I am using a SN75176AP as a DMX receiver for the arduino. It seems to work really well when plugged in, but when the DMX is not plugged in, the DMX received values change randomly as the input must be floating.
I'm not really sure what to do about it. Adding a pull-up to the output? I don't know what the arduino would think if it is reading high all the time there is no input. Adding series resistors to the differential inputs maybe?
Not really sure since I don't know a great deal about differential signals.
Adding series resistors to the differential inputs maybe?
Right... The open inputs are the problem, not the outputs. Since it's differential, one input needs to be pulled high and the other low. (The two lines are always opposite.)
[u]Wikipedia[/u] shows a typical RS-485 bias network. (DMX hardware connections are 485 compatible.)
So since this would just be another item in a DMX chain I'm pretty sure I don't want to add a 120 ohm terminating resistor, since we're not actually terminating the chain at the device.
My DMX circuit was previously like the schematic below:
Now I have 680 ohm resistors between the Vcc and pin 7 of the 75176, and pin 6 and GND. That works with the floating input problem when the device in unplugged. But now when it's plugged in it does not read the DMX values properly.
Does that suggest adding the 120 ohm resistor between pins 6 and 7 of the 75176 differential receiver. Would that create a problem further down the DMX chain? I'd just like it to hold the DMX values when the device is not plugged in at all...
I'll try and see if it works but I don't have a whole chain of devices to test it on....
If its not the end of the chain then you don't terminate. The one at the end will terminate and
bias the line so you don't see noise. However you can add some high value
resistors to bias the line too, for when nothing's plugged in.
You'll find lots of pages out there about this. Read the datasheet for your chip too.
