On using a single Ethernet cable for communication and two activity LEDs - Page 1
Forum section: electronics / projects

author: squadchannel
author_rank: Frequent Contributor
timestamp: September 27, 2024, 01:28:06 am
content: thinking about connecting ethernet cables within an enclosure.cable is a short length(about 60-70cm). from the RJ45 on the board to the RJ45 on the rear panel of the enclosure.would like to have wiring for communications and wiring for LEDs on the RJ45 port, but we would like to do this with a single Ethernet cable (don't want to increase the other wiring for the LEDs).doesn't need to be fast; 10 base is fine.So I thought of using 2 pairs for signals and assigning(=Cat3) the other 2 pairs to each pair for link and activity LED.Good idea? Wouldn't there be crosstalk or other problems?.considering cables with shields used on each pair. LAPP 2170934https://products.lappgroup.com/online-catalogue/data-communication-systems-for-ethernet-technology/industrial-ethernet-cat7/cables-for-flexible-applications/etherline-cat7-flex.html?tx_setuproducts_show%5Bguid%5D=87839343-0023-8390-ffcd-1bc9cf26bfeb&cHash=855822328aa368b03741dbc6c8d7dc22MOD:added diagram

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 27, 2024, 02:07:37 am
content: Add 100nF capacitors on the source end, after the current limiting resistors. It would slow down the edges a lot but would still appear instant visually.

author: squadchannel
author_rank: Frequent Contributor
timestamp: September 27, 2024, 02:43:47 am
content: added a diagram.I meant that I want to put the LEDs controlled by the PHY on the Ethernet cable.Now that I think of it, I might die if lightning strikes.

author: Haenk
author_rank: Super Contributor
timestamp: September 27, 2024, 10:14:29 am
content: Why not use some other multicore cable? 1m of cable is not terrible expensive.

author: squadchannel
author_rank: Frequent Contributor
timestamp: September 27, 2024, 11:20:30 am
content: Quote from: Haenk on September 27, 2024, 10:14:29 amthought about that too, but they are all thick AWGs, with outer jacket diameters of 9mm or so, and large bend radii, which is a problem for layout.bend radius of 20-30mm or less i thinking now...If Ethernet cables can't be used as they are, will think about it.

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 27, 2024, 12:48:02 pm
content: I'd definitely use a separate cable for the LEDs.Take a look at microphone cables. For example, Tasker C504 has 3.8mm outer diameter, four 0.12mm² stranded conductors (each 7 strands of 0.15mm diameter; corresponds to about AWG 26), and a 100% coverage spiral copper shielding. My local store sells it for 2.80€ per meter. Another option is Belden 9398, a bit thicker at 4.7mm outer diameter, with three 0.22mm² conductors (between AWG 23 and 24). The LED currents are typically 10mA each, something like 25mA each absolute maximum (at which point the LEDs will burn out), so even AWG 33 (about 0.7Ω/m DC resistance) would be plenty for this use case.One "trick" I'd warmly suggest is having current-limiting resistors in series on all three/four LED wires, so that in case of a spike/lightning strike, it's the resistors that blow.If we look at a typical MagJack, like sayCetus J1B1211CCD(as used onTeensy 4.1 Ethernet Kit), you'll see that the four LED pins are deliberately placed over 9mm away from the data pins. (Teensy 4.1 usesTI DP8382510/100 Ethernet PHY, which drives the LED using 3.3V and a 150Ω current-limiting resistor, so at about 9mA. Plenty bright.) For two LEDs on a long three-conductor cable driven from 3.3V, I'd use four 47Ω 0603 or 0402 SMD resistors with 62.5mW power rating: one each on the two separate A/K conductors, and two in series on the common return K/A conductor, so that if anything causes excess current to flow the 'sacrifical' resistors will burn first.The thing that I'm struggling with, is finding the most sensible within-a-shielded-chassis cable-shield grounding scheme. Just need to do some practical experiments to understand the finer points in the literature. My designs typically only consume a watt or a few watts at most of power (provided by an external AC/DC supply, 5-12 VDC), so not much EMI is needed to couple into it to fluctuate things, and I don't always have a low-impedance ground available. With a low-impedance ground available it would be obvious and easy..Apologies for the wall of text.

author: thm_w
author_rank: Super Contributor
timestamp: September 27, 2024, 11:29:41 pm
content: Some cheap products use RJ45 connectors without LEDs. You could do that.Or just have the LEDs on the main board, visible from the top or front panel or something.If your product is not cheap, don't waste your time skimping. Have a 4 or 5pin JST XH going from board to board.Is it JUST the RJ45 on the back board? Nothing else will ever be there?

author: squadchannel
author_rank: Frequent Contributor
timestamp: September 27, 2024, 11:51:31 pm
content: Is it still not recommended to put them together on the same cable considering lightning and other surges and signal integrity?It is a personal project, so only one unit is needed. No need to build it at a low cost.The backboard has only RJ45 and USB-B.I thought it was a rather good idea... but I'll be wiring it with XH and wire.

author: nctnico
author_rank: Super Contributor
timestamp: September 28, 2024, 12:17:54 am
content: Quote from: squadchannel on September 27, 2024, 11:51:31 pmNot so much signal integrity but your ethernet signals can pickup noise from the LED connections and transmit it to the outside. If your product would need to pass EMC testing (radiated, conducted, ESD, surge and immunity), the shared cable will be a source for a lot of problems because you are mixing external and internal signals in the same cable.