Analog Voltage to DMX512 Converter (Version 1.00, February 29, 2000)

Contents
    1. General Description
    2. PIC Microcontroller
    3. RS-485 Differential Transmitter
    4. BCD Input Switches
    5. Status LED
    6. Option Switch
    7. Channel Input Buffers
    8. Oscilloscope Trigger Output


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

2.  PIC Microcontroller
    2.1  The heart of this DMX transmitter 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 transmitting 250 kbps with no speed error
         (@ 20 MHz).


3.  RS-485 Differential Transmitter
    3.1  This RS-485 transceiver (75176B) provides the necessary conversion
         from the standard TTL level to the RS-485 voltage specification.  This
         signal is connected to the PICs UART transmit data output (Port C, pin 6).


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 LED
    5.1  Channel 1 Full Scale
         5.1.1  This signal is provided by output port C, pin 3.
         5.1.2  This LED illuminates anytime the voltage on input channel 1 is at full
                scale (5 volts).


6.  Option Switch
    6.1  Option Switch
         6.1.1  This signal is present on input port B, pin 7.
         6.1.2  This switch is read upon reset of the PIC.  If this signal is
                at logic high, then the data transmitted in the DMX data stream
                is sent to the UART "as is".  This results in a linear
                relationship between the channel input voltages and DMX dimmer
                values.  If this signal is at logic low, then the data transmitted 
                is complemented before it is sent to the UART.  This results in an 
                inverse relationship between the channel input voltages and DMX dimmer 
                values.


7.  Channel Input Buffers
    7.1  The inputs to each PIC ADC channel are routed through a LM324 operational 
         amplifier.  This op amp buffers the input and provides the ADC with a low 
         impedance load.  Note, the analog input range is 0 - 5 VDC.  Do not exceed the
         upper limit of 5V, as damage to the PIC could result.


8.  Oscilloscope Trigger Output
    8.1  This signal is present on output port C, pin 0.
    8.1  This output is provided so the DMX data stream may be viewed on an 
         oscilloscope.  A pulse of approximately 200 nS is output just prior to the
         break signal being generated.  This allows for the viewing of the break and
         following data.


Copyright (c) 2000 by Kelly J. Kohls
All rights reserved