Odd remark in EVSE instructions for RS-485 over CAT5 S/FTP - Page 1
Forum section: electronics / projects

author: Jeroen3
author_rank: Super Contributor
timestamp: July 29, 2025, 05:07:40 pm
content: In the installation instructions for an EVSE that has load balancing with an external RS-485 based energy meter the following strange remark is given.I don't understand this."Employ only 1 wire of each twisted pair"Thinking it was a translation error, I lookup up the german, dutch and spanish translation, they are the same.Why would they recommend using two wires from two twisted pairs for RS-485 communication in a S/FTP cable?Imho this is just as poor, maybe even worse, as running 3 singles.Or am I missing something?They also do not mention grounding the shield, but few people understand cable shields, I can forgive that.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 29, 2025, 06:35:32 pm
content: Attached is a photograph from one of our installers. Guess which terminals are the RS485 A and B.I have no idea what gives people the idea of such wiring, but it seems they have seen many weird mistakes on the field and try to avoid them with this instruction which sounds weird until you see the exact pattern of mistakes.Of course, that instruction might cause more problems than remove - but maybe they have experimental proof that exactly this text works.This is one of the things where a few pictures would work. Like, the correct install with a green checkmark, plus a few crossed-over typical failures.

author: thm_w
author_rank: Super Contributor
timestamp: July 29, 2025, 09:22:37 pm
content: The diagram is fine, the text is written by someone who doesn't quite understand.Quote from: Siwastaja on July 29, 2025, 06:35:32 pmorange/orange-white A, blue/blue-white B?Seems similar to this:https://electronics.stackexchange.com/questions/412985/divide-rs-485-simply-without-switchhttps://suntechsolar.ca/wp-content/uploads/2016/06/RJ45-to-RS485-Adaptor-instructions.pdfBut there seems to be no standard pinout as multiple exist, I don't know if the above is the most common or not.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 30, 2025, 06:43:11 pm
content: Quote from: thm_w on July 29, 2025, 09:22:37 pmYou missed the point. Pinout is irrelevant, but using separate pairs for A and B is disaster for EMI and signal integrity.That alonewould be understandable simply by the fact that the installer did not know that, but what kind of cargo cult is to parallel two wires together for no reason?And the wiring as shown is, as usual, point-to-point so just two devices in the bus, at the end of the bus.

author: thm_w
author_rank: Super Contributor
timestamp: July 30, 2025, 10:21:31 pm
content: Quote from: Siwastaja on July 30, 2025, 06:43:11 pmYes I knew why you brought it up.It looks like if you have an off the shelf T-568A cable it would properly use the twisted pairs:https://www.showmecables.com/blog/post/rj45-pinout(3,4 = A, 5,6 = B). Which makes sense..I'm guessing in your case the other end of the cable is not RJ45 but maybe just another hacked off end?

author: Siwastaja
author_rank: Super Contributor
timestamp: July 31, 2025, 07:02:04 am
content: Quote from: thm_w on July 30, 2025, 10:21:31 pmEverybody has CAT6 (or CAT5e still) with them, and that is well suitable for RS485 - of course it has excess pairs (that can be all grounded or left unconnected), a single pair + single ground wire would suffice, but then again such cable is actually more expensive.RS485 in RJ45 connector is in my experience somewhat rare, and when it happens, the pinout is always different. So far equipment manufacturers have luckily understood to use a pinout so that when standard "ethernet cable" crimping pinout is used (or, cutting a factory-made "ethernet cable"), A and B get the same pair. But sometimes it's orange pair, sometimes blue, sometimes green - for example, in Solis S6 series hybrid inverters, different model ranges (which differ in power rating) use different pinout which is pain in the ass for creating instructions for the installers.

author: PGPG
author_rank: Frequent Contributor
timestamp: July 31, 2025, 09:46:43 am
content: Quote from: Jeroen3 on July 29, 2025, 05:07:40 pmAt the end of 90s we decided to use RS485 for the first time, but we knew that easiest accessible cable is CAT5 so we should use it.I had 0 experience with RS485 those time and had to make my decisions about how to use CAT5 for RS485.What I knew:- RS485 is intended to be used with 120 ohm cable impedance,- RS485 expects third wire for GND,- CAT5 pair has 100 ohm.Using 20m CAT5, pulse generator and oscilloscope I have made some measurements and my conclusion was that the best would be to use for A and B wires from separate pairs and use second wires from each pair as GND.- measured impedance between A and B was 130 ohm (closer to RS485 standard than pair impedance 100 ohm),- I assumed that through both GND wires the right return current will be traveling to not loose to much efficiency of twisting,- in CAT5 cable the pairs are also twisted together (not so dense as wires in pairs, but my RS485 was 115.2k and not Ethernet 100M),- using 130 ohm terminal resistors (instead of 100ohm) gives 30% less voltage drop so 30% longer maximum cable length.From my point of view this solution has some advantages and I see nothing wrong in it. In any case you have to use second pair as GND so why not use for it wires from two pairs.Those time we needed to allow for as long bus as possible. Our system didn't used Ethernet at all so max bus length specified a radius of a circle (PC as center) to which the whole system installation was limited.Once installer reported that he has a problem and nothing in our system works. He made 2km bus, even we clearly (with some margin) instructed to not use longer than 1.2km buses. Practically system worked up to about 1.6km.In our next system we use Ethernet to cover the distance so I could make RS485 not isolated, using 100ohm twisted pairs and reduced bus length to 300m.Do you see something wrong in this my old solution?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 31, 2025, 11:26:05 am
content: Probably not ideal but at the low baudrates involved you could use wet string and it would still probably work.The worse crime is using thin wires in terminal blocks without ferrules

author: Jeroen3
author_rank: Super Contributor
timestamp: July 31, 2025, 09:02:40 pm
content: I know CAN is electrically similar and I have commissioned a panel with normal, straight, 4 core shielded cable. This works, but there are some failures, the bitrate is about 500k with ~50m of network.However, CAN bus protocol is very robust and hides this poor network, but still, about once a month there is a bus panic and the system re-initializes.As far as I know, RS-485/modbus does not offer the same protocol robustness, so I was confused.Sure, a bit of wet string does work for these signals on your desk. But people wire up the evse 20-30 meters away through underground cable in parallel to mains for that 20 meters.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 31, 2025, 10:09:22 pm
content: Quote from: Jeroen3 on July 31, 2025, 09:02:40 pmAnd it's probably only running at 9600 baud20 metres is absolutely nothing - I routinely run 4Mbaud over 100m of cat6 ( cat6 as there are usually multiple busses per cable, and crosstalk is often the limiting factor with cat5) .

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: August 01, 2025, 12:19:05 am
content: Quote from: PGPG on July 31, 2025, 09:46:43 amYes, the tight twisting in a pair is what provides phenomenal immunity to interference when using differential signaling. By splitting the pair, you've severely compromised that immunity.Will it still work? Yes, provided you don't have much interference around. But if someone installs something new and noisy nearby it can break your system.CAT5 100 Ohm is close enough to RS485 120 Ohm to work fine. Terminate it with 100 Ohm instead of 120 Ohm if that helps your signal integrity but I'm sure the difference is marginal.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 02, 2025, 06:24:04 am
content: Quote from: PGPG on July 31, 2025, 09:46:43 amVery interesting, thanks for sharing. I have assumed what you describe would be worse than the 100 vs 120 ohm mismatch, but maybe you are right.One point to remember though is that correct impedance is just one thing - rejecting external fields is a separate issue and that's why correct twisting is important, so I'm not sure if testing for SI using pulse generator and oscilloscope is sufficient - you probably should be also testing for EMC, and that could prove far more important.But you could be well right and your solution could be superior, I'm not sure.One thing is sure, no one has 120ohm special RS485 or CAN cable; 100-ohm cable is what ends up being used for RS485 and CAN.

author: Someone
author_rank: Super Contributor
timestamp: August 02, 2025, 06:42:40 am
content: Quote from: Siwastaja on August 02, 2025, 06:24:04 amSome people/manufacturers do, I have built and used 120 and 124 ohm runs for DMX with Belden cable. But for sure some (possibly most) cables are 110 or 100 ohm differential.

author: uer166
author_rank: Super Contributor
timestamp: August 02, 2025, 07:57:37 am
content: Quote from: Someone on August 02, 2025, 06:42:40 amYou can twist some 24AWG PVC wire with a drill and it's pretty damn close to 120 Ohms. It's what's used in Automotive (unshielded).

author: 5U4GB
author_rank: Super Contributor
timestamp: August 02, 2025, 10:36:33 am
content: Quote from: PGPG on July 31, 2025, 09:46:43 amThat was, presumably, in a relatively clean electrical environment. What happens when you subject it to electrical noise from surrounding wiring/equipment?

author: 5U4GB
author_rank: Super Contributor
timestamp: August 02, 2025, 10:37:52 am
content: Quote from: mikeselectricstuff on July 31, 2025, 11:26:05 amI know you're joking there but I've heard of RS485 run over TPS without any problems. It's remarkably forgiving, which is why you can get away with all manner of ... odd setups.