Odd notes from an old and fuddled brain.
The Standard: Actually, I'd have to argue that the standard was/is kind of crappy. I know I howled about no checksum or CRC in the packets. The counter argument was that dimmer packs should be able to use shift registers instead of UARTS and microcontrollers. Several folks showed you could do a checksum in a GAL (even then, about $1 in COG), but to no avail. Ironically, almost no 'dumb' implementations every shipped in products.
The electrical stuff was pretty much borrowed directly from international standards on RS485 and 422. Some additional measurement and work went into RDM (a talk-back for configuration on the main transmission pair), but very little into the original standard.
That said, it could be worse - we could all be running AMX 192 (which used to be printed in the same USITT standard as DMX-512).
5 vs. 3: 5 is the standard, 3 is even prohibited. Some 3 pin 'swapped' fixtures make the situation suckier still - but 3 pin fixtures and control outputs are here to stay. I'm with the purists, but even I had to cave on our entry level interface.
On the flipside, abuse of the extra two pins (4 & 5) has made the 5 pin world muckier too.
Cable: Mic cable is not twisted, and is a different impedence. So the digital signal is 'rounded' and longer runs are more succeptible to external noise. Termination causes the transmitter to produce more current, which sharpens the edges somewhat, but mic cable is still less than ideal. DMX cable should be slightly noisier in high end audio applications since it removes a natural beyond-range, low pass filter from the system.
Termination: From an engineering standpoint, it is always a good idea. A lot of folks who think they never have problems without it would be surprised to see how many bit errors can be occuring at individual fixtures. I used to always recommend building your own terminators with an LED in it (so you get used to having a signal indicator and don't fall back out of the habit of terminating), but if RDM ever really catches on, the simplest LED designs might be a problem.
-jjf
The Standard: Actually, I'd have to argue that the standard was/is kind of crappy. I know I howled about no checksum or CRC in the packets. The counter argument was that dimmer packs should be able to use shift registers instead of UARTS and microcontrollers. Several folks showed you could do a checksum in a GAL (even then, about $1 in COG), but to no avail. Ironically, almost no 'dumb' implementations every shipped in products.
The electrical stuff was pretty much borrowed directly from international standards on RS485 and 422. Some additional measurement and work went into RDM (a talk-back for configuration on the main transmission pair), but very little into the original standard.
That said, it could be worse - we could all be running AMX 192 (which used to be printed in the same USITT standard as DMX-512).
5 vs. 3: 5 is the standard, 3 is even prohibited. Some 3 pin 'swapped' fixtures make the situation suckier still - but 3 pin fixtures and control outputs are here to stay. I'm with the purists, but even I had to cave on our entry level interface.
On the flipside, abuse of the extra two pins (4 & 5) has made the 5 pin world muckier too.
Cable: Mic cable is not twisted, and is a different impedence. So the digital signal is 'rounded' and longer runs are more succeptible to external noise. Termination causes the transmitter to produce more current, which sharpens the edges somewhat, but mic cable is still less than ideal. DMX cable should be slightly noisier in high end audio applications since it removes a natural beyond-range, low pass filter from the system.
Termination: From an engineering standpoint, it is always a good idea. A lot of folks who think they never have problems without it would be surprised to see how many bit errors can be occuring at individual fixtures. I used to always recommend building your own terminators with an LED in it (so you get used to having a signal indicator and don't fall back out of the habit of terminating), but if RDM ever really catches on, the simplest LED designs might be a problem.
-jjf