APPENDIX
I / RS-232-C TechnicalInformation
Transmission ofDigital Data
The transfer
of
digital data overrelatively long distances is generally accomplished
by sending data in serial form using a single twisted wire pair to connect the
transmitting and receiving devices. One
of
two general transmission techniques is
commonly used, asynchronous
or
synchronous. The transmission technique used
in the Radio Shack system is asynchronous-bit-serial. Since we
don't
use the
synchronous technique,
we'll
not mention it again. Asynchronous transmission
does not require a synchronizing clock to be transmitted with the data and, the
characters need notbe contiguous. This means that gaps
of
varying lengths may be
presentbetween transmission
of
individual characters.
The bits which comprise a data character (generally from five to eight bits in length)
and synchronizing start and stop elements are added to each character as shown
below. The start element is a single logic zero (0) data bit that
is
added to the
front
character.
The
stop elementis maintained until the startelement
of
the next
characteris transmitted. There is no upperlimit to the length
of
the stop element.
However, there is a lowerlimit thatdepends on system characteristics. Typical
lowerlimits are
1.0, 1.42
or
2.0 data-bit intervals (although most modern systems use
1.0
or
2.0 stop bits). The negative-going transition
of
the start element defines the
location
of
the data bits in the characterbeing transmitted. A clock source at the
receiver is resetby this transition and is used to locate the center
of
each data bit.
There are several goodreasons for using the asynchronous data transmission
system. A clock signal does not need to be transmitted with the data, thus,
equipmentis simpler. Also, the characters
don't
need to be sent all
atone
time; they
canbe transmitted as they become available. This is particularly useful when
transmitting datafrom manual-entry input devices (e.g. a keyboard). The major
disadvantage
of
asynchronous transmission is that it requires a significantportion
of
the communications bandwidth for start and stop elements.
Therate at which asynchronous data is transmitted is defined as the
baud rate.
Baudrate is the inverse
of
the time duration
of
the shortest signal element.
Normally, this is onedata bit interval. The baud rate is equal to the bit rate
if
one
stopbitis used; butfor systems which use more than one stop bit, the baud rate does
not equal the bitrate.
ONE 8 BIT CHARACTER
(00100000)
STOP ELEMENT
J
START ELEMENT
I
LJLJl
I_nl..---------Jn_
/
-...
---
/
ONE 8 BIT CHARACTER START ELEMENT
STOP ELEMENT (11001000)
Asynchronous Data
1/1
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