groggy Posted July 25, 2011 Share Posted July 25, 2011 I have made a DMX controlled shutter, using a DMX to servo controller circuit board from cpc farnell. The circuit board has screw terminals on it for power and signal (in this order: +9v, Ground, 2 & 3.) when wiring the DMX into it should Pin 1 On the DMX connection be wired to the ground (-) ie have two wires going into it (Pin 1 from DMX and the - from the 9v PSU) at the moment the DMX just has pins 2 & 3 wired and the unit works fine but will it cause problems later on? Thanks in advance Link to comment Share on other sites More sharing options...
timmeh2 Posted July 25, 2011 Share Posted July 25, 2011 Hi The ground line is 0V from the PSU. Don't connect it to DMX ground. All the bestTimmeh Link to comment Share on other sites More sharing options...
timsabre Posted July 25, 2011 Share Posted July 25, 2011 I disagree, the power supply will not be earthed, so you should connect the DMX pin 1 to ground. Otherwise the 0V on your board will be floating with respect to your DMX controller 0V and the DMX receiver chip might not work (common mode voltage too high). If 0V from the PSU is connected to mains earth, then timmeh2 is correct. Link to comment Share on other sites More sharing options...
PaulDF Posted July 25, 2011 Share Posted July 25, 2011 Pin 1 connects to the screen on the cable to reduce interference. As a rule of thumb from working with instrumentation the cable screen should only be earthed at one end, usually at the source to eliminate earth loops. I haven't had any of my DMX kit to bits though to see if this is the case! Pins 2 and 3 carry the signal, as it is a differential signal i.e. a comparison between the two wires any interference which does get into the signal effects both the wires hence is cancelled out. Link to comment Share on other sites More sharing options...
timsabre Posted July 25, 2011 Share Posted July 25, 2011 Pin 1 connects to the screen on the cable to reduce interference. As a rule of thumb from working with instrumentation the cable screen should only be earthed at one end, usually at the source to eliminate earth loops. I haven't had any of my DMX kit to bits though to see if this is the case! Pins 2 and 3 carry the signal, as it is a differential signal i.e. a comparison between the two wires any interference which does get into the signal effects both the wires hence is cancelled out. All true, but if the device at the far end isn't earthed, and especially if it's powered from a switch mode power supply, it can float up to a surprising voltage above earth - 50V is not unusual. The DMX receiver will only work if pins 2 and 3 are within 12 volts of ground. So if you don't connect pin 1 the DMX may not work reliably. Of course most lighting fixtures are earthed so this only really applies to DC powered gadgets such as the device being discussed. Link to comment Share on other sites More sharing options...
groggy Posted July 25, 2011 Author Share Posted July 25, 2011 Thanks for the replys. Just to clarify the power into the board is from a 9v - 1A transformer and your all of the opinion to just leave pins 2 + 3 connected to the DMX and not to connect pin 1 to the (-) ground Link to comment Share on other sites More sharing options...
timsabre Posted July 26, 2011 Share Posted July 26, 2011 No, I said you should connect pin 1 to ground. PaulDF and Timmeh2 said you shouldn't. Link to comment Share on other sites More sharing options...
DrV Posted July 26, 2011 Share Posted July 26, 2011 No, I said you should connect pin 1 to ground. PaulDF and Timmeh2 said you shouldn't. As the votes seem to be in the wrong direction (IMHO) at the moment I will add that I concur with timsabre. At the very least you shouold put some kind of leakage path from your unit's 0v to the cable shield (say a few hundred ohms) as, without it, you might well end up swamping the common mode capability of the receiver. Dave Link to comment Share on other sites More sharing options...
scjb Posted July 26, 2011 Share Posted July 26, 2011 The (presumed) manufacturer's contact details... Link to comment Share on other sites More sharing options...
martinw Posted July 26, 2011 Share Posted July 26, 2011 From the DMX spec "Any resistance between Data Link Common's pin or contact and zero volt supply (circuit common) or the receiver circuit shall be less than 100 ohms". This is the case for isolated and non isolated receiver circuits. In practice, most implentations I've seen connect pin 1 directly to the receiver circuit ground. Martin Link to comment Share on other sites More sharing options...
timmeh2 Posted July 26, 2011 Share Posted July 26, 2011 Hi I didn't mean to cause an argument, I was just reading the instructions I had in my hand that I got when I bought mine! All the bestTimmeh Link to comment Share on other sites More sharing options...
groggy Posted July 26, 2011 Author Share Posted July 26, 2011 Thanks everyone, I ran the unit for a while today with the DMX pin 1 connected and not. For everyones info, when not connected the servo seemed to twitch and jitter slightly. But with it connected to 0v (-) that seemed to stop ... :D Cheers Link to comment Share on other sites More sharing options...
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