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10.6.0 MODEM CONTROL LINES
The modem control lines (see illustration) are
active during the up level. The following para-
graphs describe eight of these lines:
'RTS' (request to send) controls the transmit
and receive functions of the modem. When
on, the modem is in transmit mode; when off,
it is in receive mode. On a 2·wire leased line,
'RTS' is switched by the System/32. On a
4-wire leased line, 'RTS' can be (1) switched
by the System/32, (2) held permanently on
by the System/32, or (3) held permanently
on by the modem. On a switched line,
'RTS' is switched by the System/32.
'RTS' is on for the duration of each block
or frame transmitted.
'Standby' selects the switched network backup
line.
'Test' selects the modem wrap function. When
the modem wrap test is running, the transmitter
is looped back to the receiver.
'OTR' (data terminal ready) signals the modem
that the terminal is ready to send or receive
data. On a switched line, 'OTR' initiates and
holds the line connection. On World Trade
'COSTL' (connect data set to line) modems,
'OTR' is down initially, and is brought up
when a ringing signal is received in auto-
answer mode or when 'OSR' is brought up
in manual mode.
'Baud rate' selects the bit rate on modems
with the half-speed option. When this line
is down, the half-speed option is selected.
'CTS' (clear to send) indicates that the modem
is ready to transmit data. On a 4-wire leased
line, 'CTS' can be permanently wired on.
On a 2-wire leased line, 'CTS' is turned on
and off by 'RTS'.
'OSR' (data set ready) indicates that the modem
is ready. The modem is ready when power is on,
when the modem is connected to the line, and
when the modem is not in test mode; all three
conditions must be satisfied.
'Ring' indicates that the modem is receiving
a call.
Adapter
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Test
18
Baud rate
23
OTR (data terminal ready)
20
RTS (request to send)
4
Transmit data
2
Standby
11
Receive data
3
CTS (clear to send)
5
Receive timing
17
OS R (data set ready)
6
Transmit timing
15
Ring
22
Frame ground
1
Signal ground
7
Pin numbers of
t-
the RS232/CCITT
---1
connector
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Modem
Data Communications
10-41
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