piclist 1995\12\08\081337a >
Thread: Direct RS-232 Connect
picon face BY : Tony Grimer email (remove spam text)

At 17:48 12/7/95 +0500, you wrote:

>Actually Lee, Baud refers to the signalling rate. BPS is the number of bits
>transmitted per second. 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.
As an engineer who has worked in the async data transmission field for more
years than I care to remember, these discussions on terminology and modern
interpretation of standards that were established in some cases 30 years ago
without reference to the original source material bothers me....

In fact Mr Baud yes there was such a man..

Created the rate when he worked for ITT CREED LTD Brighton England. They
manufactured TELEX you remember those large lumps of electro mechanical
equipment that came before fax machines..

110 Baud as a simple example in fact refered to the transmission of an
      8 bit data field FRAMED with 1 start-bit, 1 stop-bit and 1 parity-bit

Total data transmission size for each byte of data is therefore 11 bits and
the physical transmission rate for the bits is 100 bits /sec.

In the early electro machanical days 75 Baud and 50 Baud rates - those were
the days ! refered to the BAUDIT coded method using the 5 bit data encoded
ALPHA NUMERIC transmission derived by Baud himself, for information via the
world wide Telex network.

A holding MARK the line idle situation on an async line, by the way, is
defined as a positive voltage level which is sufficient to avoid false start
bit occuring due to electrical noise. Typically the holding MARK was
originally specified to be 25% greater than the nominal or mean DC positive
level that would occur during data transmission, those old Telex machines
simply had a soleniod which drove a dolly backwards and forwards and it took
a large transient on the Line to get the soleniod to start switching.

Its a pet hate of mine that we have lost these historic definitions and that
we now allow the Baud rate and the BPS rate to be used without regard to
there real meaning.

On the subject of voltage levels on RS232 etc...

The CCITT yellow book standard (Geneva) in which V24 was formally adopted by
the world wide PTT's back in the mists of time, clearly defined the
electrical characteristics of the sihnally as ...

  Positive signal + 3v to + 30v (max) with a nominal of 15v
  Negative signal - 3v to - 30v (max) with a nominal of -15v

Therefore RS232 the ANSI standard which apart from a few words here and
there is a derivative of the yellow book standard, adopted the above.

The source current from the transmitter is specified as absolute max 5mA,
but with short circuit protection, more than can be said for RS422. Rs423
etc. which only specifies a short circuit protection for a transient period.

The major RS232 / V24 chip sets, those that quote figures related to the
standards I refer the Texas Instruements 75' series for instance, have
inbuilt hysterisis in the receiver between 3v and -3v and state that input
voltages within the -3v to + 3v range can not have defined output voltage

RS232 / V24 also has two ground connections, so any discussion on earth
current flows during transmissions must refer to both. Signal earth this is
the actual data line flow return path. Safety or Equipment earth the path
which will provide for any major distrubance on either piece of equipments
power supply current drain etc.. These two earths should not be connected
together either on the transmission path or in the equipment itself.

Sorry to take up so much space, but after all the recent discussion, on
Direct RS232 connection to a PIC I just felt I needed to air my views.


Tony Grimer ----  cm1918.....spamscitsc.wlv.ac.uk (Tony D. Grimer)

School of Computing (SCIT)  Room MC108 Tel - 0902-322764.


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