DMX512 secondary data link usage

DashNode

New Member
Hi everyone,

I'm designing a DMX512 compliant product that records a live DMX stream and allows to play it back later on.
I want to control 4 universes, and keep the system as compact as possible, so I thought : well, instead of placing 8 XLR-5 connectors on the system chassis (4 IN for the recording - 4 OUT for playback) which would be a waste of space and connectors, I could simply use 4 XLR-5 Connectors, use the primary data link pins (DATA1+ and DATA1-) to transmit the DMX playback to the gear, and use the secondary data link (DATA2+ and DATA2-) to receive the DMX incoming stream from the lighting console.
Unfortunately, as far as I know the secondary data link is almost never implemented (which is a shame because we could theoretically carry two universes in one cable), which will force the user to solder custom cables to connect the primary data link out of the console to the secondary data pins to enter my system. But I suppose there must be some kind of reason behind the industry not using this secondary data link. Am I having a bad idea ?

Thanks in advance.
 

almorton

Well-Known Member
I'd guess it's down to cost. Two cores cost less than four, and it all adds up when you have lots of long, good quality cables. Maybe not so much of an issue these days with networks and wireless, but it may have been previously.
 

SteveB

Well-Known Member
At the time DMX was implemented, not many systems were large enough to be above the 512 limits of the single universe on a single cable. The few systems that were simply added a 2nd (or however many needed) cable. When multi universe systems started to show up, the move to TCP/IP and using Cat 5 became easier to implement than bothering to utilize the 2nd pair of wires., as you could go to many more universes on TCP/IP.
 

sk8rsdad

Well-Known Member
Premium Member
Fight Leukemia
There are a lot of fixtures and installed wiring where the second pair doesn't exist. Air gaps make lousy conductors. FWIW, the second pair was for full-duplex communication per the RS-485 standard, so using it as a second transmission pair would not conform with the intended use.
 

DashNode

New Member
Thank you all for your answers !
I came up with a solution that only uses the primary data link. Since the system is, at any given time, EITHER recording OR playing back the stream, but never both at the same time, the user will simply set a switch that toggles between record mode and play mode. The microcontroller will interpret it as : if the switch is in the receive position, listen to what comes on the RX pin and record it. Don't do anything with the TX pin. If the switch is in the play position, don't care about what comes on the RX pin and just play your stream on the TX pin. The PCB traces for rx and tx will basically be tied together, it's just up to the system to decide whether it receives OR transmits on it, in a half-duplex fashion. The user plugs in the cable from the lighting board/splitter to the connector when he wants to record, and then swaps it to connect to the fixture daisy-chain when getting ready to play.
 

egilson1

Senior Team
Senior Team
CB Mods
Premium Member
Thank you all for your answers !
I came up with a solution that only uses the primary data link. Since the system is, at any given time, EITHER recording OR playing back the stream, but never both at the same time, the user will simply set a switch that toggles between record mode and play mode. The microcontroller will interpret it as : if the switch is in the receive position, listen to what comes on the RX pin and record it. Don't do anything with the TX pin. If the switch is in the play position, don't care about what comes on the RX pin and just play your stream on the TX pin. The PCB traces for rx and tx will basically be tied together, it's just up to the system to decide whether it receives OR transmits on it, in a half-duplex fashion. The user plugs in the cable from the lighting board/splitter to the connector when he wants to record, and then swaps it to connect to the fixture daisy-chain when getting ready to play.
You will run into the issue of "gender" on the connectors. plug from the console, pins to the fixtures. Another thing to consider is the programmer will likely want to see what the lights are doing when recording to the device. So only having one set of connectors might limit the usefulness of your device.
 

RonHebbard

Well-Known Member
Premium Member
You will run into the issue of "gender" on the connectors. plug from the console, pins to the fixtures. Another thing to consider is the programmer will likely want to see what the lights are doing when recording to the device. So only having one set of connectors might limit the usefulness of your device.
Consider an XLR 5 barrel with a female on one end and a male on the other.
Add a burr free hole in the center of the barrel to accommodate your cable.
Toodleoo!
Ron Hebbard
 

Harrison Hohnholt

Well-Known Member
Another thing to consider is the programmer will likely want to see what the lights are doing when recording to the device. So only having one set of connectors might limit the usefulness of your device.
This 100%!

A device similar to what you are working on is the CueServer Mini from Interactive Technologies. They only have the ability for a single universe in/out but having the ability to make sure what you are programming looks good while you are programming it is very beneficial.
 
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DashNode

New Member
having the ability to make sure what you are programmer looks good while you are programming it is very beneficial.
I totally agree ! But with a simple splitter they can simultaneously send the DMX stream from the console to the fixture and a copy of it to the recorder. I feel like it's down to the programmer's convenience because of having to use a splitter or not. As for connector gender, a good quality gender adapter should make it ok I suppose.
 
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Harrison Hohnholt

Well-Known Member
I totally agree ! But with a simple splitter they can simultaneously send the DMX stream from the console to the fixture and a copy of it to the recorder. I feel like it's down to the programmer's convenience because of having to use a splitter or not.
I would say that depends on who your target audience is and how they are going to be setting the lights. And most splitters are a single universe in and multiple outs of that single universe. Once you get into the multiple universes in you are now talking about a splitter per universe.

This again depends on your target audience but have you thought about a network based input like sACN or Art-Net and then outs as DMX?
 

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