This is just an experiment to see if a very simple USB powered circuit could be used to test long DMX cables where the other end is remote. It basically uses a PIC microcontroller to light LEDs in the passive receiver plug that indicate correct termination and then scans them all for a short time to allow cable wiggling tests to detect broken cores.
This is strictly experimental, so I’m not sure how the PIC will handle being coupled directly to long runs of cable. It does have resistors on its output pins and internal protection diodes to the supply rails, so it should be OK. (Only one way to find out.)
The tester isn’t designed to be used through the data in and out of lights, but shouldn’t harm them as its levels are similar to the 485 levels used in DMX data transmission. Some cheaper lights may actually react randomly to the test signals if they don’t check for data validity.
All the resistors are 220 ohm. This provides protection to the PIC when it is connected to a shorted cable, reduces the peak current if connected to a line with a residual charge or if connected to a signal source and also partly limits the current through the LEDs in the receiver.
The receiver also has three 220 ohm resistors to limit the current through the LEDs if the receiver is plugged into an active network.
The transmitter contains a programmed PIC12F635 microcontroller, an optional (but recommended) 100nF 50V decoupling capacitor across it’s power supply pins and three or five 220 ohm resistors between its output pins and the socket connections. Power is supplied by a hacked USB lead. The pins are connected to the socket via a 220 ohm resistor as follows.
channel 1 = pin 7
channel 2 = pin 6
channel 3 = pin 5
channel 4 = pin 3
channel 5 = pin 2
+5V goes to pins 1 and 4
0v goes to pin 8
The receiver has an identical circuit connected to each pin. A 220 ohm resistor and then an LED and generic 1N4148 diode in inverse parallel with all the LED cathodes (and 1N4148 anodes) commoned.
I recommend the use of high output red LEDs as the older red technology is more rugged and also has a slightly lower forward voltage.
You can find the 3 and 5 channel hex files for use with the PICkit2/3 programmers at:-
plus also the assembler files so you can play with the code.
If you enjoy these videos you can help support the channel with a dollar for coffee, cookies and random gadgets for disassembly at:-
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