Which RJ45 pin are safe for my non ethernet device - Page 1
Forum section: electronics / projects

author: drakejest
author_rank: Regular Contributor
timestamp: January 08, 2021, 01:36:59 pm
content: I am going to use an Rj45 (wire UTP) connector to connect two of my own custom board,Now i only need 2pairs / 4 wires and would like to atleast put some strategy on which of the 4 pairs am i going to place it with.What im hoping that in a scenario that i get ultra dumb, and plug an RJ45 that is connected to a switch/computer on the other end, which pair would least likely damage the computer/ switch and by extension my device too (but more importantly the more expensive network switch/ computers).The signals are 5v, 2x non differencial signal, ground. The IC on my board recommends that i pair 5v with 1x signal and 1x signal with ground.

author: jfiresto
author_rank: Frequent Contributor
timestamp: January 08, 2021, 01:52:58 pm
content: Personally, I would use an RJ12 and not use the two, popular RJ11 pins. Over the last few decades, I have unplugged and plugged RJ45 ethernet devices far more often than RJ11 telephone equipment.

author: madires
author_rank: Super Contributor
timestamp: January 08, 2021, 02:00:36 pm
content: Most devices come with 1000BASE-T which uses all four pairs. Maybe a large "not for networking" label?

author: madires
author_rank: Super Contributor
timestamp: January 08, 2021, 02:14:12 pm
content: Quote from: jfiresto on January 08, 2021, 01:52:58 pmIsn't bulletproof either because some older phones use the outer pins. And don't underestimate creativity! I've seen RJ10/11/12/14 plugged into RJ45 sockets too many times.

author: SeanB
author_rank: Super Contributor
timestamp: January 08, 2021, 02:22:41 pm
content: Use the blue and brown pairs, and add in a trio of 6V8 transorbs on all 3 non ground wires, or simply use the pairs doubled up, as that way almost all reasonably intelligent POE equipment will sense the line is shorted, and not power it. If possible use series resistors between the inputs and outputs of the data lines and the socket, and put the transorbs on the equipment side. Preferably flame proof resistors or fusible ones.

author: Pack34
author_rank: Frequent Contributor
timestamp: January 08, 2021, 04:24:17 pm
content: I don't typically like to advocate using a standard connector for a non-standard protocol. But I have occasionally seen RS485 over Ethernet cable and RJ45 connectors. What you could do, would be to use a twisted pair as sense pins. The Master/Primary device would look at the state of those pins before attempting communication.I believe RS485 over Ethernet uses pins 4/5 (Blue/Blue-White) that don't seem to be used by T568A/T568B. The pair could be grounded on the Slave/Secondary device and weak pull-ups on the Master/Primary device. Then use the typical Rx/Tx pairs for your data, while keeping pins 7/8 (Brown/Brown-white) for ground.If the Master/Primary sees the sense pins in the correct state for some amount of time, attempt communication. For redundancy, you could also have the Slave/Secondary send an expected packet down one of the data lines. Just something simple like (0xAA, 0x55, 0x00, 0xFF). Once the Master/Primary sees the state change on the sense pins it'll know the Slave/Secondary is connected and the received packet will let it know that it's the correct one. You could perhaps use pin 7 (brown/white) for this since that's typically grounded.

author: TheMG
author_rank: Frequent Contributor
timestamp: January 08, 2021, 05:43:14 pm
content: Quote from: edavid on January 08, 2021, 04:35:04 pmIf you happened to put power and ground on the same pair and there is enough current capability from the device you could blow open the winding on the transformer, rendering that ethernet port useless.Granted, that would probably require a substantial amount of current given the typical DC resistance of ethernet NIC transformers is below 0.5 ohms. But if the power source inside your device is not current limited, it is a possibility, as is the possibility of damaging your device due to the short circuit if there is not adequate overload protection in place.Placing the ground and supply voltage(s) on what would be separate pairs on an ethernet jack would avoid this scenario, as the separate pairs are galvanically isolated from one another.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 08, 2021, 05:47:29 pm
content: You can use any pairs, they are isolated in all Ethernet devices using a transformer.Just do not supply one pair of 5V, divide this voltage between the pairs.See the ethernet socket diagram for clarity.

author: viperidae
author_rank: Frequent Contributor
timestamp: January 08, 2021, 08:41:01 pm
content: How much current can your signal sink/source? If ground and 5V are on different pairs, they pose no problem for an Ethernet port.

author: drakejest
author_rank: Regular Contributor
timestamp: January 09, 2021, 01:45:45 pm
content: So as long as i do not place 5v and Gnd on the same pair then i should be fine right? It may be important to not that that the signal lines are pulled up by a 500 Ohm resistor, so i am worried about that too

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 09, 2021, 02:55:19 pm
content: Quote from: drakejest on January 09, 2021, 01:45:45 pmYes, but don't forget about the line resistance and possible voltage drop. And I didn't understand about resistors. Free pairs are not good to leave flying in the air - this is an antenna, it is better to connect them loosely to the ground.

author: drakejest
author_rank: Regular Contributor
timestamp: January 09, 2021, 06:02:25 pm
content: Yes i have about 25meters for this setup (according to the datasheet).Quote from: S. Petrukhin on January 09, 2021, 02:55:19 pmThe signal lines are pulled up by a resistor that means that those lines are defaultly 5v.Quote from: S. Petrukhin on January 09, 2021, 02:55:19 pmEven if those free pairs are not connected on both ends?

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 09, 2021, 06:59:49 pm
content: Quote from: drakejest on January 09, 2021, 06:02:25 pm25m is already a tangible distance, study the resistivity of the cable and calculate its resistance.Pulled up resistor is not the best solution for the line, better hard control.Yes, it is desirable to ground the free wires on one side, remove the potential, most often not rigidly, through a 1MOhm resistor, for example.

author: drakejest
author_rank: Regular Contributor
timestamp: January 10, 2021, 01:27:09 pm
content: Quote from: S. Petrukhin on January 09, 2021, 06:59:49 pmAll that are what the datasheet of the IC i am using, for a 25meters of twisted pair i must use this value of resistance.Quote from: S. Petrukhin on January 09, 2021, 06:59:49 pmOh ~! why on one side only?

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 10, 2021, 05:49:03 pm
content: Quote from: drakejest on January 10, 2021, 01:27:09 pmUsing a soft ground to remove potential is not a big deal, but it is good practice to ground the line (most often a protective shield) on just one side. This is for the purpose of safety of the potential difference on the grounded wire on both sides. There may not always be good grounding on both sides. If there is a different potential on both sides on wire, this wire will align it, current will flow and it will become a radiating antenna. Even in industrial installations, where the grounding of all devices is very tight, I never risk grounding the shield on both sides.

author: eb4fbz
author_rank: Regular Contributor
timestamp: January 10, 2021, 07:32:36 pm
content: Quote from: drakejest on January 09, 2021, 01:45:45 pmNo, you can burn the bob-smith termination resistors on non-PoE Ethernet equipment.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 10, 2021, 08:10:20 pm
content: Quote from: eb4fbz on January 10, 2021, 07:32:36 pm75... ~0.06A, ~0.3W (0.075W for each)...

author: drakejest
author_rank: Regular Contributor
timestamp: January 11, 2021, 07:07:35 am
content: Quote from: S. Petrukhin on January 10, 2021, 05:49:03 pmI have actually seen this being done on a few circuit a 1M stitch resistor shield and ground. A frequently seen it also comes parallel with a capacitor the value i can remember is 4.7nF . Is the capacitor necessary ?Also which end should this be done ? If it was a master slave relationship between device should i do it on the slave incase something went wrong it will be cheap to replace

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 11, 2021, 01:39:38 pm
content: Quote from: drakejest on January 11, 2021, 07:07:35 amYes, a capacitor is needed, it passes sharp bursts from the penetration of static charge. Any installer can touch while carrying a charge. Since static discharges have a very steep rise side, a small-capacity is a good conductor for them.It does not matter in principle from which side the shell is grounded. But, most often, it is easier to do this from the head unit and the grounding is better there. But sometimes several devices are connected to the same bus and then the incoming side is grounded to the side of the previous device, and the outgoing side is here. So it is more familiar to those who write from left to right. I do that.