0
\$\begingroup\$

I want to recreate this circuit (only show a small relevant portion):

enter image description here

And my XLR plug for input (K2) looks like this:

enter image description here

I found a reference how to connect K2, +, - and GND to the correct pins of the XLR plug, but what about the 4th pin (with the hole in it)? I think I have to connect it also to GND (?)

Update

The scheme is part of this (which I want to create about equally, but with the following changes:

  • Separate AC/DC adapter per DMX output and one for DMX input
  • Four DMX Outputs instead of three DMX outputs
  • MAX487CPA instead of LTC485's

Website: DMX Splitter

Full schematic can be found on page 2 but here is a copy:

enter image description here

\$\endgroup\$
  • 1
    \$\begingroup\$ Hm, this looks like a cable shield connector. \$\endgroup\$ – Marcus Müller Aug 12 '18 at 13:58
  • 1
    \$\begingroup\$ If it is, it should be connected to GND at only one side of the cable. \$\endgroup\$ – τεκ Aug 12 '18 at 14:01
  • 1
    \$\begingroup\$ @τεκ that's been up for discussion between audio techs for decades. Standards says "should never be connected to GND", logic says "should be connected close to amplifier", practical considerations say "connect everywhere, because otherwise we'll have to mark devices that connect to GND"… see en.wikipedia.org/wiki/XLR_connector#Three-pin_in_audio_use \$\endgroup\$ – Marcus Müller Aug 12 '18 at 14:04
  • 1
    \$\begingroup\$ @τεκ but realizing this is for digital signal usage (by the way, the DMX512, according to wikipedia, explicitly states you MUSTN'T use XLR for DMX), I'd say that the thing should probably connected on either end, and that the end devices should be galvanically disconnected from audio hardware... \$\endgroup\$ – Marcus Müller Aug 12 '18 at 14:09
  • 3
    \$\begingroup\$ If you are going to use XLR connectors, at least use 5-pin specified by DMX standard. \$\endgroup\$ – Maple Aug 12 '18 at 19:00
1
\$\begingroup\$

What follows is TL;DR kinda rant. You can skip it.

It is interesting to note how the answer depends on a way the question is formulated when it shouldn't be.

For example, there are tried-and-true wiring methods for pure RS-485 networks. Industry standard Honeywell security systems use STP cables with ground wire. See Figure 3 in this bulletin for how the shield is earth-grounded at one side only, while the common "ground" wire is actually isolated from it. So, when asked about RS-485 connection I'd usually give similar advice.

In RS-485 systems with isolated transceivers the shield can be used as floating common ground, in which case it shouldn't be connected to device chassis. However, since accidental contact with ground is possible, it is better to not have it at all, relying on virtual ground instead. This would be my advice for isolated applications.

Then there is DMX, which uses RS-485 signalling while at the same time specifically prohibiting some typical EIA-485 grounding techniques. As a stickler for the standards I said "OK, let's see what is supposed to be there". What I found is disconcerting.

Standard requires one connector, everybody uses different one. The same connectors also used for line level audio, microphone connections and even power distribution. The recommended isolation of the receivers is hard to achieve in practice, especially in temporary theatrical installations. How do you guys manage doing business without electrocuting yourselves?!

Sometimes designer has full control over controller, receivers and cabling in between. This gives some leeway as we can fall back to familiar RS-485 methods. However, if there is even a slightest chance that your device will be connected to 3rd party component then you have to follow DMX practices (almost said "standard", but stopped myself in time).

Now, to move on with actual answer. There are tree important things here to consider:

  • The schematics in question has 4 isolated floating grounds.
  • The DMX wiring often uses STP cables with single twisted pair. Since common wire is required it leaves designer no choice but to use shield as ground wire.
  • There is high probability for accidental contact of the connector shroud of one network segment's cable to the other or to the device chassis.

Which basically means that you have only one option: You have to leave 4th pin unconnected, exactly as in the article you've referenced.

This does not mean you are out of the woods yet! Note that all four connectors in the article are mounted on the chassis, i.e. the shrouds are electrically tied. If some devices (or cables) on the network have shield connected internally to their ground then plugging them into thoroughly isolated repeater will effectively neutralize the isolation. The lack of attention to grounding issues is what bugs me most in DMX approach to wiring.

\$\endgroup\$
  • \$\begingroup\$ Thanks for this answer. From what become a 'short' question seems to turn in a lot of answers which talk about what you say. Btw, in my case the enclosure is plastic and if I understand correctly from your and other answers I can leave GND unconnected. \$\endgroup\$ – Michel Keijzers Aug 13 '18 at 22:26
  • 1
    \$\begingroup\$ Yes, it is better to leave it unconnected. Also, plastic case makes it safer than original design, less chances to cross grounds of separate sub-nets. But do add TVS as shown in last picture here. \$\endgroup\$ – Maple Aug 13 '18 at 22:37
  • \$\begingroup\$ Just to clarify, GND is not connected to 4th pin. It still must be connected to pin 1. Note that all sub-circuits are isolated, so the grounds marked GND1, GND2 etc for a reason, they should not be connected to each other, only to their own pin 1. \$\endgroup\$ – Maple Aug 16 '18 at 10:39
1
\$\begingroup\$

Ok, so this thing is a fully isolated DMX splitter, in which case you have a few choices to make.

If you are using a metal case then the choice is made for you and all of the connector shells are connected to the local chassis ground, otherwise the connector shells may or may not be grounded at your discretion. DMX cables do NOT normally connect the connector shell to the screen, so what to do with the metal connector bodies in the splitter is actually not that critical in terms of connecting them to chassis and local safety earth (I usually would).

However, RS485 (Which is what DMX512 really is) has a limited common mode range (+12, -7V) with respect to line receiver local ground, so probably the GND signals should NOT be connected to the connector shells, so that all the inputs and outputs are floating (I might add a few 1 meg resistors to bleed off static).

Some ESD diodes and clamps would be a nice addition.

\$\endgroup\$
  • \$\begingroup\$ Also, you show a three pin connector, it should be 5 pin per the spec! Three pin DMX is evil as there are two competing wiring standards! \$\endgroup\$ – Dan Mills Aug 13 '18 at 19:00
  • \$\begingroup\$ Thanks for the answer. I'm using a plastic case, do the 1 meg resistors, ESD diodes and clamps still apply? (Btw, where to put these resistors? And I will check myself what ESD diodes and clamps are, sorry for my lack of electronics knowledge). \$\endgroup\$ – Michel Keijzers Aug 13 '18 at 22:18
  • \$\begingroup\$ Btw, I cannot use 3 pin XLR, because my DMX controller is 3 pin, my DMX lights are 3 pin and all cables I already have. \$\endgroup\$ – Michel Keijzers Aug 13 '18 at 22:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.