Jan 082019

We were contacted by an integrator whose DMX fixture installation was large and complicated. They were using a Control4 system and chose our RS-232 DMX Engine as the master output controller in their system. However, in their case the standard practice of daisy-chaining large groups of fixtures* together was impossible, due to the required physical layout, access to connected conduit, etc.

During a telephone consultation, we discussed installing multiple DMX Engines throughout their venue. Because of the required layout, this would have been quite expensive. When we learned that their timeline was fairly relaxed, it made sense to commission a rack-mount system with 12 outputs, 12 isolated output drivers, and genuine Neutrik EtherCON jacks for easy termination of their installed wiring. Our friendly metal shop across town returned two pair of blue-anodized chassis sets a few weeks later:

Click to Enlarge
Click to Enlarge

The system follows the now-standard pinout for DMX over CAT5 cable. We use the DATA 1 pair (orange) with both brown conductors grounded:

CAT5 cables wired to the 568-B standard can carry DMX data

This rack-mount DMX Engine functions exactly the same as its smaller red brethren. The command set is identical; only the form factor and output count have changed. Need one? Let us know.

*This was the subject of a tech support call earlier in the week. When playing in the DMX world, there are two numbers to keep track of: channel count and fixture count.

Channel count is fairly obvious: a full universe of DMX contains 512 separate channels. This means that 512 single AC dimmers could drive 512 discretely connected incandescent light bulbs. When the author got started in technical theatre, the entire venue was controlled by large, dusty, hum-emitting 6 KW dimmers. About 30 worked properly. The command ‘set all to full time zero’ provoked a physical, visceral reaction from offstage left.

If using RGB LED fixtures, a universe can discretely control 512 / 3 = 170 fixtures without any overlap. Some modern moving lights à la rock concert gobble up 100+ channels each, which means only five can be individually driven on a single universe.

Recently, we completed a Christmas art installation where a single tree required nearly 23,000 channels of data running at 50 frames per second. We used e1.31 / sACN (DMX over ethernet, more or less) as the control backbone.

Fixture count is a different beast altogether. It relates to the total electrical load on the differential bus (D+ and D-, the DMX data signals). Each fixture connected in a daisy chain increases the load, and if it’s too high, signaling can turn erratic and be difficult to troubleshoot. Here’s a great article which dives deeper into the math. In any case, the ‘standard’ load for an RS-485 receiver (and by extension, a connected DMX device) is 1/32, which means that 32 devices can be safely daisy-chained together and driven successfully by a single master controller. If fixture load isn’t explicitly called out in a device’s datasheet or instruction manual, assume 1/32 load.

For slightly more money, some manufacturers (including us, as has been required by clients from time to time) design DMX input stages with 1/256 load receivers, which allows up to 256 devices in a single daisy chain without introducing signal issues.

Bottom line: make sure both your channel count and fixture count are within acceptable ranges. If fixture count is too high, consider using what’s called an optosplitter, which receives a signal and then regenerates it multiple times across multiple outputs. Or call us and we can help with something custom.