Rs-232-C Technical Information - Radio Shack TRS-80 Model 3 Operation And Reference Manual

APPENDIX
I /
RS-232-C
Technical Information
Transmission of Digital Data
The transfer of digital data over relatively 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 not be contiguous. This means that gaps of varying lengths may be
present between 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 element is maintained until the start element of the next
character is transmitted. There is no upper limit to the length of the stop element.
However, there is a lower limit that depends on system characteristics. Typical
lower limits 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 character being transmitted. A clock source at the
receiver is reset by this transition and is used to locate the center of each data bit.
There are several good reasons for using the asynchronous data transmission
system. A clock signal does not need to be transmitted with the data, thus,
equipment is simpler. Also, the characters don't need to be sent all atone time; they
can be transmitted as they become available. This is particularly useful when
transmitting data from manual-entry input devices (e. g. a keyboard). The major
disadvantage of asynchronous transmission is that it requires a significant portion
of the communications bandwidth for start and stop elements.
The rate at which asynchronous data is transmitted is defined as the
baud rate.
Baud rate is the inverse of the time duration of the shortest signal element.
Normally, this is one data bit interval. The baud rate is equal to the bit rate if one
stop bit is used; but for systems which use more than one stop bit, the baud rate does
not equal the bit rate.
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
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