THEORY OF OPERATION
3-5
001-3282-101/102
3.5.4 DELAY PERIOD
When the RTS input of the RS-232 port goes high, a delay period tRC (On) is initiated (refer to Table 3-1).
During this delay period, a carrier is transmitted to alert the other modem that data is coming. After this delay
period expires, the Clear-To-Send (CTS) output of the RS-232 port goes high to indicate to the data equipment that
it may start sending data. Data is fed into the modem on the Transmit Data (TxD) pin of the port.
3.5.5 DATA FORMAT
NOTE: The data from the data equipment is inverted by U240. However, it is inverted again at the receiving end, so
the logic level at the RS-232 port is the same on both ends.
The data from the data equipment is converted to audio tones by U230 using FSK (Frequency-Shift-
Keying) modulation. A “Mark” tone is produced by a logic 0 (low) input to the RS-232 port, and a “Space” tone is
produced by a logic 1 (high) input. The frequency of these tones is shown in Table 2-1. When the TxD input
changes states, the tone frequency changes without any interruption in the signal (modulation is phase continuous).
The audio tones are synthesized by U230 using digital techniques.
Transmit filtering in U230 attenuates sideband energy present in the square-wave digital signal generated
by U230. This transforms into a sine wave. Transmit filtering also prevents interference with the back channel (not
used) or the other duplex channel.
3.5.6 COMPLETING DATA TRANSMISSION
When the data equipment is through sending data, it brings the RTS input of the RS-232 port back low
(Off). The TxD input is then disabled and a timing period tRC (Off) begins (refer to Table 3-1). With Bell 202
modes, soft turn-off and squelch occur during this timing period. Other modes do not utilize either function. At the
end of this period, the CTS output of the RS-232 port goes back low (Off).
The following is a brief description of soft turn-off and squelch:
Soft Turn-Off
When the RTS input of the RS-232 port goes low at the end of a message, transients may be generated
which could cause erroneous data to be received at the other modem. During soft turn-off, a 900 Hz Soft Carrier
Turn-off Tone is transmitted for a period tSTO after RTS goes low. This provides a steady “Turn Off” tone
followed by a silence at the other modem so that transients are not interpreted as data. The 900 Hz Soft Carrier
Turn-off is not available in Bell 103 mode.
Squelch
The squelch period tSQ is initiated when the RTS input of the RS-232 port goes low and if in Bell 202
mode the 900 Hz Soft Carrier Turn-off Tone is transmitted for tSTO (25 ms). During this period, the CD output is
clamped low (Off) which forces the RD output low. This disables the data receiver between the time RTS goes low
and the start of carrier detect so that transients are not demodulated.