Should there be any voltage present on DMX in?

[beware]

So I bought a cheap FTDI RS485 USB adaptor to see if I could make it work for DMX.

 

It kind of works but there seems to be a problem with the differential circuit - you have to wiggle the power jumper to make the DMX send.

 

While trying to figure out where the 5V power was going I discovered 1.2V coming out of the LED par I've been using to test (when not connected to anything).

 

Is this normal or a feature of a dodgy chinese LED Par? Is it causing the power issue?

 

I've been testing with lightput.

 

Pic of USB adaptor: http://www.aliexpress.com/item/free-shipping-2-pcs-lot-USB-TO-TTL-RS485-double-function-double-protect-USB-485-modules/1256240702.html

[timsabre]

Yes this is normal for any DMX device. The RS485 receiver chip has a bias on the lines so that when nothing is connected, it won't pick up noise.

 

But also check that the LED par is not in some sort of stand alone mode which would make it transmit DMX.

[beware]

Yes this is normal for any DMX device. The RS485 receiver chip has a bias on the lines so that when nothing is connected, it won't pick up noise.

 

But also check that the LED par is not in some sort of stand alone mode which would make it transmit DMX.

 

 

Excellent.

 

FYI, I did get it to work, but it requires soldering a wire from 5V to one pin of the IC (pin 3 I think). Whilst doing that, I melted the IC. Considering buying another one since they're only £6.

[DickyT]

Slight hijack - is the voltage used in the same way that MIDI uses it? i.e. to create square wave pulses?

Rich

[Brian]

Slight hijack - is the voltage used in the same way that MIDI uses it? i.e. to create square wave pulses?

That's not really how MIDI uses it.

 

MIDI is, technically, a current loop. The current either flows or doesn't to indicate the signal state.