DMX motors/ chainhoists

[JSFox]

You may also want to check out John Huntington's book. Not sure if it's been updated since last time I flipped through it, but some good info.

 

[icewolf08]

Not to pick on MSC, but MIDI is also a unidirectional protocol without error checking. I'd have to dig up the MSC spec again to confirm, but I believe that it uses a universal SysEx (System Exclusive) message to send show 'commands'. I don't recall error checking being added to the command structure. Without that, and without an ack/nack mechanism from devices (not possible with MIDI topography), I would not use MSC for safety sensitive control applications without taking the same sorts of precautions and enhancements I described above.

I do not claim to be an expert on MSC, but the way commands are transmitted is inherently safer than a protocol like DMX. In a MIDI system commands are transmitted only once as opposed to a constant repeating data stream like DMX. Therefore if a device sends a command (say a lighting console to a computer running SFX) the command is transmitted only once (Cue X Go). If there is any data loss or the receiver can't interpret the command nothing happens. Some devices require more than one MSC command, like "Load Cue X" followed by "Cue X Go".

Compared to DMX for many situations this is an ideal setup. If there is a bit error in a DMX stream you might not notice if a dimmer kicks to full for one 44 thousandth of a second, but that is enough to trigger a relay or a cue.

MSC is very widely used from amusement parks to Broadway to Vegas shows. I know, from working in the cruise industry that lighting, sound and pyro are tied together via a combination of SMPTE timecode from the click track tape and MSC from the lighting console.

 

[What Rigger?]

Please clarify. I am assuming that you are talking about AMX, the automation company. Not AMX the old Strand Lighting Control signal....
DMX may be bad, but AMX is very very very very very very bad.
Jeff

Hey Jeff, nice catch! I'm talking about AMX as a way of communicating between (for lack of better terms) "show control" and "automation". My current gig uses this as a way of keeping one of the elements of the gig synch'ed up and running safely. SMPTE also figures in, but as I've had to say before to folks, I can't say much more. Current conditions of my employment forbid me to speak in specific terms. Sorry.
Perhaps Fisher Technical's website can add further illumination to the issue: www.fishertechnical.com

 

[jfitzpat]

In a MIDI system commands are transmitted only once as opposed to a constant repeating data stream like DMX. Therefore if a device sends a command (say a lighting console to a computer running SFX) the command is transmitted only once (Cue X Go). If there is any data loss or the receiver can't interpret the command nothing happens.

The most likely form of error is a line bit error. Without a checksum or CRC, how does the receiver know to discard the command instead of executing a corrupt one?

If there is a bit error in a DMX stream you might not notice if a dimmer kicks to full for one 44 thousandth of a second, but that is enough to trigger a relay or a cue.

Read my post above, most the approaches that I am aware of do not use DMX this way. Much as MSC uses a SYSEX message to embed a different protocol message on the MIDI uart stream, most applications that I am aware of use bandwidth on the DMX stream to implement a more robust protocol for certain types of control applications.

MSC is very widely used...

DMX is widely used as well, but it is still a lame protocol.

Forgive me if I sound jaded, but I made many of the same arguments when MIDI was standardized that I made when DMX was standardized. And the response to no error checking, no duplex support, etc., was similiar. Much as DMX was only envisioned replacing the 0-10V snake from front of house, MIDI was envisioned connecting two keyboards together. A bit error, the wrong note plays, no big deal...

Two technical advantages that MIDI does have for these sorts of applications are:

1. It is opto isolated by specification.
2. It uses a dramatically lower baud rate (31250).

However, most complex theme park installs that I am aware of use proprietary protocols for master show control. Show control implies smart clients (ex. lighting board reading MSC and producing DMX). MIDI's electrical specification makes even rudimentary remote device management and monitoring (like the RDM specification) impossible in a multi device rig.

-jjf

 

[PadawanGeek]

You could also use something like an ETC Unison.

 

[Kelite]

<Current conditions of my employment forbid me to speak in specific terms. >
I've got to remember that one!

 

[TimMiller]

i'd personally try to stay away from running chain hoists over AMX. Esp on wireless. Maxx technology did an install over at second baptist and they love to use AMX. They have AMX running 4 chain motors that lift up a big piece of truss with screens in them. Well one sunday the wireless remote lost signal with the reciever and the motors kept on moving and they smart people at maxx tech did not set up any limit switches. They had to run up and kill the power to the motors. The truss was about 1/2" before it was going to go through the roof.


Now on another note of using AMX
How about using some projectors that has lens shift. So you just lens shift them to every position. Lens shift can easily be carried out using an AMX system.

Also as far as using DMX for lasers.
There are quite a few high output lasers on the market that use DMX. Last one I used was a 30 watt. There were interlocks around and mirrors carefully place so no matter what happened the laser beam would not shine into the audience.

 

[jfitzpat]

people at maxx tech did not set up any limit switches...

There were interlocks around and mirrors carefully place so no matter what happened the laser beam would not shine into the audience...

I think this sums up nicely what I was trying to say before. Safety generally involves a multi-part system with some redundancy. A communication protocol is just one part and, in of itself, may or may not matter.

-jjf

 

[Old School 512]

AHHHHHHHHHHHHHHH... NO THEY DO NOT EXIST NOR SHALL THEY EVER EXIST. DMX is the worst motion controller ever. There is no redundancy or checksum, therefore there is no feedback or real control. DMX works great for lights, not good for show control. What you can do is get a motor controller that takes DMX cues and through a dead man switch the motors will move, i.e. if the switch is not pressed they will not move. Thats the safest way to do it. Think about it, do you really want to be pageing through cues and have the US truss move?

I most definitely agree! Back in the late 80s umm early 90s ahh whatever. When you actually had to know electronics and how to mod them fast. I built a black box to control 2 CM 1 ton winches in my shop and tested them in the warehouse in a controlled 10ft rig. Oh the disaster it was almost funny but in real life it would kill people. There was no protocol for the winches except relays on the death box. Even having both motors hardware patched to the same channel was a failure. 1 Though they look and are rated at the same speed that is not always the case after running them up and down a few times they would start to tilt the test truss. 2 no safety at all at that time locking out a channel on a console could be done but overwritten causing umm death. Please take it from a couple of techs that tried this IT Does Not Work don't bother trying you can end up killing people. I changed my design back then to not touch the winches but as was stated before. It operated a red and green light that a human using a lift controller operated. I did expand it but those were the solder and duct tape days. Today buy or rent a real motor controller with mechanical safety covers over the actuator switches. I am sure many here could lead an interested party to a proper system. My last install was with JR Clancy equipment a bit pricey but I cannot put a value on the fans under practically tons of aluminum and live power. Oh my DMX to motor controller it sits on a shelf to remind me and teach others the importance of lift control.

 

[Pie4Weebl]

It took me way too long to realize I was reading a 10 year old thread.

 

[Old School 512]

It took me way too long to realize I was reading a 10 year old thread.

I have done that before. I think we all have, todays technology has to have a prebuilt answer for you check Applied ( http://www.appliednn.com/index.php ). They may have an appropriate answer for you.