Specifications

6.3.1 Circuit Description
The heart of the DAI-2 is a Motorola MC68HC711 microcontroller. It contains the CPU, program ROM and RAM,
timers, parallel I/O and an A-D converter. The CD22202 (U7) DTMF decoder, the ULN2803 (U8) octal open-collector
driver, the MAX7624 multiplying converter, and the status inputs connect directly to it. The microprocessor operates
off a 3.579 MHz crystal oscillator and a portion of this signal is used to drive the DTMF decoder as well.
Figure 6.1; DAI-2 block diagram
D5 is a 15 volt, 5 watt zener which protects the DAI-2 from over-voltage and reverse-polarity on the power supply
input. U9 is a standard 5 volt regulator. U-1 is a "low voltage interrupt" (LVI) device which resets the microprocessor
if its supply voltage falls below 4.5 volts.
U5b and U5c comprise the balanced audio output section. U5a serves in the balanced audio input section. U4 is a
multiplying D-A converter which is configured as a programmable gain amplifier (PGA). U5d buffers the output of the
PGA. This drives the level detection circuit and a third order low pass filter using U2c. The DC output of the level
detection circuit is fed to the analog-to-digital (A-D) converter in the microprocessor. U3 is a CMOS switch with four
single-pole-single-throw sections and is used to route audio. U2a and U2d comprise a hybrid circuit that lowers the
amount of "send" audio arriving at the DTMF detector.
OC1 detects ring and hang-up signals. A hang-up is detected by a step voltage change on the telephone line caused
by a momentary "battery" interruption (loop break). Capacitors C2 and C3, and inductors L1 and L2 form a low pass
filter to attenuate radio frequency interference (RFI). SP1 is a three terminal gas surge protector.
DAI-2 Troubleshooting and Repair page 6. 5