User guide

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41
Circuit
Operation
The AL4007 and 74LS138 provide chip selects and address outputs,
used to address a single demultiplex channel from the 72 available.
The analogue multiplex output is then routed through that channel to
the sample and hold circuit.
The application drawings which follow show this process using 4051, 8
channel de-multiplexors and TL084 op-amps. The TL084 has been
chosen in order to provide a very high input impedance for the
sample and hold circuit.
It should be noted that the analogue multiplex bus and the 4051's
are all run at 5V supply in order to avoid converting the address and
chip select signals to CMOS voltages.
The final conversion to a 10V output is produced by using a gain stage
in the output amplifier.
DMX512
Base
Address
The BCD inputs are used to set the DMX base address. The inputs
are set up for connection of three standard Binary Coded Decimal
Thumbwheel switches. The BCD inputs are active low, i.e. to select a
base address of 001, connect UNIT1 to ground and pull up all other
BCD inputs. Valid addresses range from 001 to 512.
Test Mode
The BCD inputs are also used to enter test mode.
When Test mode is active, the DMX512 input is disabled.
When the Hundred inputs are set to either ‘6’ or ‘7’ Test mode is
active. The Hundred input sets the intensity, while the Ten and Units
inputs select the channel.
The following examples show the operation:
Hundreds Tens Units Output DMX
Input
0 0 0 All Off Off
0 0 1 Controlled by inputs On
5 1 2 Controlled by inputs On
7 0 1 Output 1 @ 100% Off
6 0 1 Output 1 @ 50% Off
7 6 9 Output 69 @ 100% Off
6 1 2 Output 12 @ 50% Off