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modem to synchronize. Following a change of half-duplex
transmitting direction, time must also elapse for data to
clear from the channel and from the modem. 'Clear to
send' stays up (on) until the DTE has turned off 'request
to send' and until all data accepted by the transmitting
modem has been transmitted.
• 'Data carrier detected' - The modem raises (turns on) this
line to signal to the DTE to monitor the 'received data'
line. The 3872 turns on this line when the modem is
fully synchronized, and clamps off 'received data' in the
interim.
• 'Data signal rate selector' - When the 3872 Modem Mode
switch is set to EXTERNAL, the modem's ability to trans-
mit and receive at half-speed is controlled by this line
from the DTE interface. An up (on) level causes full-
speed (2400 bps) operation; a down (off) level causes
half-speed ( 1200 bps) operation. Other positions of the
mode switch provide manual control of speed selection.
(See "Switches and Indicators" in Part 2.)
• 'New sync' (not standardized by EIA or CCITT) -The
control station DTE (Figure 1-6) controls this line to the
control station modem. In a centralized multipoint du-
plex configuration, 'new sync' can reduce the interval re-
quired between the end of transmission by one tributary
modem and the start of transmission by another tributary
modem. As the control station DTE transmits an invita-
tion to the next tributary station to transmit, it momen-
tarily raises (turns on) the 'new sync' line to its modem.
This causes the control station modem receiver to reset
all receiver synchronization controls. A shorter 'clear to
send' delay can then be used at the tributary station, and
the control station modem receiver will be waiting to
synchronize.
• 'Test' (not standarized by CCITT) - This line from the
DTE interface controls the local test loop function of the
modem (see "Modem Test Concepts," in Part 1). When
this line is up (on), the modem transmit signal (Figure 1-4)
is attenuated (reduced in power) and connected to the
input of the receiver. This local loop can also be controlled
by the 3872 Test Operate switch for manual testing.
• 'Signal ground' and 'protective ground' - These lines con-
nect the electronic ground references and the frames,
respectively, of the DTE and the modem. The circuitry
must be connected to provide a reference in both devices
for up (on) and down (off) levels of the other DTE inter-
face lines. Local restrictions may prohibit the use of the
'protective ground' line. In such case, grounding is accom-
plished through the ground conductor in the power cords
of the respective devices. Power cord ground wires should
be connected together through earth or building ground.
• 'Ring indicator (Calling indicator)' - A modem in a switched
network configuration raises (turns on) 'ring indicator' to
the DTE during the time the ringing signal is being received
from the DAA. See "Special Feature Descriptions" in
Part 3.
• 'Data terminal ready' - The DTE raises (turns on) this line
to a modem in a switched network configuration to allow
the modem to be connected to the communications chan-
nel. If the modem is equipped for automatic answering or
automatic call origination (see "Special Feature Descrip-
tions", following) 'data terminal ready' must be up (on)
to answer a call or to automatically originate a call. With
either feature, the DTE drops (turns off) 'data terminal
ready' to disconnect the channel.
• '(Select standby)'(not standardized by EIA) -The DTE
raises (turns on) this line to the IBM 3872 modem in a
dedicated channel configuration. (The 3872 Modem
requires the Switched Network Backup feature. See
"Special Feature Descriptions," following.) When 'select
standby' is up (on), the modem disconnects from the
dedicated channel and connects to the switched network
access (DAA in the USA and Canada).
It
is then necessary
to complete a switched network call before data can be
transferred. Modem operation can also be manually trans-
ferred to the switched network by the 3872 Mode switch
(see "Switches and Indicators" in Part 2).
3872-DTE Interface
The digital (binary) interface between the DTE and the IBM
3872 Modem conforms to the specifications of EIA RS-232-C
and RS-334 and of CCITT Recommendation V.24, with the
considerations described in the following text. Nonstandard-
ized interface lines provided to the DTE interface by the
3872 are also described.
In
this section of the manual, the
following illustrations show the DTE interface:
Figure 3-2 shows the EIA and CCITT designations and
names of interface lines, together with the standard pin
number of each line.
Figure 3-3 shows the standard interface connector, with
pin locations and numbers, and the line that connects to
each pin.
Figure 3-4 shows the interface connections for both the
3872-DTE interface and the 3872-DAA interface. The
ACO (automatic call origination) interface has a physical
arrangement corresponding to that shown in Figure 3-3;
the line names and functions are different. (See "Special
Feature Descriptions".)
Figure 3-5 shows interface timings.
USING MACHINE CONSIDERATIONS
The 3872 provides bit synchronization; the using DTE must
frame characters. Other considerations are discussed in the
following paragraphs.
New Sync Line
Use of the 'new sync' interface line by a multipoint control
DTE permits the shortest clear-to-send delay (8.Sms) at the
Modem Environment
3-3
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