Opto Splitter

[mstaylor]

This may be a QOTD but here it is. I use opto-splitters all the time, understand why and where to put them but I have no clue how it works inside. I don't really need to know but thought it would be interesting for the students around here.

 

[techno89]

This may be a QOTD but here it is. I use opto-splitters all the time, understand why and where to put them but I have no clue how it works inside. I don't really need to know but thought it would be interesting for the students around here.

all I know is that the chip in my ETC OptoSplitter went bad because whenever I use it, it causes the lights to flash randomly if I put it on my conventional dmx chain and if I use it with my scrollers or movers they will either move randomly or scroll randomly. So its sending stray dmx.

 

[sk8rsdad]

At the heart of a DMX split is the opto-isolator. This device is an integrated circuit that contains a light source and a photosensitive receiver. The purpose of the device is to provide electrical isolation between the circuit with the light source and the circuit containing the photosensitive receiver. Wikipedia's article has a pretty good survey of the various technologies.

Opto-isolators are useful in DMX circuits since there is an upper limit (32) the number of devices that can be connected to a single run, as well as a limit on the maximum length of a run. An opto-isolator acts as a repeater to enable longer runs. When configured into an opto-splitter it acts as a multiplier by repeating the signal on several branch circuits.

 

[dramatech]

This may be a QOTD but here it is. I use opto-splitters all the time, understand why and where to put them but I have no clue how it works inside. I don't really need to know but thought it would be interesting for the students around here.

I will give it a start, and others can jump in and correct, or add to my explanation. Almost all DMX devices, both receiving and transmitting devices, do their communicating with each other device, by means of a 8 pin dual in line integrated circuit, or 8 pin DIP chip. Most use a 75176. There are other versions, but the most common chip is the 75176. The chip can be wired to transmit, such as in lighting consoles, or it can be wired to receive, such as in scrollers, dimmers, gobo spinners and others. Each chip can tranmit a signal that can drive 32 more of the same chips in the receive mode. An opto splitter, uses one of those chips to receive a signal, presumably from a console or other tranmitting device. The balanced input signal, (RS485/DMX) is changed to a single wire unbalanced signal in the chip. That signal is then connected to another chip, that is essentially an LED that drives an output with it"s illumination. The ouput then drives a series of the 75176 chips in the transmit mode. The signal is also changed from a single wire unbalanced to a balanced, two wire output. The fact that the LED is using light to send the signal in a digital form, from the output of one transceiver chip to the input of an other tranceiver chip, means that there is no electrical signal connection. Or in other words, the signal is coupled optically from the input to several outputs. In order to really make this signal completely isolated electronically, both the input chip and the output chips have seperate power supplies. In many of the Iso/splitters, each output chip has it's own power supply. This power supply is usually a stepdown transformer going through four diodes (full bridge rectifier) filtered by a capacitor and fed into a voltage regulator. This is usually a fully integrated circuit that regulates the voltage to 5 volts DC. This chip is usually a 7805. Some splitters also incorporate additional digital chips that drive LEDs to indicate the presence of both plus data and the minus data. Each output chip is now a reproduction of the input totally and electrically isolated from the input, and capable of driving another 32 devices.
Where many of the differences in one manufacture to another, is the quality of the transformers used and the quality of the printed circuit board, they are basically the same in what they accomplish. The real beauty of an opto/splitter is the fact that if something were to short or put damaging power on one of the outputs, it won't damage any devices on the other outputs, or the input/console. This may not always be true, because some manufactures use one tranformer to drive all of the regulators, and some use one tranformer for the input and one for all of the outputs. To be truly protecting, it needs separate supplies of both tranformers and regulators for the input, and each of the outputs.
This is as I know it, and there will probably be others who can add to what I have given.

 

[Dionysus]

Doug Fleenor (aka Dr. DMX) has on his website some various info on this topic.
DMX Basics

A quote from Doug's DMX Primer page:

Isolation can do wonders.

I designed our first DMX isolator to protect consoles against dimmer failures. Since that time I keep hearing from customers about how it solved this or that problem. Isolating the console from the control line eliminates ground loops, reduces static electricity problems, provides high drive current, and cures warts. I don't try to explain all the solutions, I just listen to my customers. If they say it works, it must work. I will say isolators protect against dimmer failure, electrical storm damage, and plugging the intercom supply into the console wall plate. But as for exorcizing gremlins? Hey, if it works, do it.

- First an isolator, optically isolates each 'circuit' which prevents all sorts of bad things.
- Each line has as much "power" as the original output, so you can hook up more devices.
- Nice clean, powerful signal. Less degradation, less chance of "blowing up" other devices in the chain if their is an issue.
- Opto-Isolators are in general a very good idea.

 

[JD]

An LED and a photo-transistor in the same package, one for each output.

Signal comes in, gets buffered and drives the LEDs. The photo-transistors receive this light signal and output their signals to a buffer and then to each output.

Oversimplified, as in some cases, the signal is reprocessed, but the basic concept is:
Electrical data > Light data > Electrical data.

opto is short for optical.

 

[mstaylor]

As I say, I know what they are used for and the advantages it brings but I just had no idea how it actually did it. In my area I am the guy that answers the odd questions like this so I always like to have an answer. Just because I have been doing this for a long time doesn't mean I want to quit learning, thanks guys.

 

[church]

Some key information has been omitted.

You can Have a DMX splitter which receives a differential DMX signal converts it to a single ended signal and then uses this as the input to multiple 75176 devices configured as transmitters, each providing a DMX signal capable of driving 32 loads. Alterntively you can have an optically isolated splitter which as mentioned above uses an optoisolator chip between the receiver and transmitters. However to achieve full electrical isolation the input circuit and output circuit each need a seperate power supply and the PCB needs a 1/4 gap between all of the board tracks and the optoisolator chip straddles the gap. This provides full electrical isolation to protect the console and dimmers/moving-heads from damage should a mains voltage level accidently be shorted to the DMX cable.

Both designs are useful but you only get limited electrical isolation from the first approach.