GAI-Tronics IDR1000A DC Remote User and Installation Manual - Manual (Page 23)

19 02/12
Theory of Operation
Digital Circuits
On power-up, microcontroller U1 reads switch SWA through tri-state buffer U3. Switch readings
determine certain operating characteristics of the desk set.
Analog-to-Digital Conversions
Each keyboard button is associated with a particular voltage level at microcontroller inputs AN0 and
AN1. These inputs are periodically scanned to determine whether a key is being pressed or released.
The microcontroller determines whether the handset is on-hook, off-hook, or whether the handset PTT
bar is being pressed by reading the voltage level at input AN2.
Serial Peripheral Interface
The microcontroller uses a synchronous serial interface to control LEDs, provide audio control logic, and
to interface non-volatile memory U2. LEDs are turned on or off by the outputs of serial-to-parallel latch
U6. The microcontroller manages audio circuits through the outputs of serial-to-parallel latch U4.
Operating parameters such as control current magnitudes and polarities, etc. are stored in serial NVRAM
U2.
DC Current Generation
B+ is stepped up to a relatively high ac value by T2 and rectified by D21, D23 and filtered by C86 to the
level required for the line current into the wire line connected to the desk set. Q19, a power MOSFET, is
driven by U22, a pulse width modulation regulator operating at approximately 50 kHz. U22 controls the
drive to Q19 according to the feedback received from U21b, which effectively bridges the line, forming a
servo. U22 is switched by the HVON signal when control currents are demanded by the microcontroller.
When dc current is sensed on the line, the BUSY signal (output of U19c) is sensed by the microcontroller
at input AN3. Normally, when this signal is sensed and the desk set is not generating current, the
microcontroller inhibits operation of keys which would interfere with parallel transmissions.
POT2 is used to calibrate the dc control currents, its supply being referenced by zener diode D28.
The microcontroller demands specific current polarity by the DIR output of serial-to-parallel latch U20.
This signal controls an opto-isolated H-bridge (consisting of U23 and U24) which forces the required
polarity. The microcontroller selects specific current magnitudes via 6 output lines of U20.
The regulated dc current is fed to split-winding line transformer T1. C87 ensures a low impedance audio
path.