Hi Mario,

You are doing nothing wrong.  The start and stop bits must have opposite
polarity, and this requirement arises from the way in which asynchronous
serial communications work.

There are two ways to look at this.  I'll try the simple way first and then
give a slightly fuller explanation.  The start and stop bits mark the start
and end of each transmitted character, right?  Imagine a stream of
characters: you get a start bit, the character, a stop bit(s), a start bit,
the character, a stop bit(s), and so on.

If you look at that sequence it should be obvious that the stop and start
bits between the two characters must have opposite polarities otherwise you
couldn't tell them apart, right?  If they were both the same polarity you
couldn't tell when the stop bit finished and the start bit started.  In
other words, the receiver needs an "edge" or a "transition" to signal the
commencement of the start bit.  So it's gotta be of opposite polarity to the
stop bit preceding it.

Incidentally, before proceeding, I need to tell you about marks and spaces.
Basically, with serial comms you talk about marks and spaces rather than 1s
and 0s, or highs and lows.  A mark represents a logic 1, a space represents
a logic 0.  A stop bit happens to be a mark, a start bit is a space.

Just to confuse you even further, the polarity of marks and spaces on an
RS232 serial transmission line seems the wrong way round: marks are sent as
a negative voltage, spaces as a positive voltage.

I'll just summarise that again, as a sanity check: a mark represents a logic
1 and is a negative voltage, a space represents a logic 0 and is a positive
voltage.  A start bit is a space (positive), a stop bit is a mark
(negative).

In the eight bits of the transmitted character, the 1s are negative (marks)
and the 0s are positive (spaces).  I said it seemed the wrong way round!!

One more thing, which I'm sure you know.  The duration of the bits is
determined by the bits-per-second rate of the line (often, but not always
correctly, called the baud rate).  So, on a 1200 baud link each bit lasts
1/1200th of a second.  This is relevant later.

The thing about asynchronous comms is that there is, or can be, any amount
of idle time between the characters transmitted.  So the receiver can't
synchronise itself to the incoming bit stream (hence "asynchronous").  When
the line is idle, the transmitter sends out a continuous "marking"
(negative) voltage.  You might like to think of this as a continuous stream
of stop bits.  The receiver sits there drumming its fingers, having no idea
when the next character is going to arrive.

Eventually a character is transmitted.  Here is the crucial part: the start
bit is of opposite polarity, so the first thing the receiver sees is a
transition from a mark to a space.  The receiver now knows the precise
moment in time at which this transition - the leading edge of the start
bit - arrived.  It also knows how long each bit (including the start bit)
should last (i.e. 1/1200th of a second).  So, it waits
for the duration of half a bit and samples the voltage on the line again.
This puts it right smack in the middle of the start bit.  If it still sees a
'space' it knows the start bit is real (i.e. it wasn't just a noise blip on
the line).  It then waits 1/1200th of a second and samples the line yet
again.  This puts it smack in the middle of the first bit of the character.
It continues to sample at 1/1200th of a second intervals thereafter until it
gets to the stop bit(s).  Once it has registered the stop bit it then goes
into continuous monitoring mode in order to pick up the next transition to a
start bit.

You will see that the receiver's internal clock (for measuring out the
1/1200th of a second sampling intervals) doesn't need to be particularly
accurate.  All that's required is that it stays within half a bit over the
duration of the received character.  In other words, an accuracy of a few
percent if fine.  This means that asynchronous comms are cheap and easy to
implement.

Just a couple of odds and sods.  It is not uncommon to transmit two stop
bits.  Interestingly, in this case you can configure the receiver to expect
either one or two and it will still work.  If you tell it to expect just
one, it simply assumes there is a slight gap between the characters.
Remember, the stop bit - a mark - looks like the line idle condition.
Sometimes a parity bit is transmitted.  What this means is that, according
to how you set things up, a character may be anything from 10 to 12 bits
long.

At one time some systems were set up to transmit just seven bits for the
character, that being sufficient for the entire ASCII character set.  So
these could be as short as 9 bits in length.

You might be interested to know that the worldwide telex standard uses a
five bit code, with a stop bit one-and-a-half bits long!  This gives 7 1/2
bits per character.

Phew!!  Sorry for the awesome length of this email.  I hope you will find at
least some of it of use.

Regards,

Steve

Steve Thackery
Suffolk, England.
Web Site: http://www.btinternet.com/~stevethack/