GAI-Tronics ITA2000A ITA2000A Series Tone Remote Adapter with Scanning User and Installation Manual - Manual (Page 40)

Theory of Operation ITA2000A Series Tone Remote Adapter
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Accessory Transmit
Accessory TX audio input J3-12 expects a nominal input level of 80 mV ac. U18 buffers the signal and
U19 routes the signal to the radio TX and the telephone line output based on the state of the accessory
PTT J3-6. For radio TX, the audio is routed to amplifier U16, then output to the radio TX. For
telephone line output, the audio is routed to amplifier U23, then output to the telephone line connection.
Desk Mic Transmit
Input at J4-5, TX audio from the desk mic jack is controlled with U19 based on the state of J4-6 (desk
mic PTT input) and routed to radio TX output, telephone line output, and the accessory RX output.
For radio TX, the audio is routed to amplifier U16, then output to the radio TX. For telephone line
output, the audio is routed to amplifier U23, then output to the telephone line connection. For accessory
RX output, the audio is routed to amplifier U27, then output to the accessory RX output.
Reset Circuit
An internal watchdog circuit consisting of U3, Q1, C5, and associated components provides reset to the
DSP. In normal operation U1 generates an IRQ signal on pin 30 that drives Q1 keeping C5 discharged.
If the IRQ signal is not present, C5 will charge and force U3 pin 6 low. U29, low voltage monitor IC
resets U1 whenever the 5 V dc supply is out of tolerance and provides approximately 200 ms of reset
upon power up.