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DMX512 Termination

• Theoretical introduction to the need to terminate
• Just what is a terminator, anyway?
• Termination in practice
• How can I determine where hidden terminators are?

The book Recommended practice for DMX512 states: Incorrect or missing termination is probably the single most common reason for faulty DMX512 systems.

We agree.

Yet the termination for most DMX512 systems is not designed into the rig, but just "happens". We aim to debunk the mystique of termination, and give simple guidelines on how to terminate.

Theory - The need to terminate

The data rate for DMX512 signals is 250,000 bits per second. This is fast enough to put a DMX512 signal into the category of signals knows as "radio frequency", or RF. RF signals behave strangely, if you are not used to the characteristics of these signals. For instance, if you have a piece of wire carrying an RF signal, and the end of the wire is just a cut end, then that cut end acts as a "mirror", and "bounces" the signal back down the wire. Radio amateurs and CBers are familiar with this phenomenon, and it's effects, including (the curse of) VSWR.

A somewhat odder effect also occurs, and that is dependent upon the length of the cable. This is particularly noticeable when you have a tee wiring arrangment, like this:

With this arrangement, there is an unterminated length of cable hanging off the spliter ("twofer", to use that great Americanism), and this can cause all sorts of problems. The technical term for this bit of wire hanging off the main bus is a "stub". With just the right length of cable, that stub becomes effectively a short circuit at just the right frequency. Thus although the circuits all check out OK with your multimeter, when DMX is flowing, all sorts of inexplicable odd things happen.

Just what is a terminator, anyway?

A terminator in DMX512 terms is a simple resistor, of about 120 ohms, connected between the two DMX512 data lines.

Thats it. Nothing fancy, just a few pence (or cents) resistor. A few pennies worth of component are the single most key link in the operation of your entire multi-thousand (in whatever currency) light rig.

Frightening, isn't it?

You can build your own terminators, here's how.

Termination in practice

The simple rule is: At the end of each chain of DMX512 receivers there should be a termination resistor.

Thats it, thats all there is to it. But wait - if thats all there is to it, how come most of the time line termination is done just plain wrong?

The main problems are

• Remembering the simple rule, above, and
• spotting where terminators are

Take a look at this diagram of a very simple lighting system, and try to work out where the terminator(s), if any are:

How did you do? - zero or one terminators is probably correct, two is possible but less likely, and three or more is unlikely.

So for just a simple system there are lots of possibilities. Lets look at them in a bit more detail...

(But before diving into that, it's helpful if you understand a bit about DMX512 networks, which (paradoxically) is the next chapter in the series...)

It is unlikely that there will be a terminating resistor in the console; since consoles generally drive DMX512 lines directly, and is the only driver on the DMX512 "bus" (thats the technical term for a DMX512 cable that snakes it's way to multiple receivers), it doesn't need terminating.

That just leaves us with the dimmer pack. It may have an internal terminator, or it may not. How can you tell? Deductive method:

• If there is just one DMX512 input socket, and no "daisy chain" output then you probably have an integrated terminator. If not, it's an annoying design, 'cos to put a terminator on you need some fancy cablework, which is not pretty, and not obvious to the user of the system.
• If there is a switch marked "terminator", "end-of-line", "last rack" or something similarly obvious then what you have is switchable termination. Thats very easy to understand. This arrangement can be found on systems with (perversely) just an input socket, or (more sensibly) systems with both an input and daisy chain output.
• If you have a daisy chain output then you probably don't have internal terminators, and have to add them externally.

The exception to prove the rule: Systems that actively regenerate DMX512 also have an input socket, and a daisy chain output, and some fancy footwork under the covers. When the dimmer is on, then the input socket is internally terminated in the unit, and the output socket is supplied with DMX512 from electronics in the dimmer. This is a freshly regenrated output, and has lots of ability to drive a chain of devices.

When the power fails, a relay completes the circuit between the input and daisy chain sockets, allowing devices further down the chain to continue to function. In this mode the internal terminator is disconnected. It's also worth noting the the DMX512 line power has now dropped down to that supplied by the driving device up the chain, and half your rig may have just plunged into full mirror jitter mode.....

How can I determine where hidden terminators are?

No end-of-line terminating switch or labels on the device? Bummer. Then you have to get a bit scientific, and use a testmeter. A modern, digital testmeter is best, 'cos these can accurately measure resistance of a hundred ohms without frying the circuitry of the device under test.

Just a single DMX512 socket, ie an input but no output or daisy-chain

Use the testmeter to measure the resistance across pins 2 and 3 of the input socket. If it is about 100 ohms, your device is terminated, if it is several K ohms or greater, you are unterminated. Check with the power to the device on and off, it'll probably be the same result in both cases. Now label the device.

One input and one output (daisy-chain) socket

Start at step one.

Notes

• Type 1: This device is a well behaved device, which can be daisychained. Nothing nasty happens if this device looses power. This device can also be connected using "daisy chain past" (see Type 3) or from an active splitter (see Type 4)

• Type 2: This device is well behaved most of the time, however under power failure there will be a wibble as they daisychaining switches from being an electronic repeat to a relay bypass. Since electronic repeat extends the theoretical length of the chain to a infinite number of devices, then if a bunch of these in series loose power, then the amplifying effect will be lost, and that could impact devices further downstream. This device can also be connected using "daisy chain past" (see Type 3) or from an active splitter (see Type 4)

• Type 3: These class of devices are difficult, for if they loose power (or fail) they will isolate all downstream daisy-chained devices from DMX512 control. Do not daisy-chain through, ignore the DMX512 output socket and instead daisy chain past, like this:
  

• Type 4: This devices are nightmarer, as they apparently have a daisy-chain socket (and capability), yet also terminates the line. This puts a terminator in the middle of a DMX512 chain, and as such daisy-chaining with this device is not supported. Use a splitter to drive this, like this:
  

• Type 5: This device is evil. Sometimes it is terminated, other times it isn't. Treat is as Type 4, and prey for the best.