Letter Pwb - SAC SS-800AVS Manual

2°2°4 Keyer
The Southern Avionics Keyer is an all solid state International
Morse Code generator. The Keyer is designed to generate an identifier
of any sequence of up to three letters or numerals automatically
at a rate of approximately seven words per minute. The Keyer consists
of a Recycle PWB and Letter PWB's plus inter-connecting wiring.
The Recycle PWB generates the gated audio tone, and recycles the
ident by initiating the interrogating pulse for the first Letter
PWB approximately 3.5 seconds after the last letter has been formed.
Each Letter PWB contains a number of serially connected monostable
multivibrators connected in such a manner as to form the dots,
dashes, and spaces required to form a letter.
Each PWB provides an interrogating pulse to the following Letter
PWB socket, thus cycling the Letter PWB's in a sequence determined
by their socket positions. Where a special code is required, such
as two or more sequences of letters and then a long pause or a long
dash, etc., the keyer can be modified to supply as many letters
and/or numerals as required with any timing desired.
2.2.4.1 Letter PWB
These PWB's consist of a series of monostable multivibrators, or
one-shots. Each one-shot can generate a dot or dash depending on
its R-C time constant.
For example, consider the letter "D" (_..). There are only three
characters so the first one-shot is not used. A jumper is placed
at J3 to couple the input trigger to the second one-shot. A negative
trigger signal is applied to pin 19. This signal comes from the
Recycle PWB if "D" is the first letter, or from a letter PWB
otherwise. It is a negative pulse supplied through a 10K resistor
in series with a 1 ~F capacitor. This switches Q3 off which makes
the collector voltage positive. This positive voltage is coupled
through CR2 to the logic line which goes to the Recycle PWB and
starts the dash for the letter "D". The dash will terminate after
the I~F capacitor is charged through R9, and Q3 is switched on, and
Q4 is switched off. The pause signal will remain until C3 is charged
and Q4 is switched on again. This switches Q5 off. The positive
voltage at the collector of Q5 is coupled through CR3 to the logic
line and the second character of the letter "D" (a dot) begins.
To generate a dot, RI5 is in the circuit. This causes the time
constant of the charging circuit for C4 to be smaller. When C4 is
charged through the parallel combination of RI5 and RI6, Q4 is
switched on again, Q5 is switched off, and the pause signal is
generated. When C5 is charged, Q6 is switched on, Q7 is switched
off, and the third character of the letter "D" (a dot) is begun.
SS-800AVS Southern Avionics
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