MODBUS RTU T junction boxes - Page 1
Forum section: electronics / projects

author: mikerj
author_rank: Super Contributor
timestamp: September 19, 2024, 08:42:10 am
content: Modbus RTU is supposed to be wired as a daisy chain, and this is simple within an electrical cabinet where everything is using screw connections. However there are numerous external MODBUS enabled devices (e.g. combined sensor/transmitters) that provide no facility to enable this i.e. two connection ports, two cable glands etc. The only practical way to deal with this is to Tee the RS485 cable and create as short a stub as possible. Are there any commercial solutions to help with this for typical three core+screen RS485 cables, that maintain the shielding and provide water resistance?

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: September 19, 2024, 10:13:42 am
content: Hi mikerj,something like this:https://www.buerklin.com/de/p/phoenix-contact/interfacesteckverbinder-bus-steckverbinder/2708999/64P7010/https://www.buerklin.com/de/p/phoenix-contact/interfacesteckverbinder-bus-steckverbinder/2902325/64P7001/https://www.buerklin.com/de/p/weidmueller/interfacesteckverbinder-bus-steckverbinder/1550590000/51P6397/.....prices are high.Good luck

author: mikerj
author_rank: Super Contributor
timestamp: September 20, 2024, 09:39:47 am
content: I don't need the D-Sub connector, and it's not important to have the cables connectorised for easy removal (though I don't mind if they are). I just want to splice into a three core+screen cable whilst maintaining the screening as far as possible and having it water proof (cable glands). I was originally planning to manually splice them, using solder and glue lined heatshrink but it's a fiddly operation and I'd prefer something easier/faster with screw terminals if it's available. Seems like this must be a problem many industrial automation folk have run into.EDIT: I was using "modbus" in my searches, changing this to RS485 yielded this which is more or less what I was after. No price shown of course, probably ridiculous.https://www.controltechnology.com/products/2500-Series-Classic/Remote-Base-Controllers/2500-TAP-RS485-Network-Tap

author: ajb
author_rank: Super Contributor
timestamp: September 21, 2024, 06:07:04 pm
content: Quote from: mikerj on September 20, 2024, 09:39:47 amThat looks a lot like a standard conduit box likethiswith cable glands stuck into the ends. If you have something similar available in the UK it wouldn't be hard to put something like that together, as long as you can find glands with compatible threads. In the US, NPT or NPS threads are common on conduit fittings and cable glands -- as well as plumbing fittings, so it's possible to cobble together all sorts of monstrosities.

author: mikerj
author_rank: Super Contributor
timestamp: September 22, 2024, 11:58:44 am
content: Great spot, it does look very much like the same housing. The inspection tees I have seen in the UK are either plastic and designed for glued joints(no threaded ends), or heavy galvanized metal for steel conduit. If I'm going to roll my own a small cast aluminium project box with a gasket would likely be easier.

author: 5U4GB
author_rank: Super Contributor
timestamp: September 22, 2024, 12:02:48 pm
content: I just use these (google "lever wire connector" on Aliexpress):Then put them inside whatever waterproof junction box you prefer.

author: victorhooi
author_rank: Regular Contributor
timestamp: January 14, 2025, 04:47:45 am
content: I have a pretty similar problem to solve - i.e. how to physically daisy-chain a bunch of Modbus/RS485 sensors together. For my use-case, this is for various types of sensors to monitor a greenhouse (e.g. soil sensors, temperature/humidity sensors, PAR light sensors etc.).In my case, I'm using CAT6 cabling, with RJ45 connectors for my Modbus network (since that's what I have lying around, and can crimp/terminate those easily).The vendors for theModbus soil sensorI bought actually sell this RJ45 (technically 8P8C, I guess) T-interconect:https://www.tindie.com/products/miceuz/rs485-t-interconnect/which seems to be their own custom PCB. However, I'm seeing if there's any other factory-made options I can get locally as well.Looking only, you can also get various RJ45 (i.e. 8P8C) "splitters"/adapters -1,2,3,4.However, they're designed for computer networking, and if you look at the wiring diagrams on the datasheet - they seem to do some weird trickery with the 4 x wire pairs, and give you 2 x 100 Mbps connections:so I assume they wouldn't work here for Modbus/RS485.I've found a few RJ45 splitters which seem to be for telecom use - which I think are "pass through" (or straight-through), in which all pins are passed-through to all the terminals -1,2,3,4.Would anybody be able to confirm that these should work?Eaton also seems to make one specifically for Modbus for RJ45/CAT cabling, but it's stupidly expensive - i.e. $50 per connector vs $5. (datasheet,RS Components link).As well as several others that seem to be Modbus (or Modbus RTU) specific, so I assume should work - but are a lot more expensive, and not as common or widely available -1,2,3,4.There's other people online, who are also successfully using these RJ45-style splitters for daisy-chaining sensors - so I'm assuming there must be some types/models that will work:The other thing to consider is waterproofing - in my case, the sensors are outdoors, and the connections will be exposed to the elements - I don't want to have to put each connection into it's own separate waterproof box. Do you think just putting dielectric grease in the RJ45 (8P8C) port will be enough?Apart from the RJ45 splitters, a second way I thought of might be to use those twin-wire end-ferrules (e.g.Knipex examples), and then put those ferrules directly into say, a WAGO 221 inline connector?Also, for my own reference, found some useful links on Modbus wiring - I'll need to read/digest these later, particularly the TI/Analog ones =).https://electrical-engineering-portal.com/correct-cabling-modbus-rs485https://www.ti.com/lit/an/snla049b/snla049b.pdfhttps://www.analog.com/en/resources/technical-articles/rs485-cable-specification-guide--maxim-integrated.htmlI still need to find a way to add the termination resistors to my network - how to people tend to physically attach the resistors to the cables? Do they just solder them on? Or is there a quicker way to do it in the field?

author: 5U4GB
author_rank: Super Contributor
timestamp: January 16, 2025, 10:43:59 am
content: Quote from: victorhooi on January 14, 2025, 04:47:45 amThat's a kludge for running two lots of 100Mbps ethernet down a single wire. Those things are all over your favourite crapvendor sites as a magical means of getting two logical ethernet runs down a single physical ethernet run.There are, needless to say, a number of blog posts and Youtube videos which express opinions on the sanity of anyone doing this.

author: 5U4GB
author_rank: Super Contributor
timestamp: January 16, 2025, 10:50:25 am
content: Quote from: victorhooi on January 14, 2025, 04:47:45 amThat's what I use for the Chinese Wago clones (well, variants) I mentioned in my previous post, crimp a ferrule on the end of each wire for which the collar mostly fills the hole in the C-Wago. However you're still going to have to use some sort of waterproof enclosure, neither the Wagos nor C-Wagos are anywhere near moisture-tight.Quote from: victorhooi on January 14, 2025, 04:47:45 amWith the C-Wagos for the last one in the chain I put the EOL resistor across the terminals where the A/B wires would otherwise continue on.

author: 5U4GB
author_rank: Super Contributor
timestamp: January 16, 2025, 11:10:29 am
content: Here's an example of one in action, in this case for 12V power at the start of the run.I don't have one of anything along the run itself since they're all inside IP65 boxes.