>
> > maybe a stupid question.
>
> The only stupid question is the one you don't ask. :-)
>
> But I've set followups to sci.electronics.basics, since this is a basics
> question.
>
> > I know rs232 data flow normally consists of 1 Start bit, 8 Data bits, 1
> > Stop Bits and 1 Check bits for a Byte data.
> >
> > Can someone tell me whether this Start Bit is high or low, and stop
> > bit? If this depends on the hardware implementation, then please give
> > me the answer under the condition of using PC and windows.
>
> Maybe some of these links could be helpful:
> http://www.google.com/search?q=rs-232+specification
> http://www.google.com/search?q=rs232+specification
>
> Cheers!
> Rich
>

In article <(E-Mail Removed)>,
Rich Grise <(E-Mail Removed)> wrote:
>On Wed, 08 Mar 2006 05:27:41 -0800, Steven P wrote:

>> maybe a stupid question.

>The only stupid question is the one you don't ask. :-)

To a point. If the answer is easily obtainable, then the question can
be construed as superfluous.

>But I've set followups to sci.electronics.basics, since this is a basics
>question.

Shoot.


>> I know rs232 data flow normally consists of 1 Start bit, 8 Data bits, 1
>> Stop Bits and 1 Check bits for a Byte data.

Actually the format of the stream is all over the place. 7 or 8 bits of data. 1 or 2 stop bits. even/odd/no parity. It all depends on what the two devices
communicating agree on.

>> Can someone tell me whether this Start Bit is high or low,

Given that negative voltages in RS-232 represent a 1 bit, and positive voltages
represent a 0 bit, the start bit is a 0 bit.

It's important to understand the differences between the actual voltages and
their representation. Most of the time the RS-232 voltages are converted into
what is called a serial TTL format. This serial TTL is the interface to most
UARTS and microcontrollers.

Bt default a RS-232 transmit line idles on the 1 bit. So if no transmission
is occuring, then the line shows a 1 (which is a negative voltage in the range
of -12V).

So now it makes sense that when the device is ready to send something that
some change should occur to the line to signal that something is coming down
the track. Since the idle line is a 1, then the start bit, which is that
signal, needs to be a 0 bit (represented by a +12V on the line).

>> and stop bit?

The opposite. The stop bit is designed to give the line some time to get
back into an idle state. Since idle is a 1 bit, the stop bit is a 1 bit too.

>> If this depends on the hardware implementation, then please give
>> me the answer under the condition of using PC and windows.

Not really. The bit representations and the voltages are a part of the
standard. Now many PC implementations cheat by using 0-5V to represent
the 1 and 0 bits respectively. But the 0V is technically out of specification
because it's not negative.

BAJ