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RECEIVE TTY CHANNEL 2
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TM-11-5805-387-34-2 Modem Radion Teletypewriter MD-522A/RC (NSN
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RECEIVE DC LOOP
TM 11-5805-387-34-2
1-31. RECEIVE MARK-SPACE PULSE GENERATION
(fig. FO-5)
The mark-space pulse generation circuitry consists of discriminator differential amplifier A4A1Z1, dc coupled bistable
multivibrator designated normal mark A4A1Q5, Q6, and ac coupled bistable multivibrator A4A1Q7, Q8. Previously
discussed frequency discriminators A4A1Q9, Q10 and A4A2Q7, Q8 could be included in this listing also. The output of
either tty channel 1 and/or tty channel 2 is applied to discriminator differential amplifier A4A1Z1. The signal applied to
A4A1Z1 is of one polarity for a modem mark tone input, and of the other polarity for a modem space tone input; with no
input to the modem, the differential output of the frequency discriminators is 0 volt.  With respect to ground, the
frequency discriminator output is a pulsating dc. Discriminator differential amplifier A4A1Z1 converts the analog input to
a pulse (digital) output to drive dc-coupled bistable multivibrator A4A1Q5, Q6. Multivibrator A4A1Q5, Q6 serves as a
buffer, isolating A4A1Z1 from following circuitry, and converting the varying dc input to an approximation of a square
wave output.  The output of dc-coupled bistable multivibrator A4A1Q5, Q6 drives ac-coupled bistable multivibrator
A4A1Q7, Q8. The output of the frequency discriminators is balanced and floating. One side of this balanced dc signal
path has been designated to normal mark buss, the other reverse mark buss.
Operation of Mark-Space Pulse Generation Circuitry for Mark Tone input to Modem. The inputs to discriminator
differential amplifier A4A1Z1 are labeled A (normal mark buss) and B (reverse mark buss). The outputs are labeled C
and D. When A is negative with respect to B, corresponding to a normal mark tone input to the modem, the voltage of C
with respect to D rises, switching output E of dc-coupled bistable multivibrator A4A1Q5, Q6 into a low state. This in turn
switches output G of the ac-coupled bistable multivibrator into a high state, and output H into a low state. If RECEIVE
switch S2 is at NORM, a positive pulse corresponding to a normal mark is applied to the receive sensing circuitry. If S2
is set at REV, ground is applied to the receive sensing circuitry, corresponding to a reverse mark. Thus, if the mark tone
has been transmitted higher than the space tone, correction can be made at this point for proper printout on the attached
receive tty equipment.
Operation of Mark-Space Pulse Generation Circuitry for Space Tone Input to Modem. When the A input to
A4A1Z1 is positive with respect to B, corresponding to a normal space tone, the voltage of C with respect to D falls,
switching the output E of dc-coupled bistable multivibrator A4A1Q5, Q6 into a high state. This in turn switches output G
of the ac-coupled bistable multivibrator into a low state, and output H into a high state. If RECEIVE switch S2 is at
NORM a ground is applied to the receive sensing circuitry, corresponding to a normal space. If S2 is set at REV, a
positive voltage is applied to the receive sensing circuitry, corresponding to a reverse space. Thus, compensation can
be made at this point for mark-space tones which may have been transmitted in reverse relationship by properly setting
S2.
1-32. RECEIVE SENSING CIRCUIT
(fig. FO-5, fig. FO-6)
The receive sensing circuit consists of receive oscillator A5A2Q2, T2, envelope detector-doubles A5A2CR5, CR6, and
loop current switch A5A2Q1. The receive sensing circuitry isolates the previously discussed stages of the receive tty
signal path from the external dc loops, preventing interaction of grounds between equipment attached to the dc loop and
the modem. The output of the mark-space pulse generation circuitry is applied to the receive sensing circuitry through
RECEIVE switch S2. A positive pulse, corresponding to a normal mark, or a reverse space gates receive oscillator
A5A2Q2, T2 on. The output of A5A2Q2, T2 is detected by envelope detector-doubler A5A2CR5, CR6, which causes
loop current switch A5A2Q1 to conduct. When ground is applied to the receive oscillator, there is no output; therefore,
loop current switch A5A2Q1 is biased into nonconduction.
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