JANDS DD-8 DMX SPLITTER
Revision 2.0 - 5 July 1998
OPERATING MANUAL
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5.0 Technical Aspects
5.1
Circuit Description
The DD-8 copies the incoming DMX-512 signal into eight identical opto-isolated
outputs. A separate power supply is provided for the input and each output to maintain
complete isolation. Refer to Figure 2.1 (over).
The input signal is received by a standard RS485 line receiver (IC1) via protection
components PTC1, PTC2, MOV1, MOV2. Note the input shield is lifted from chassis
via a parallel RC network R1, C1. R4 terminates the incoming DMX line.
The output of IC1 drives the opto-isolator LEDs. These LEDs are arranged in two series
strings of four, and are driven via 20mA current sink transistors Q2 and Q3. The use of
current sinks ensures power supply voltage fluctuations do not affect the opto drive
current.
The receiver output also drives the signal indicator LED L2 via positive edge detector
network C8, R26, R8, D3, transistor driver Q1, and pulse lengthener C5. This circuit will
respond to signals with high repetition rates, and may light in the presence of non DMX
signals.
Eight identical output circuits drive the DMX outputs. The opto-transistor and pull-up
resistor R9 etc drive the DMX line driver directly. The line driver output is protected by
PTC3, PTC4, MOV3, MOV4, etc. The output shields are tied hard to the DD-8 chassis.
The DD-8 uses a custom-wound toroidal power transformer with nine separate but
identical secondary windings. Each winding is used to provide an isolated linear power
supply for one of the output circuits or the input circuit. A polyswitch protects each
output from local overloads.
The primary of the transformer is wound with a combined 240/110V winding, and the
orientation of the mains fuse determines which winding is connected to the mains input.
Note that when the mains voltage is changed, a fuse with the correct rating must be used.
