DMX robust - RDM not?

[Milton Davis]

Hello Steve,

The DFD 4 pin terminators are built by request. The design of the Wybron products is such that you are supposed to cable out to the first scroller, daisy chain to the nex, etc. until you get to the last one. The last open connector on the last scroller is supposed to cable back to the power supply. This serves two purposes:
1. The additional cable serves as "doubling up" of the power wires so voltage drop is better controlled.
2. There is a data terminator in the power supply so the act of connecting this loop also terminates the signal.

On those occasions when it is not practical to run the final cable from the last scroller to the power supply, a terminator can be plugged into the end of the line. While this addresses the termination issue, it leave the possability for low voltage particularly at the end of the string of scrollers. Also, just for reference, it is a 2 Watt, 120 ohm resistor connected between pins 2 and 3. We use a 2W part because a 1/4 or 1/2 Watt part might be destroyed in the event of certain connector mis-wiring configuration.

I have not gotten a good sense of the issues prompting this thread, but is is possible that the real cause of problems is one or more scrollers resetting due to a power "brown out"? Given that there is no return cable, the last units may be getting voltage "starved".

Regards,
Milton Davis, Engineer
Doug Fleenor Design, Inc.

[svanciel]

Hello Milton,

I'm going to have to go back and re-read the Wybron equipment manuals (yes, I actually read them before) to hunt for the loop back wiring scheme you mentioned. This does not sound at all familiar from prior reading, discussions with Wybron reps, nor a good deal of prior scroller experience.
What I do recall is a through discussion of the "head-feet" calculations that are required in order to manage the line loss issues for power.

I understand how a loop back could be helpful to the power side of the equation, but don't recall ever, or anywhere, seeing such a recommendation for a data line, DMX or otherwise. Should I go back and re-read Recommended Practice for DMX512 as well?

The discussion basically started when I was trying to track down the cause of one set of three scrollers constantly being reported in InfoTrace as being Dropped, yet they continued to function correctly in cues. What I discovered was a NEMA wet-location rated electrical box that was full of water. The box contained a 4-pin Neutrix output jack with an ETC printed circuit board soldered to the terminals with a Euro-connector to mate up with the cable (in conduit) from the power supply. There was a lot of corrosion at this junction. Replacing the whole jack resolved the issue. My conclusion was that DMX could blast through alright but RDM suffered. Scott B essentially confirmed this.

We have other RDM error message issues. One of the suggestions from Wybron reps was to install the terminators on the scroller runs. The previous "dumb" scrollers that were in place for 8 years had worked quite well without them. We have fourteen separate runs to groups of three and four scrollers. The distances are within the head-feet requirements but it would be a major project to attempt rewiring each of these runs with a second cable to the dimmer room where the power supplies are housed. The terminators seem to have helped cut down on spurious error messages. My next project is to check and rework all of the output junctions with a more water tight junction to see if that eliminates more of the reported errors such as device dropped, initialization failure, and/or RFID read errors.

One nice feature of the Aquaram IT scrollers is a report back of the voltage and motor current values. As I recall, these readings indicate that power line loss is not a major issue but it is something I will double check.

Hope you have a nice Memorial Day weekend.
SteveV

[Milton Davis]

Hello Steve,

On the return path with termination built into the power supply, perhaps I am thinking of the Chroma Q rather than the Wybron stuff. I thought the Wybron equipment followed the same practice, but I don't have it in front of me and you do.

If the scrollers are simply daisy chained with no return path or built-in termination, the 4 pin terminators are definitely a good idea. This would relate to the requirements of EIA-485 as much as DMX512.

Water + RDM = poor performance. Water + DMX512 = poor performance masked over by lots of redundant updates of the information. Is DMX more robust? Technically, no, you just can't detect the problems as easily giving the perception that the DMX signal is error free.

Regards,
Milton