DMX Termination Fail

D Oz

Member
Joined
Nov 5, 2014
Location
California
My theater recently purchased a used Rosco Model 1750 Fog Machine for immediate use. I briefly tested it upon arrival, to make sure that it was responding to DMX. I then put it in its perch for the production, making sure to put a DMX terminator in (as the manual demands).

During the preshow check, however, the fogger did not respond to DMX control. After several troubleshooting steps, I removed the terminator, and the device started responding to DMX signal again.

Now, I figured the terminator might have caused the jankiness, since it's a 3-pin terminator and I had to put it on an adapter to connect to the fogger's 5-pin DMX out port. But testing continuity showed that both adapter and terminator are wired correctly. (The only wrinkle is that the terminator has a 110-ohm resistor - as opposed to the standard 120 - but it has worked fine in the past.)

I didn't have a chance to test the DMX out of the fogger, so I can't speculate on its role in this mystery.

Have you had a problem like this in the past? I'd appreciate your insight.
 

JD

Well-Known Member
Joined
Jan 1, 2005
Location
North Wales PA
You also want to make sure that the loop is not double terminated. If the end unit is the machine and there is no other termination, then it should be, but terminators can be faulty. Check with a meter to see if there is any resistance between pin 1 any other pins and the shell of the terminator. It should all be open circuit, but sometimes the resistor may be leaning against the shell.
If you are not using and opto-splitter, unplug the connector at the board. Check with the meter that you are reading somewhere around 120 ohms between pins 2 and 3. If it's closer to 60, you are double terminated. If it much higher, then you have no termination. Again, there should be a near open circuit between pin 1 and either pin 2 or 3. Qualifier: Some DMX chips have gate protection diodes that will give you a reading between pin 1 and the others, but the reading SHOULD NOT be a short. If you get a reading, reverse the meter leads. If the second reading is different, then it's gate protection and nothing to worry about.
 

cbrandt

Well-Known Member
Joined
Dec 7, 2011
Location
Michigan
Especially in older gear, or in less specification compliant gear I've seen wackiness start to show up from termination. That is a minority of the time, but it is something to be aware of. I love my portable Leprecon dimmer racks, but *never* put a terminator in their 48 channel racks. Due to the internal wiring, it plays all kinds of havoc with the system. I tend to isolate those racks with an opto.