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This chapter describes the interfaces between the
2702/03 and the communications facilities.
INTER FA C£ LINE DESCRIPTIONS
EIA RS232B Interface Description
The line designations of the interface with the com-
mon-carrier data sets conform to EIA Recommended
Standard 232B (EIA RS232B). The interface lines
used by the 2702/03 are shown in Figures 3-1 and
3-2.
Line Descriptions
Protective Ground (Circuit AA). This conductor,
where used, is electrically bonded to the machine
frame.
Signal Ground (AB). This conductor establishes the
common ground reference for all interface lines
except circuit AA.
Transmitted Data (BA). This circuit transfers data
from the 2702/03to the data set for transmission to
the remote terminals. The 2702/03 holds circuit
BA in the Mark condition during any time interval
between characters or words or when no signals are
to be transmitted.
Received Data (BB). This circuit transfers data from
the data set to the 2702/03. Signals on this circuit
are generated by the data set in response to data
signals received from the remote terminal. The
data set holds this line in the Mark condition when the
line is idle or carrier is not detected.
Request to Send
(CA).
Signals on this circuit are
generated by the 2702/03 to condition the local data
set to transmit. The On condition is maintained
whenever the 2702/03 has information ready for
transmission on information being transmitted. The
2702/03 transmits data on circuit BA (Transmitted
Data) only when the On condition is maintained on
circuits CA, CB, and CC. In half-duplex service,
the Off condition holds the data set in the Receive
Data condition, and the On condition holds the data
set in the Transmit Data condition.
CHAPTER 3. COMMUNICATIONS INTERFACES
Clear to Send (CB). Signals on this circuit are gen-
erated by the data set to indicate that it is prepared
to transmit data. The On condition on circuit CA
(Request to Send) is delayed as long as maybe
required to establish a connection to a remote ter-
minal. When Request to Send is turned off, Clear to
Send is also turned off.
Data Set Ready (CC). Signals on this circuit are gen-
erated by local data sets to indicate that it is ready
to operate. The Off condition indicates one of the
following:
1.
An abnormal or test condition occurs that dis-
ables or impairs the normal function associ-
ated with the class of services being furnished,
2. The communications channel is switched to
alternate means of communication (e. g., alter-
nate voice telephone).
3. The local data set is not connected to a com-
munications channel (e. g., the data set is on-
hook).
The On condition appears at all other times.
Data Terminal Ready (CD). The signals on this cir-
cuit are used to control switching of the signal con-
verter to the communications channel. However,
when the station is equipped for call origination only
by a means external to this interface (e. g.
manually
or via an automatic call-originating unit), the On
condition serves only to maintain the connection
established by the external means. When the station
is equipped for automatic answering of recei ved calls,
connection to the line may be made only in response
to a ringing signal. The Off condition removes the
signal coverter from the communications channel for
such reasons as:
1.
Freeing the line for alternate use (e. g., voice
or use by other terminals).
2. Permitting use of data processing equipment
for an alternate function.
3. Terminating a call
(i.
e., going on-hook).
Data Carrier Detector (CF). Signals on this circuits
are used to indicate that the data carrier is being
recei ved. The Off condition indicates the end of the
present transmission activity or a fault condition.
Communications Interfaces
3-1
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