IBM 3872 User Manual page 19

Receive
Refer to Figure 1-5.
1. The modem synchronizes on modulated carrier, raises
'data carrier detected', and presents 'received data' to
the DTE.
2. The DTE accepts 'received data' at the rate of 'received
signal element timing'.
3. When the modem no longer senses carrier, it drops 'data
carrier detected'.
Point-to-Point, Four-Wire, Duplex Sequences
Both DTEs in this configuration (Figure 1-5) hold 'request
to send' permanently on. 'Clear to send' is delayed by the
modem only when modem power is first turned on. There-
after, 'clear to send' remains up at both modems, both
modems generate and transmit carrier, and both modem
receivers remain synchronized. 'Data carrier detected' stays
on at both modems. Either DTE can send data at any time
allowed by the data system.
Transmit
1. The DTE presents serial b!nary data at the rate of
'transmit signal element timing'.
2. The modem modulates carrier to transmit data.
3. The DTE stops presenting serial data when it finishes
sending.
Receive
1. The modem presents 'received data' to the DTE.
2. The DTE accepts 'received data' at the rate of
'received signal element timing'.
Centralized Multipoint, Four-Wire, Duplex Sequences
The control station DTE in this configuration (Figure 1-6)
may hold 'request to send' permanently on. 'Clear to send'
is delayed by the control station modem only when modem
power is first turned on. 'Clear to send' remains up, and the
control station modem continuously generates and transmits
carrier.
Tributary station modem receivers are always synchronized
and have 'data carrier detected' raised to the tributary DTEs.
Tributary DTEs raise 'request to send' only when they have
been invited to transmit by the control station DTE. "Con-
trol station transmit" and "tributary station receive" are
identical to "point-to-point, duplex transmit and receive".
Tributary station transmit is identical to "point-to-point,
half~duplex
transmit". "Control station receive" is described
as follows:
1. The control station modem synchronizes on modulated
received carrier, raises 'data carrier detected' when data
arrives, and presents 'received data' to the control station
DTE.
2. The DTE accepts 'received data' at the rate of 'received
signal element timing'.
3. When all received data has been presented to the DTE and
the modem no longer senses carrier, it drops 'data carrier
detected'.
4. The control station DTE, if equipped to control the 'new
sync' line, may momentarily raise 'new sync' at this time
to reset the modem receiver synchronizing circuits
quickly, in expectation of receiving a carrier signal from
a different tributary modem.
Switched Network, Manual Call, Manual Answer Sequences
In this configuration (Figure 1-7), it is assumed that the
respective DTE operators will arrange by voice who is to
transmit first. For this discussion, assume that the calling
party transmits first.
Note: Once a dialed connection is complete and the
channel has been turned over to the modems by the
DTE operators, switched network operation is point-
to-point, half-duplex. The DTEs have full use of the
channel (via the modems) until one or both operators
disconnect.
Transmit
1. The DTE operator readies the DTE and, using the tele-
phone set attached to the DAA (data access arrangement),
dials the telephone number of the modem that is to
receive.
2. The receiving station operator answers, readies the receiv-
ing DTE, and activates the exclusion key.
3. The transmitting station operator, hearing a tone, raises
the data key on the telephone. The transmitting modem,
now connected to the channel, raises 'data set ready'.
4. Sensing 'data set ready' and not 'clear to send', the trans-
mitting DTE raises 'request to send'.
5. The modem clamps 'received data' and 'data carrier
detected', generates carrier, delays for the receiving
modem to synchronize and equalize, and raises 'clear
to send (ready for sending)'.
6. The DTE presents serial binary data to the modem at
the rate of 'transmit signal element timing'.
7. The modem modulates carrier to transmit the data.
8. The DTE drops 'request to send' when it has finished
sending.
9. When all data has been sent, the modem stops generating
carrier and drops 'clear to send'.
Receive
1. The DTE operator answers the ringing of the telephone
set attached to the DAA, readies the DTE, and activates
the data (exclusion) key on the telephone.
2. The receiving modem synchronizes and equalizes on the
received carrier, raises 'data carrier detected' when data
arrives, and presents 'received data' to the receiving DTE.
3. The DTE accepts 'received data' at the rate of 'receive
signal element timing'.
Introduction to Modems 1-9