In my proposed application, I will have approximately 30 RS-485 nodes configured in 3 chains spaced ~1m apart. There is a need to pass through 240v L-N-E, +5v, A, B, GND in each node. The physical layout is shown below, as well as an example of the cable.

I am worried about the interference on my signal lines. There will be approx ~1.6A Triac switched current on each chain.

How do I connect the the signal shield to offer the best protection?



  • \$\begingroup\$ Why not just do an experiment to see how bad the situation is. I guess this is a follow-up to your previous post on the subject. Set-up a simple system that has the 1.6A switching current in the power cable and see whether it can be solved with screen on data grounded at 1 end or both ends \$\endgroup\$ – Andy aka Mar 19 '13 at 10:47
  • \$\begingroup\$ Sure I will definitely do that, but I feel that problems might not arise until the whole system is in place - and just want to make sure I am on the right track before investing in this. I've never dealt with AC and data in proximity before. \$\endgroup\$ – davivid Mar 19 '13 at 10:54
  • 1
    \$\begingroup\$ you might want to check that it's OK for 230V ac and not just 110V ac - it doesn't state anything in the spec about voltage rating i could see. Nice cable though. \$\endgroup\$ – Andy aka Mar 19 '13 at 11:46

Is there an equivalent to this cable where the data member is a "star quad" cable? A differential signalling standard on correctly wired "star quad" cable is more resistant to interference.

Star quad cable has 4 conductors in a circle (I numbered them clockwise below)
1 2
4 3
Use 1 and 3 in parallel for one leg, and 2 and 4 for the other. This gives much better noise cancellation than a regular twisted pair.

The downside of star quad is a much higher capacitance between legs, but over short (1m) runs that won't be such a problem.

edit : this tight twist is what I think of as star quad; though ordinary quad used the same way will also provide reasonably good immunity to interference. Image from the Canford catalogue.

Star Quad cable

| improve this answer | |
  • \$\begingroup\$ Looking at the picture of the cable i think the data section is quad and he is saying he's using 485 data which is balanced (you knew that didn't ya!!). He can run 0V and 5V down the "opposing" pair too and the screen he's free to tie at one end or the other or both. I still think it might not work though!! \$\endgroup\$ – Andy aka Mar 19 '13 at 11:37
  • 1
    \$\begingroup\$ The picture is not star quad. That has a very tight twist, designed for routing audio signals around triac dimmed kilowatt lights in TV studios. \$\endgroup\$ – Brian Drummond Mar 19 '13 at 12:51
  • \$\begingroup\$ I didn't realize there was a difference between star-quad and "quad". Hey I've learned something - thanks \$\endgroup\$ – Andy aka Mar 19 '13 at 13:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.