DMX Start Code - a cautionary tale for casual DMX users

RFH

Member
My work is all volunteer, I'm a poser, and I'm getting old. The offspring who used to help have moved away. To keep the hobby going I had to tighten up the efficiency of setup and strike. A lot of time was devoted to cabling DMX. The venues I'm in have sturdy suspended ceilings so it is easy (but time consuming) to pin cables up with "barnacles"... you know, those two piece hooks that lock onto T tracks with impressive tenacity.
With all these considerations roiling in the back of my mind I contacted Control Booth member Jim@RC4Wireless. After some soul searching, it was a simple purchase: 4x RC4M-900SX.
While rehearsing in my basement "lab" prior to my first RC4 show, everything tested fine using my Elation SDC 12 dimmer as the DMX source. BUT when I changed the DMX source to my trusty Durand Interstellar 4201-B DMX controller , everything stopped. None of my shoebox dimmers were seeing DMX from the RC4 receivers. Keeping the 4201-B as the DMX source, I replaced the RC4 units with cables... Everything ran perfectly over cables.
I contacted Jim at RC4, and he provided customer service of the highest caliber. He figured out that I had inadvertently programmed my 4201-B controller to send a start code of 1 instead of 0. He went on to explain that my shoebox dimmers ignore start codes, so they ran fine with start code 1 over cable. But the RC4 gear knows better than to propagate a message with a non-zero start code, so the shoebox dimmers were properly denied DMX. Truth is, this old poser didn't even know there was such a thing as a start code!
So if your a dabbler like me, beware of start codes. If you're using a programmable DMX controller, keep the start code set at 0 to stay out of trouble. I recommend looking up start codes, its fascinating to see who reserved them and why.
 
I guess with the link being one direction there may be little point forwarding packets where a response is required. I'm no RDM expert, but dont most messages elicit a response?

That is the point of RDM. Non zero start codes are supposed to indicate non level information. Not all of that requires return responses.

RDM compatible gear is more complex and expensive. Such products have a place, but it's a rapidly shrinking place.
 
That is the point of RDM. Non zero start codes are supposed to indicate non level information. Not all of that requires return responses.

RDM compatible gear is more complex and expensive. Such products have a place, but it's a rapidly shrinking place.
Gotcha. So some RDM messages would still work fine over a unidirectional link. I've learnt something :)

I'd be interested to know why you think RDM usage will fall, too.
 
Gotcha. So some RDM messages would still work fine over a unidirectional link. I've learnt something :)

I'd be interested to know why you think RDM usage will fall, too.
I think rather the idea is that not all non-zero start codes are RDM messages. There are other classes of messages (besides dimmer levels and RDM) that can go via DMX512: various proprietary setup/control/patch messages (which often don't need bidirectional communications), system information packets (SIPs), etc. Support for these sorts of things, at least to the extent of having non-zero start codes for various purposes, has been a part of the specification since its inception.

RDM cannot work over a unidirectional link, practically speaking--all non-broadcast RDM messages require responses, and it's not possible to individually configure devices using broadcast messages alone unless by some bizarre luck they each come from a different manufacturer.
 
The important message in this thread is that RC4 products (and all other devices compliant with the DMX specifications) do NOT treat non-zero start-code packets as channel level data.

In our current implementation, RC4Magic systems do not wirelessly transmit non-zero start codes. The transmitter itself does respond to RDM messages (start code 0xCC) for configuration, but due to patent infringement issues we do not provide a wireless RDM link in the RC4Magic product line. (That's a whole other story: in my opinion, patenting RDM over wireless should never have passed examination, and that patent should never have been granted.)

We do, however, provide wireless RDM in our RC4 LumenDim and RC4 W-DIM product lines, provided you are using an RDM-capable transmitter. For example, if you are using an RC4 LumenDimM4 wireless dimmer with a LumenRadio TX1 transmitter, you have full wireless RDM access to configure the RC4 LumenDimM4 dimmer.

Many manufacturers, including RC4 Wireless, have registered for a "private" manufacturer-specific start code that we can use for proprietary purposes. At present, RC4 devices are not passing any of them through our wireless links. So far, with 30+ years experience, we have not had a single complaint about this -- nobody is trying to get a legitimate non-zero start code through our system. Nonetheless, it is trivial for us to "open that gate" if needed, and I am happy to do so on request.
 
The important message in this thread is that RC4 products (and all other devices compliant with the DMX specifications) do NOT treat non-zero start-code packets as channel level data.

In our current implementation, RC4Magic systems do not wirelessly transmit non-zero start codes. The transmitter itself does respond to RDM messages (start code 0xCC) for configuration, but due to patent infringement issues we do not provide a wireless RDM link in the RC4Magic product line. (That's a whole other story: in my opinion, patenting RDM over wireless should never have passed examination, and that patent should never have been granted.)

We do, however, provide wireless RDM in our RC4 LumenDim and RC4 W-DIM product lines, provided you are using an RDM-capable transmitter. For example, if you are using an RC4 LumenDimM4 wireless dimmer with a LumenRadio TX1 transmitter, you have full wireless RDM access to configure the RC4 LumenDimM4 dimmer.

Many manufacturers, including RC4 Wireless, have registered for a "private" manufacturer-specific start code that we can use for proprietary purposes. At present, RC4 devices are not passing any of them through our wireless links. So far, with 30+ years experience, we have not had a single complaint about this -- nobody is trying to get a legitimate non-zero start code through our system. Nonetheless, it is trivial for us to "open that gate" if needed, and I am happy to do so on request.

Thank you for the detailed response.

These things have a way of being more complex than it seems at first.
 

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