31
Digital Circuit
There are three microprocessors on the digital schematic page. U402 controls the keypad interface and also talks
to the LCD. In addition it generates the keyboard beeps to the speaker amp, it controls the speaker mute when
the handset is taken off-hook. Additionally, it controls the Line 2/Supervisor to the line relay on the analog page.
Microprocessor U401 generates the Guard Tone, function tones and signaling tones on its B0-B7 port. These
tones are fed into U405 the DAC output over to the analog page. U401 also controls PTT disable, PTT mic enable
and ear mute. Both of these microprocessors get their voltage from 5-volt regulator U601.
Microprocessor U403 controls the printer option U404 and decodes the high and low level guard tone, as well as
the function tones.
Microprocessor U407 is the real time clock chip with battery back-up.
