485/DMX Problem at Power Up

**ebrady** · 05-16-2012, 11:19 PM

Originally Posted by David_AVD

Might be a case of the cheap 3 channel controller seeing part of the DMX frame and going into channel setting mode?

Looks like this may be the case. I connected 3 other controllers to the splitter board, and they all worked fine at powerup, only the cheap 3 channel controller has problems. I did not verify if it was reprogramming the start address as my test pattern broadcasts to all 512 channels.

**mschell** · 05-21-2012, 02:38 PM

You shouldn't need a common ground between systems. In fact, many times the ground of the DMX cable is lifted on one end so as to not have a group loop in theaters. The beauty of RS485 is that it's a differential signalling method, and the receiver is looking at the difference between A and B, not necessarily relative to earth ground.

If the cheap 3 channel controller isn't using a standard interface chip, such as the ones used on a number of the DIY controllers, then it may not be following the spec as it should...

**ebrady** · 05-21-2012, 03:24 PM

Originally Posted by mschell

You shouldn't need a common ground between systems. In fact, many times the ground of the DMX cable is lifted on one end so as to not have a group loop in theaters. The beauty of RS485 is that it's a differential signalling method, and the receiver is looking at the difference between A and B, not necessarily relative to earth ground.

If the cheap 3 channel controller isn't using a standard interface chip, such as the ones used on a number of the DIY controllers, then it may not be following the spec as it should...

I have come to the conclusion that the 3 channel DMX controller is the culprit and not this design. I have not been able to repeat the problem on any of my other control boards. I am considering putting a resistor / cap on the 485 tx enable to cause a slight delay between power-up and transmit to see if the 3 channel boards will work better.

**CaptKirk** · 05-21-2012, 05:10 PM

Where did you get the cheap 3 channel DMX controller? Sounds like one to avoid!

**David_AVD** · 05-21-2012, 05:30 PM

Originally Posted by mschell

You shouldn't need a common ground between systems. In fact, many times the ground of the DMX cable is lifted on one end so as to not have a group loop in theaters.

The common reference is part of the spec and always has been. If you need to left it to fix a ground loop, then one or both devices is not properly designed.