DMX - Connecting your DMX device.

Connecting your DMX device.

Most DMX devices have an “XLR” type connector, some have RJ45, but they’ll all have a D+ and a D- pin. (Note that this doesn’t mean the same as positive(+) and negative(-) voltage, it’s to do with the “differential”, in this case, if the voltage on D+ is higher than the voltage on D-, that’s a logical 1, if D- is higher than D+ it’s a logical 0.)

For a three pin XLR, pin 2 is the D+ while pin 3 is D-. We don’t need to worry about ground for transmitting data.

               

 

For my generic RS485 dongle, the connectors are unhelpfully labeled “A” and “B”. A is D+ and B is D-. To connect the RS485 dongle to the DMX device you’ll need to use a twisted pair cable. This isn’t a special cable, you can make your own (I did). If you wrap a standard wire around a post/door knob/whatever, you can then stick the loose ends in a drill, pull the cable tight and drill-baby-drill your own twisted pair cable. (drill slowly, too fast and you’ll end up not twisting the cable evenly). 

Not a very good diagram – but you get the idea – I hope. Alternatively, buy some twisted pair or use CAT5e (Ethernet).

For our set up, we connect “A” to pin 2 and “B” to pin 3. Hopefully whatever DMX device you’ve bought has enough documentation to let you know which pins/cables to connect.

If you’re connecting multiple devices, then you’ll use twisted pair between them all, and you’ll be connected daisy chaining them all. (ie, pin2 to pin2 and pin3 to pin3 on each device).

The DMX spec calls out for a 120 ohm terminating resistor. If you’re connecting to one device over a short length, then you’ll probably not need the terminating resistor. I’d advise getting a collection of resistors in the 100 ohm range, as I’ve found that a lot of issues can be solved by using a terminating resistor. You put the resistor between pins 2 and 3 in the last device in the link.