Receiver Signal Element Timing (DD). Signals on
this circuit are used to provide the 2703 with sig-
nal-bit timing information. The transition occurs at
the center of each bit.
Transmitter Signal Element Timing (DB). Signals
on this circuit are used to provide the 2702/03 with
signal-bit timing information. The 2702/03 changes
the Transmit Data signals with transitions on this
Transmitter Signal Element Timing (DA). Signals
on this circuit are used to provide the data set with
Signal-element timing information. This signal is
on and off for equal periods of time, and a transition
from on to off indicates the center of each bit time
on the Transmit Data line.
Supervisory Transmitted Data (SA). Signals on this
circuit are used to communicate from the 2702/03
data set to the 2712 data set Simultaneously with the
normal data channel. This line signals the 2712 to
wrap test data received from the 2702/03 back to the
The 2702/03 data-set interfaces comply with the elec-
trical requirements of the RS232B as described in
All voltages are measured at the connector with
respect to Signal Ground (circuit AB). The output
line delivers between 5v and 25v into a resistance of
at least 3000 ohms. The input circuit will respond
to voltage between 3v and 25v. The polarity of the
signals depends on the following conditions:
The terminating impedance at the receiving end
of the interchange circuit must be at least 3000 ohms
but not more than 7000 ohms. The capacitance mea-
sured at the interface connector should not exceed
2500 pf. The open-circuit voltage of the input cir-
cuits should not exceed 2v of either polarity.
For the data and timing circuits, the rise and fal
time through the +3v to -3v range should not exceed
3 percent of the nominal bit time.
The following control lines, when used, are con-
sidered "failsafe" lines:
Request to Send (CA)
Data Set Ready
Data Terminal Ready (CD)
The power-off source impedance of the sending end
of the "failsafe" lines should be at least 300 ohms
measured at an applied voltage of not greater than
Automatic Calling Unit Interface
To accomplish automatic dialing, the 2702/03 con-
nects to the Western Electric Automatic Calling Unit
B01A1 or equivalent. The electrical characteristics
conform to RS232B described earlier. The interface
lines are shown in Figure 3-3 and described in the
Call Request (CR
This line is used to initiate an
automatic call origination. Call Request remains on
throughout the dialing procedure; it is turned off when
the dataset's interlock line turns on or when the 2701
terminates the call attempt.
Digit Present (DPR); This line indicates to the dialer
that the digit, as presented, is valid. Once the DPR
line is turned on, it stays on until the Present Next
Digit (PND) line goes off. When this occurs, the
DPR line is immediately turned off and is not turned
on again until the PND line has been turned on and the
next digit is valid.
Digit Leads (NB1, NB2, NB3, NB4). These leads
represent the dialing digit in true binary form to the
Automatic Calling Unit; the low-order position is
Present Next Digit (PND). This line is turned on
when the dialer is ready to accept a digit and the
DPR line is off. The PND line must remain on until
the DPR line is activated and the digit has been used
by the dialer.
Power Indicator (PWI).
This line is on as long as
power is available in the dialer. The dialer indicates
that it is inoperative when PWI is off.
Abandon Call and Retry (ACR). This line is turned
on by the Automatic Calling Unit to indicate that the
call completion was unsuccessful.
The physical connection is made through a cable and
a connector identical to those for the data sets de-
scribed in Chapter 4. The pin assignment is shown
in Figure 3-4.