Local Soft Key; Hand Soft Key; Baud Rate Switch; General Rs-232-C Information - Wavetek 75 Instruction Manual

3.30.2 BUFFR Soft Key (3)
For remotely operated units the BUFFR soft key will
display the last 16 ASCll characters received by the
serial interface. Rotating the Control knob will scroll
through the last 80 ASCll characters received by the
RS-23242 interface. This feature can be used as a debug-
ging tool for remote instrument programming.
3.30.3 LOCAL Soft Key (1)
Pressing this soft key from the SETUP menu brings the
Model 75 into local control providing that the controller
has not locked out the key (with the ' * ' character).
3.30.4 HAND Soft Key (2)
Pressing the HAND soft key (2) from the SETUP menu
allows the user to change the handshake mode from
XONIXOFF toCTSlDTR or vice versa by rotating thecon-
trol knob (5).
3.30.5 Control Knob (5)
Allows the user to scroll through the ASCll characters
received over the serial interface bus when the BUFFR
soft key has been pressed. It also changes the hand-
shake from CTSIDTR toXONlXOFF when the HAND soft
key is pressed.
3.30.6 Baud Rate Switch (6)
Changing the position of each individual switch section
(and therefore the overall combination) in the Baud Rate
switch produces adifferent baud rate for the serial port.
See Section 2.2.8.
3.31 GENERAL RS-2324 INFORMATION
EIA standard RS-232-C specifies the electrical
characteristics and pin out of a serial communication
standard for connecting "data terminal equipment"
(DTE) to "data communication equipment" (DCE). Data
terminal equipment is usuallydevices such as terminals,
computers, or printers that are the final destination for
data. Data communication equipment, on the other
hand, is usually a modem or other device that converts
thedata to another form and passes it through. Because
RS-23242 signal lines defined as inputs on a DCE device
are outputs on a DTE device and vise versa, connection
of a DCE to another DCE or of a DTE to another DTE will
require a special cable with many of the signals inter-
charlged. These cables are often called "modem
eliminators" because they are used to eliminate a pair
of modems between two computers. The Model 75 can
be configured as either a DCE or DTE device, so in most
cases it can be connected with a straight through cable
to either a computer or to a modem.
The baud rate is the bit rate during the transmission of
a word in bits per second. Different devices use many
baud rates but the baud rates of the twodevices that are
connected must be the same. The Model 75 can be set
to 14 different baud rates ranging from 50 to 19,200 as
described in paragraph 2.2.8 and table 2-4.
Data signals over the RS-23242 use a voltage of
+
3 to
+
25V to represent a zero (also called a space) and a
voltage of
-
3 to
-
25V to represent a one (also called
a mark). Handshake and control lines use
+
3 to
+
25V
to indicate a true condition and
-
3 to
-
25V to indicate
a false condition.
When nodata is being transmitted, the idle state of the
data lines will be the mark state. To transmit a byte, the
transmitting device first sends a start bit which is a space
for one bit time(1 lbaud rate) tosynchronize the receiver.
Next, the data bits are sent LSB first (the Model 75 uses
8 data bits), then at least one bit time of the mark state
(stop bits) before initiating the transmission of the next
byte. Some devices use an optional parity bit between
the data bits and the stop bits. The Model 75 does not
support this and it should be suppressed on the con-
nected device (see figure 3-9).
The RS-23242 standard is not very specific about many
of the handshaking signals and it is therefore usually
necessary to refer to the manuals for both of thedevices
being connected to determine the exact pin out, signal
definition, and signal direction for the devices (see
paragraph 2.2.7, 2.2.8, and table 2-3) for the Model 75
pin outs).
Handshaking is used so that each device can tell the
other device when it is ready or not ready to receivedata.
The Model 75 supports two different types of hand-
shaking, CTSIDTR (hardware handshaking) and XONI
XOFF (see table 3-4). With hardware handshaking, the
DCE device pulls the CTS line positive or negative to in-
dicate whether it is ready or not ready respectively and
the DTE device pulls the DTR line positive or negative
to indicate whether it is ready or not ready respectively.
With XONIXOFF handshaking either device sends an
XOFF ($1 3) over the appropriate data line when it is no
longer ready to receive data, then sends an XON ($1 1)
when it is again ready.
When the Model 75's DTR (DCE configuration) or CTS
(DTE configuration) line is pulled negative, it will send
a maximum of one character before it stopstransmitting
and waits for the handshake line to return positive. When
the Model 75 receives an XOFF character it will send
a maximum of 3 characters before it stops transmitting
and waits to receive an XON character.
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