GAI-Tronics ILD1000A Local Deskset User and Installation Manual - Manual (Page 19)

15 02/12
Theory of Operation
Digital Circuits
On power-up, microcontroller U1 reads switch SWA through tri-state buffer U2. Switch readings
determine operating characteristics of the desk set. Periodically, U1 reads the state of keyboard buttons
via its bidirectional I/O ports.
Monitor is asserted by the microcontroller through Q10 and associated components. The circuit
consisting of Q11, U3e, and associated components provides indication that the monitor function is
asserted (whether by this desk set or a parallel-connected device). This indication is used to light the
MONITOR
LED through U3d and Q2.
PTT is asserted by the microcontroller through Q12 and associated components. The circuit consisting
of Q13, U3f, and associated components provides indication that the PTT function is asserted (whether
by this desk set or a parallel-connected device). This indication is used to light the
TRANSMIT
LED
through U3c and Q1. Also, this indication is input to the microcontroller to allow tracking the status of
parallel units.
Comparator U4a outputs a logic “low” when the handset is taken off-hook. This logic signal selects
either the handset or internal microphone, and it is input to the microcontroller where it is examined it to
determine proper speaker handling.
Comparator U4b outputs a logic “high” when the handset PTT bar is depressed. This signal is ORed
with the
TRANSMIT
button to allow it to be read by the microcontroller.
Audio Circuits
Audio from the internal microphone or handset microphone is amplified by U12b and scaled to the
required level by U12a. POT4 is adjusted during installation to ensure that the level presented to the TX
audio port is correct for the associated radio. Whenever PTT is asserted or the Intercom button is
pressed, the circuit consisting of U15b, Q5, Q6, and Q7 gates microphone audio to the radio’s TX audio
port.
Radio receiver audio is input through differential amplifier U10d. Intercom and transmit audio from
parallel units is input through differential amplifier U10c. During installation, POT1 and POT2 are
adjusted to ensure that the internal intercom and receive audio levels at test point TREF are
approximately 800mV
RMS
.
Summed receiver and intercom audio is scaled through U10a and fed to power amplifier U17. It is also
gated through U15a to the handset earpiece. POT3 adjusts the handset earpiece volume. The handset
earpiece is automatically disabled through Q4 whenever the handset PTT bar or Intercom button is
pressed.