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Thread: Direct RS-232 Connect
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face BY : Lee Jones email (remove spam text)



Hello Dana,

>> The "baud rate" is the number of bits per second when the line is
>> running continuously.  The difference between bits per second and
>> baud rate is that baud rate may vary from 0 bps to maximum.  Thus
>> the baud rate is the correct term for an async line while BPS (bits
>> per second) is correct for a sync line.  [Misuse of this term is a
>> pet peeve of mine.]  Data is sent least significant bit first.
>
> Actually Lee, Baud refers to the signalling rate. BPS is the number
> of bits transmitted per second.

Agreed.  But I was speaking in the context of an asynchronous serial
channel using a 2 state RS232 voltage oriented interface for each
each data line.  Consider the following.

From the "Encyclopedia of Computer Science" by Ralston & Meek:

"A baud is a unit of signaling speed and refers to the number of
times the state (or condition) of a line changes per second.  It is
the reciprocal of the length (in seconds) of the shortest element in
the signaling code.  Historically, it is a contraction of the surname
of the Frenchman J.M.E. Baudot, whose five-bit code was adopted by
the French telegraph system in 1877."

"The fastest signaling rate of a communications channel is called
the 'baud rate'. [...]  When only two-level signaling is used, the
baud is also equal to the [maximum] rate of information transfer
in bits per second (bps)."


To me, bits per second (bps) implies a continuous stream of bits.
Async serial is not continuous since there are usually long periods
with no transitions (particulary if it's someone at a keyboad :-) ).
During those idle times, the bps rate is 0.

I still prefer baud rate for an async connection from RS232 DTE (Data
Terminal Equipment) to DCE (Data Circuit-terminating Equipment).  I
believe it better implies the fact that the signalling rate can vary
between 0 bps and whatever maximum has been configured (e.g. 9600).


> The distinction only comes into play when more than
> one bit at a time (per baud) is transmitted, as in BPSK, QPAM, etc.
> Bell 212 modems transmit two bits at a time, as 4 quadrant phase shift. The
> signalling rate is 600 Baud, the bit rate is 1200BPS.

In a 2 state scheme, such as RS232, the baud rate and maximum bps rate
are identical.  But there is still a distinction, admittedly tenuous,
between baud and bps for an _asynchronous_ digital interface.

You're introducing that portion of the communications channel that I
hadn't addressed, namely the link between 2 DCE devices (aka modems).
Up through 300 baud modems, each bit on the digital interface caused
one transition on the analog (aka phone) side.  After that, starting
with 1200 baud modems, the inter-modem link was usually synchronous
and the baud and bps rates diverged (due to phone bandwidth limits).

For example, Bell 212s ran the digital connection (RS232 interface)
at 1200 baud async.  And, as I recall, it ran the analog connection
(telephone side) at 1225 bps synchronous using a 4 level PSK signalling
scheme.  This allowed the modem to accomodate timing variations on the
async interface even if running at maximum rate without losing data.

                                               Sincerely,
                                               Lee Jones
<9512081847.AA02277@MIT.EDU>

See also: www.piclist.com/techref/io/serials.htm?key=rs%2D232
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