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Transmission
Request to Send
Direction
Up level
Down level
from DTE
on
off
Transmission is initiated by 'request-to-send'. When
'request-to-send' switches from off to on:
• If
the continuous-carrier option is not selected (see
"Strapping Options"), 'request to send' causes the
transmitter to send the sync pattern on the telephone
line, then to raise 'ready for sending' (see Figure 19).
• If
the continuous-carrier option is selected, 'request to
send' causes the transmitter to:
a. Send the sync pattern on the telephone line after the
first equalization period, then to raise 'ready for
sending'.
b. Raise directly 'ready for sending' at all other times.
On switched network operation, the receiver remains
clamped for an additional 125 ms delay (echo clamp delay),
after 'request to send' goes off.
Ready for Sending (Clear to Send)
Direction
Up level
Down level
from modem
on
off
'Ready for sending' on informs the DTE that the modem is
ready to transmit data. 'Ready for sending' turns on:
• After 35 ms each time that 'request to send' turns on for
a point-to-point or a multipoint control or tributary
station.
If
the continuous carrier option is selected for a
point-to-point or control station, the 35 ms delay for
'ready for sending' applies only at the beginning of
transmission; thereafter, 'ready for sending' rises with
'request to send'.
• 160 ms after 'request to send' turns on for a switched
network station (see Figure 19).
'Ready for sending' turns off after 'request to send' goes
off; all data remaining in the modem is, however,
transmitted to the telephone line.
Note: If a re-equalization is initiated while 'request to send'
is on, 'ready for sending':
• Remains on at a control modem during the equalization.
• Drops at a tributary modem during the equalization.
• Remains on or drops, according to the strapping option
selected,
at
a point-to-point modem during the
equalization.
42
3874 User's Guide
Transmitter Signal Element Timing
Direction
Up level
Down level
from modem
on
off
'Transmitter signal element timing' signal serves the DTE as
a clock for the transmitted data. This signal, which is a
square wave with a frequency of 4800 Hz or 2400 Hz (for
4800 bps or 2400 bps), is generated when the modem has
power on.
Transmitted Data
Direction
Up level
Down level
from DTE
space (logical 0)
mark (logical
1)
The DTE uses the 'transmitted data' line to send data to the
modem for transmission over the telephone line. The
modem samples the 'transmitted data' line during the on to
off transition of the 'transmitter signal element timing'
(Figure 21 ).
The modem ignores any spurious signals that may appear
on the 'transmitted data' line during the delay between
'request to send' coming on and 'ready for sending' coming
on.
Sampling Times
I
+ +
+
i
On -
Transmitter Signal Element Timing Off _
__f1__F1__fl__f
I
I
I
I
I
I
I
I
I
I
I
I
:::·:::t•d
~
j
I
l l
{
4800 bps: 209 µs
Nominal Bit Time
2400 bps: 418 µs
Notes:
1.
The timing of 'transmitted data' presented to the modem must
be such that the time interval between any transmitted data
transition and the sampling times of 'transmitter signal element
timing' is not less than 25% of the nominal bit time.
2.
The frequency tolerance of 'transmitter signal element timing'
is ±0.01 %.
3. The maximum asymmetric jitter of 'transmitter signal element
timing' is ±1%.
Figure 21. Transmitted data sampling
[11666]
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