User guide

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36
DMX512
Input
The DMX512 input connects directly to the output of a standard
RS485 or RS422 receiver such as the 75176.
Good Data
The GOOD signal is driven low to show that valid DMX512 data is
being received. The output is suitable for direct connection (via a
current limit resistor) to the cathode of a 20mA LED.
Power
Supply
The AL4006/9 requires a 5V DC supply at approximately 6mA
All Ground and Vcc pins must be connected.
The device is CMOS so normal handling precautions should be
adhered to.
Good decoupling and pcb layout conventions (i.e. fill all unused pcb
areas with grounded copper planes) should be followed.
Particular attention should be paid to routing the crystal
connections.
Crystal
The AL4006/9 requires a 16MHz, fundamental mode crystal. A
tuning capacitance of approximately 12pF is suggested between each
crystal connection and Ground. The inherent pcb and lead
capacitance must be included in this.
EMC
There are two major considerations in reducing the electromagnetic
radiation generated by the AL4006/9.
The device tends to generate noise at the third harmonic of the
crystal (48MHz). Adding the crystal capacitors detailed above
reduces this to acceptable limits.
Broad band noise can be reduced by good decoupling. A range of
22uF aluminium capacitors and 0u1 ceramic capacitors should be used
on the circuit board. A 560pF ceramic capacitor should also be
connected across the AL4005 power pins as close as possible to the
device.
The use of 150uH inductors in series with all three DMX512 signals
is also an effective way to reduce conducted emissions.
Application
The following circuit shows the AL4006/9 application drawing. This
is also the circuit diagram for the evaluation PCB.
The evaluation card provides 8 relays on board, plus an 8 channel
darlington drive output for an additional 8 relays.