How should master handle receiving DMX after sending request?

[berntd]

Thanks everyone.

It is more of a question as to how to handle it and not as much as finding an actual cause. IU was not able to find out in the spec of what shoudl happen in such a case.

However, there are certain fixtures out there that when they loose the DMX input signal (for example if the line goes quiet, like waiting for RDM activity), they start sending DMX like packets. I am not sure what they are but our programmers have encountered them before and now that we are developing a new RDM driver, we want to be sure on how to deal with stuff like that.


Regards
Bernt

[ericthegeek]

Originally Posted by berntd

IU was not able to find out in the spec of what shoudl happen in such a case.

The standard tells you what you should do when things are working properly. For the most part though, it doesn't tell you what to do when there are errors or faults. That's up to you.

Originally Posted by berntd

However, there are certain fixtures out there that when they loose the DMX input signal (for example if the line goes quiet, like waiting for RDM activity), they start sending DMX like packets.

Yes, some poorly implemented fixtures try to automatically start a master/slave mode when there's no DMX. This lets a string of lights flash and change color in coordination with each other when there's no controller. A proper DMX implementation shouldn't start this this automatically, but there is equipment out there that does it.

Realistically, if there's a device on the '485 wire that is talking when it's not supposed to, it's not going to be polite and only talk between RDM packets. It's going to talk whenever it feels like it. This will almost certainly create contention on the 485 bus that will not make sense to any receiving UARTs. If you detect this, your best bet may be to notify the user that something is talking when it shouldn't, and give up. Then check the line every 10 seconds or so to see if the offending device has been removed.

You might get lucky, for example, if you have a strong 5V line driver, and the jabbering device has a weak 3.3V driver. Then you might be able to overdrive it. But in most cases you'll just end up with oddly flashing lights and upset users...