The Resource Magazine For Apple, Atari, and Commodore ...

28 COMPUTE! September/Ociober. 198O. Issue 6
The figure below illustrates a 2-wire, point-to-point,
half duplex, and a telephone leased line which is
always available to the customer.
Computer Data Set Set Terminal
Assume that the computer needs to transmit a message
to the terminal. The computer's software brings up
the "data terminal ready line" to its data set.
If the data set is "ON" it will return "Data
Set Ready" (interlock it) to the computer. When the
computer wants to transmit, it raises the "request
lo send" level which tells the data set to turn on the
carrier wave. The carrier wave is sent from the
computer's data set over the telephone circuit to the
terminal's data set. The terminal's data set, upon
detecting the carrier, will raise the "Received Line
Signal Detector" level to inform the terminal, in
effect, that a message is aboul to be received. After
a fixed delay time (strappable in some data sets)
and after raising "Request ToSend," the computer's
data set will return. "Clear To Send." The computer
upon receiving the "Clear To Send" signal, can
now start transmitting the message, as marks and
spaces, on the "Transmitted Data" line to its data
set. The data set converts the digital signals into
frequency or phase-- shifted signals for transmission
over the leased line to the terminal's data set.
Most data sets contain a damp circuit which
clamps the "Received Data" line. "Received Line
Signal Detector" level is not generated until after
the carrier is detected. The clamp delay masks out
all [he possible noise on the line which occurs during
the switching from either transmit to receive or
receive to transmit.
If this was a synchronous operation, the clocking or
synchronization of each bit would be done by the
computer's data set. So that the computer knows
when each bit must be placed on the "Trans
mitted Data" line, the data set sends clock to the
computer on the "Transmission Signal Element
Timing" line. This clock will be coincident with
the leading adge of each data bit on the "Trans
mitted Data" line.
Ai the terminal end of the system, the computer's
data set turns on its carrier; the terminal's data
set detects it and sends "Carrier Detected" level
to the terminals. Several milliseconds later (length
determined by the "Clear To Sent!" delay in the
transmitting data set), the first message bits arrive
and are converted to a digital signal, which is
passed from the data set to the terminal on the
"Received Data" line. In synchronous operation,
clocking for the data is generated by the receiving
data set and is passed to the terminal on the
"Receiver Signal Element Timing: line in order that
the terminal can correctlv clock (he bits into its
buffer or memory as they arrive. The clock pulse is
timed to occur at the center of the data bit on the
"Received Data" line.
The following is a list of the definitions of the RS232-C
signals which are listed in order of pin number. To
simplify the definitions, the transmitter of the message
will be identified as the "transmitting terminal"
and the receiver as the "receiving terminal."
PROTECTIVE GROUND PIN 1: This ground is
electrically connected to the equipment frame. It may
be connected to external grounds, as required.
TRANSMITTED DATA PIN 2: This signal is
generated by the transmitting terminal and is trans
ferred to the local transmitting data set for trans
mission of data to the receiving terminal. The
transmitting terminal will hold "Transmitted Data"
in marking condition during the intervals between
characters or words, and at all times when no data
are being transmitted.
In all systems, the transmitting terminal will not
transmit data unless an ON condition is present on all
of the following four signals:
1. Request To Send
2. Clear to Send
3. Data Set Ready
4. Data Terminal Ready
RECEIVED DATA PIN 3: This signal is generated
by the receiving data set in response lo data signals
received from transmitting terminal via the trans
mitting data set. "Received Data" will be held in the
binary one (marking) condition at all times when
"Received Line Signal Detector" is in the OFF
condition. This is called clamping the line.
On a half-duplex channel, "Received Data"
signal will be held in the binary one (marking)
condition when "Request To Send" is in the ON
condition and for a brief interval following the ON
to OFF transition of "Request To Send'' signal to
allow for the completion of transmission and the
decay of line reflections. This is called squelch.
REQUEST TO SEND PIN 4: This signal is used
to condition the data set for data transmission. On
simplex channels or duplex channels, the ON
condition maintains the data set in the transmit
mode. The OFF condition maintains the data sei in
a non-transmit mode.
On a half-duplex channel, the ON condition
maintains the data set in the transmit mode and in
hibits the received mode. The OFF condition main
tains the data set in the receive mode.
A transition from OFF to On instructs the data
set to enter the transmit state which turns on the
carrier. The data set responds by taking such action
as may be necessary and indicates completion of
such actions by turning ON "Clear To Send,"
thereby indicating to the terminal that data may be
transferred on the interchange signal "Transmitted
Data."