familiar think about the output of U3, a 4 line signal that represents the DTMF signal received and then the 16 different DTMF signals. You guessed it, U2 constantly looks at the 4 output lines from U3 and sets one of it’s 16 outputs to a low level, whenever the data valid signal is activated from U3, based on which DTMF signal was received. While this device is not as complex as U1 and U3 it is still important. This circuit functions all the time no matter how the DCI2 has been configured. This means that you can always tell which DTMF signal is received. One of the 16 output lines will be activated, set to a low level, whenever a valid signal is received.
The 4 data input lines are on pins 20, 21, 22 and 23 of U2 and the data valid signal, supplied thru Q7 to pin 19. Q7 is necessary because the data valid signal is the opposite level of what U3 provides. It simply inverts the level so a the high level from U3 for data valid is changed to a low for use by U2. Other Information When the micro-controller, U2, receives valid data from the user, it flashes LED D4 (the activity indicator) and sends a feedback tone back over the phone line to let you know the command was received. Components R19, R21, Q6, C5, D10 and D11 work together with the micro-controller to generate the tone pulse.
The pulsed tone output from U1:pin 11 is amplified from 0 to 5 VDC up to 0 to 12 VDC to ensure that enough signal will couple through to the phone line. Protection diodes D10 and D11 limit the incoming signal on the phone line (between +12.7 and - 0.7 V) so that potentially damaging voltages like the 90 Vp-p ‘Ring’ signal does not blow up Q6! A detailed description of each of the configurations is provided later in this manual The Power Supply So far we have covered most of the circuitry on the board with exception of the power supply. VR1 and the surrounding parts C3, C4 and the polarity protection diode D1 form a simple voltage regulator to supply a steady 5 VDC to the ICs on the board.
By using this scheme, we can operate the unit with a single 12 VDC (positive tip on J5) source and have access to the multiple voltages needed for the unit to operate. DCI2 6