Driving each line in twisted pair differently? - Page 1
Forum section: electronics / projects

author: Fulcrum
author_rank: Regular Contributor
timestamp: July 15, 2016, 08:02:58 pm
content: Hi everyone.I was wondering what would happen if I treated a twisted pair as two separate lines? I know this is not at all what twisted pair is meant for, but I started thinking about it today. I need to run a single twisted pair for RS485 communication between 16 boards, and I thought of using a standard ethernet CAT5 cable since they are so common. However, a CAT5 ethernet cable does not have a single twisted pair, but instead they have four! This is of course not a problem, but what a waste. Three pairs will just be sitting there, not being used... Poor guys.Then I realized that I might be able to use them for something. See, I also need to remotely reset these boards in case they freeze up. For this, they all have a reset pin, which when pulled high resets the board. Instead of pulling 16 separate wires to each board, I could use 4 wires from the ethernet cable connected to an inexpensive 16:1 multiplexer on each board. These multiplexers would each have 15 inputs connected to GND, and one input to logic high. The pin connected to logic high would be different for each board. Thus, I can just set the four ethernet wires to a specific configuration in order to reset the board I want to.My only problem is that the wires in the ethernet cable are twisted pairs, and I do not know what the effect will be of me driving the lines in each pair differently. I expect there will be strong interference between the wires in each twisted pair, but how strong? And can I mitigate this by simply driving the lines arbitrarily slow?Thanks for any answers!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 15, 2016, 08:16:09 pm
content: Sounds like an ideal method to generate random resets in the presence of EMI.Tim

author: Fulcrum
author_rank: Regular Contributor
timestamp: July 15, 2016, 08:19:02 pm
content: Quote from: T3sl4co1l on July 15, 2016, 08:16:09 pmHmm, yes. Would need to put some heavy filtering between the ethernet wires and multiplexer inputs. Not really a problem, RC filters are inexpensive. I worry more about what happens if I drive one wire in a pair high while the other is held at gnd.

author: Ian.M
author_rank: Super Contributor
timestamp: July 15, 2016, 08:52:29 pm
content: There are various ways to get multiple independent outputs from one signal without anything more complex than MSI logic chips. Seehttp://www.romanblack.com/shift1.htmfor one method that controls seven independent outputs with a 74HC595 8 bit SIPO register with latch. You should use a proper balanced line driver and receiver to trasport the Shift1 signal over oner twisted pair of the cable, increase the caps to 10nF to slow it down approximately 4.5 times increasing the '1' pulse length to 5us low in a 68us window, the '0' pulse to 68us low in a 205us window and the 'latch' pulse to 910us low in a 1360us window. That will keep the maximum bit rate over the twisted pair down to 200KHz which shouldn't be a problem over moderate distances. It would be advisable to add Schmitt input buffers (or comparators biassed to mid rail) between the RC networks and the '595.You then have two spare pairs left in the cable, which if you use them all for power, gives you about 700mA available. Supply excess voltage via a polyfuse and use a well decoupled switching regulator at the far end.

author: David Hess
author_rank: Super Contributor
timestamp: July 15, 2016, 08:56:34 pm
content: Quote from: Fulcrum on July 15, 2016, 08:19:02 pmYou have already identified the solution; drive the lines through an RC filter to limit high frequency crosstalk. The low slew rate at the receiving end may require debouncing.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 16, 2016, 01:52:49 am
content: Yup, as long as you have a common ground and a lot of filtering (and something to deal with the slow edges, usually a schmitt trigger), you're fine. Heck, use one ground and the N-1 remaining as slow signals!Tim

author: penfold
author_rank: Frequent Contributor
timestamp: July 16, 2016, 05:53:19 pm
content: You can also do some interesting things with common mode chokes if you want to preserve a bit of HF. If you run each side of the pair through a separate CM choke against ground then you get the differential impedance to filter out cross talk from the opposite leg but minimal impedance on the desired signal.Of course I'm sure strange things will happen if you have to work with electrically long distances.