piclist 1995\12\08\144018a >
Thread: Direct RS-232 Connect
face BY : Lee Jones email (remove spam text)

On Fri, 8 Dec 1995 11:21:39 0000, Tony Grimer wrote:

> In fact Mr Baud yes there was such a man..
> Created the rate when he worked for ITT CREED LTD Brighton England.

Sounds implausible.

> 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.

I beg to differ.  When I first started working on computers, terminals
were Teletype Corporation models 33, 35, and 37 (with lowercase).  I've
worked on both the ASR (automatic send/receive) and KSR (keyboard only)
versions.  The ASR was the one with a paper tape punch & reader.

In 110 baud, each bit was 1/110 second wide.  A character was 7 bits
encoded in US ASCII.  The 8th data bit was for parity (odd, I believe).
There was 1 start bit and 2 stop bits (to give the mechanical clockwork
drive some breathing room).  Thus, a total of 11 bits per character.

I never worked on TELEX, but I thought it used a 7 bit code too.

They would do 10 characters per second (with quite a noise & vibration).
The rate was quoted as 100 words per minute (since they were competing
with secretaries taking dictation over the phone or telegraph operators).

> 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.

I disagree.  RS232 came from the Electronic Industry Association as
Recommended Standard 232 (thus the well known term EIA RS232).  The
suffix letter denotes the revision.  RS232C was current as computers
started to become afordable; I think it's now up to RS232D.  I don't
believe it was derivative of any CCITT (now ITU) work.  ANSI might
have coordinated with them later...

> 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.

"Should not" is the operative phrase.  Almost all terminal equipment
in the US has the earth ground and signal ground tied together inside
the unit (and I do include PCs).

I've had to repair equipment where the computer was in one building
and the terminals were in another (30 feet apart).  50 pair phone cable
had been run between the buildings.  RS232 3 wire (TxD, RxD, & signal
ground) was used.  When the equipment blew, the current path was from
earth ground in building A, up the power cord's 3rd wire, over the
RS232 signal ground wire, down other power cord's 3rd wire, and into
the ground in building B.  Spectacular -- chips fragmented, tops blown
off, etc.  Happened in good weather (no storms).

Building B had an excellent earth ground for the medium size mainframe
computer (it would be considered huge by today's standards).  Building
A had normal power grid ground system.  Building A floated tens of volts
above other buildilng when the ground got really dry.

                                               Lee Jones

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