Making a crossover cable

algerdes

Al Gerdes
Joined
Nov 29, 2012
Messages
132
Location
Lebanon, Illinois
AAH said:
You're playing with RS485/DMX and there is no distance requirement other than a max total distance of 1.2km and a maximum number of boards connected of 32. The video is much more likely referring to ethernet and I can't say if there is a minimum distance but the max is definitely 100m.

You are correct. There is a minimum of .5 meters to help prevent electrical bounce between Ethernet devices. I have been unable to find any such minimum for RS485/DMX. There is a formula in the Ti sheet "RS-485 Design Guide", but no specified constant minimum length. (See http://www.ti.com/lit/an/slla272b/slla272b.pdf)
 

AAH

I love blinky lights :)
Community project designer
Joined
Dec 27, 2010
Messages
3,343
Location
Eaglehawk
There's no need to a minimum length for the prevention of bounce/reflections as the rs485/dmx line is designed to be terminated into a 120 ohm resistor which is there for that exact purpose.
 

algerdes

Al Gerdes
Joined
Nov 29, 2012
Messages
132
Location
Lebanon, Illinois
Agreed. In practice I've had occasion to not have a terminating resistor on a DMX line, yet everything seemed to work very well. Not that I would advise taking the chance, but the show went on and no one (tech crew or audience) knew the difference. And some of those lines were quite long. (Several city blocks.) RS-485 is one heck of a spec, for sure.
 

BundyRoy

Dedicated elf
Joined
Apr 9, 2014
Messages
1,026
Just realised, even if it was Ethernet I could have got away with the short crossover length as it goes into a 10m length of straight cat5e cable so the total length would be well over 0.5m anyway.
 
Top