C.E.S. Model SDI-50 Manual, Circuit Description
event there is a voltage drop to maintain system integrity.
This function is provided by CR1, Ql, and Q3.
9.1.6 Watch Dog Timer
The parts consisting of Q2 and U5 form a missing pulse detector.
Both halves of U5 are re-triggerable one shots. Durring normal
operation, Ul constantly sends pulses to U5 to keep it from
timing out. If for any reason the first half of U5 isn't re-
triggered before it's time-out time (set by the time constant of
Cl and Rl), it will in turn trigger the second half of U5 which
will cause a reset to the microcomputer.
9.1.7 Telephone Line Circuit
The telephone line is connected directly to the back of the
Model SDI-50 via a cable equiped with RJ11-C connectors. A ring
detecter (U10) provides a square wave output at the ring voltage
frequency to Q8. This 20 to 40 Hz signal is filtered at the
gate of Q8 to próvido a single pulse for each ring to the
microcomputer.
Ull, Q9, and Q10 detect an "off hook" condition on the telephone
line. Q12, Q13, Q14 and diodos CR18 through CR21 provide an
electronic holding coil to sink off hook current. Line
transformer TI couples telephone and radio audio. This single
transformer provides 35dB of isolation as a fuli duplex hybrid.
Audio from the radio receiver is filtered by a high pass filter,
U17, that removes frequencies below 300 Hz. The level of audio
is adjusted by R60 and is de-emphasized by U13. Receive audio
is then amplified and directed into the hybrid by U16. Where
de-emphasis of RX audio is required, JMP2 is installed, which
places C31 into the feedback path of U13.
Audio from the phone line is amplified and limited by U15. The
level to the transmitter is controlled by R75 and is then
amplified and buffered by U13.
Repeat audio level is controlled by R61 and returned to the
transmitter by U13.
9.1.8 VOX Detect Circuit
U15 amplifies telephone line audio and drives a detector network
that gives a peak DC reading on the amount of audio set by R99.
This DC voltage is fed to comparator U19 which provides a logic
level output to the microcomputer.
9.1.9 Noise Circuit
The noise circuit consists of U16 and U18. Audio from the
receiver is coupled to U16, which is a high pass filter with
cutoff of 8 Khz. This filter eliminates all voice signals and
allows the noise circuit to monitor channel
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