DMX512 to Analog Voltage Converter (Version 1.12, December 31, 2001)

Contents
    1. General Description
    2. PIC Microcontroller
    3. RS-485 Differential Receiver
    4. BCD Input Switches
    5. Status LEDs
    6. Relay Outputs/Option Switch
    7. Digital to Analog Converters
    8. DAC Reference Voltage
    9. Channel Output Buffers


1.  General Description
    1.1  The circuit presented here implements a DMX512 to analog voltage
         converter.  


2.  PIC Microcontroller
    2.1  The heart of this DMX receiver is a Microchip PIC16F876
         microcontroller.  The 16F876 was chosen for primarily two reasons.
         One, it has a built in UART; and two, the UART is capable of
         receiving 250 kbps with no speed error (@ 20 MHz).


3.  RS-485 Differential Receiver
    3.1  This RS-485 transceiver (75176B) provides the necessary conversion
         from the RS-485 voltage specification to a standard TTL level.  This
         signal is connected to the PICs UART receive data input (pin 18).


4.  BCD Input Switches
    4.1  BCD Switch Decoding

              PIC
             Output         BCD Switch Common
         ------------------------------------
         Port B, pin 6      Start Address Hundreds
         Port B, pin 5      Start Address Tens
         Port B, pin 4      Start Address Ones

         BCD Switch          PIC
           Input            Input
         ----------------------------
          Binary 1      Port B, pin 0
          Binary 2      Port B, pin 1
          Binary 4      Port B, pin 2
          Binary 8      Port B, pin 3

    4.2  DMX Start Address Switches
         4.2.1  These three BCD switches provide input for setting the
                starting DMX address.


5.  Status LEDs
    5.1  Data Received LED
         5.1.1  This signal is provided by output RB7 (pin 28).
         5.1.2  This LED illuminates anytime a DMX signal is being received.
    5.2  Hardware Overrun LED
         5.2.1  This signal is provided by output RC4 (pin 15).
         5.2.2  This LED will illuminate if data is being received faster than
                the hardware UART is configured to handle.  During normal
                program execution this LED should never illuminate.
    5.3  Software Overrun LED
         5.3.1  This signal is provided by output RC5 (pin 16).
         5.3.2  This LED will illuminate if data is being received faster than
                the interrupt handler/main program loop can handle.  During
                normal program execution this LED should never illuminate.


6.  Relay Outputs/Option Switch
    6.1  Relay Outputs
         6.1.1  These signals are provided by outputs RC0 and RC1 (pins 11,12).
         6.1.2  These outputs are for future use.  At the present time, they
                provide a logic high when a DMX signal is being received,
                logic low otherwise.
    6.2  Option Switch
         6.2.1  This signal is present on input RC2 (pin 13).
         6.2.2  This switch is read upon reset of the PIC.  If this signal is
                at logic high, then the data received in the DMX data stream
                is sent to the DACs "as is".  This results in a linear
                relationship between the DAC output voltages and DMX dimmer
                values.  If this signal is at logic low, then the data
                received is complemented before it is sent to the DACs.  This
                results in an inverse relationship between the DAC output
                voltages and DMX dimmer values.


7.  Digital to Analog Converters
    7.1  The digital to analog conversion is provided by a pair of Analog
         Devices DAC8800s.  Each DAC8800 provides eight outputs, thus
         bringing the total number of channels to sixteen.
    7.2  DAC/PIC Interfacing
         7.2.1  Clear
                7.2.1.1  The clear pins (pin 12) of both DAC8800s are tied
                         together and are driven by output RA0 (pin 2) of the
                         PIC.
         7.2.2  Clock
                7.2.2.1  The clock pins (pin 9) of both DAC8800s are tied
                         together and are driven by output RA1 (pin 3) of the
                         PIC.
         7.2.3  SDI
                7.2.3.1  The serial data in pins (pin 8) of both DAC8800s are
                         tied together and are driven by output RA2 (pin 4)
                         of the PIC.
         7.2.4  Load
                7.2.4.1  The load pin (pin 13) of the first DAC8800
                         (channels 1-8) is driven by ouput RA5 (pin 7) of the
                         PIC.
                7.2.4.2  The load pin (pin 13) of the second DAC8800
                         (channels 9-16) is driven by ouput RA3 (pin 5) of the
                         PIC.


8.  DAC Reference Voltage
    8.1  The voltage reference provided to the DAC8800s is obtained from a
         resistive voltage divider and buffered by an Analog Devices OP284
         operational amplifier.  The same voltage reference is provided to
         both DAC8800s.

         Use of other op amps (less expensive) is possible.  Use of these was
         due to samples on hand.


9.  Channel Output Buffers
    9.1  The outputs from each DAC channel are routed through an Analog
         Devices OP484 operational amplifier.  This op amp buffers the DAC
         output and provides the DAC with a high impedance load.

         Use of other op amps (less expensive) is possible.  Use of these was
         due to samples on hand.


Copyright (c) 2001 by Kelly J. Kohls
All rights reserved