RS-485 via RJ-45 ???

[TwoBlocked]

So I am upgrading a PLC cabinet. The decision was made to control a couple VFD pumps with modbus on a RS-485 network. I knew this Allen-Bradley Powerflex 400 had onboard RS-485, so I use a spare analog cable and pull in a 4 conductor Belden like usual. THEN I look to see where to land the wires. What the ... I have to use the RJ-45 port, with the SHIELD going to a screw terminal? I call up the manual and it says I have to use only positions 4 & 5 on the RJ-45 plug because other positions are used for other functions. The manual also gives the typical RS-485 layout, including daisy chaining and end of line resistors. Ok ... now how would I daisy chain with RJ-45 connectors let alone install EOLs ??? Not to mention landing the SHIELD at a field device instead of just the PLC cabinet. And gee wizz! I have CAT 5E cable, but it doesn't have a shield anyway. And the comm port on the controller isn't RJ-45, it's wire terminals!

So I did what I could with what I had. Used the 4 conductor Belden to pull CAT 5E, put on a connector with just two wires in the positions called for, kept the solid green to go to the "shield" terminal (I'd call it 0 volt reference ...), and landed these three flimsy 24 ga solid wires on the controller's comm port. Felt disgusted so put in just 10 hours instead of the usual 12.

Who else has dealt with this? Is there a better way?

 

[LarryFine]

I would have to see this situation with my own eyes to make any useful comments.

(Even then, there's no guarantee on the "useful." )

 

[TwoBlocked]

I would have to see this situation with my own eyes to make any useful comments.

(Even then, there's no guarantee on the "useful." )

 

[TwoBlocked]

 

[LarryFine]

Okay, that helps. In my opinion, CAT-5 is the wrong type of cable for this purpose.

I would use a 2-conductor-plus-shield cable, with a foil shield with a drain wire.

 

[TwoBlocked]

Okay, that helps. In my opinion, CAT-5 is the wrong type of cable for this purpose.

I would use a 2-conductor-plus-shield cable, with a foil shield with a drain wire.

View attachment 2573709

But how do you put it in a RJ-45 connector? It has to be about 24 gauge and you gotta get it into positions 4&5.

 

[LarryFine]

But how do you put it in a RJ-45 connector? It has to be about 24 gauge and you gotta get it into positions 4&5.

Get plugs made for stranded wire, and use the largest wire size the plug takes.

Inserting two wires in the two center-most positions should be easy.

https://www.cableorganizer.com/learning-center/articles/rj45-plugs-boots.php

 

[hbiss]

https://www.amazon.com/Poyiccot-Net...or+with+screw+terminals&qid=1727823821&sr=8-9

-Hal

 

[LarryFine]

https://www.amazon.com/Poyiccot-Network-Ethernet-Terminal-Converter/dp/B07LCJ4V5C/ref=sr_1_9?adgrpid=1342504727596859&dib=eyJ2IjoiMSJ9.Q_rqVaSgiyRlYSwJCqARslt-Ea-JxoklCPe37FFKNdXJX_sqkM7VbRxsWhGcIVafEoYjIVozmVbHuGitJrDb51AabSmVZF8z2-0MiyQPIB-Aw2ufk26SWxC6BzooR-ON9gdwMjRtwF8kedC3mqU3NRO6luhOEghOn5JLutS3iBNiBZGTH1C1_Dm7bXCW6u04ip7l2PR-JjdnvfX7rzqgZSFHOD1PrnxrsygGPtrW8W8.Ht7DFa3EWIMLXCilJ_GcgT9UB6FJKdfCdNcrrNrdHWs&dib_tag=se&hvadid=83906731234106&hvbmt=be&hvdev=c&hvlocphy=99543&hvnetw=o&hvqmt=e&hvtargid=kwd-83907225559816:loc-190&hydadcr=18013_13444571&keywords=rj45+connector+with+screw+terminals&qid=1727823821&sr=8-9

Those would work, too. I see them as unnecessary, but useful if you want to use larger wire.

 

[TwoBlocked]

https://www.amazon.com/Poyiccot-Network-Ethernet-Terminal-Converter/dp/B07LCJ4V5C/ref=sr_1_9?adgrpid=1342504727596859&dib=eyJ2IjoiMSJ9.Q_rqVaSgiyRlYSwJCqARslt-Ea-JxoklCPe37FFKNdXJX_sqkM7VbRxsWhGcIVafEoYjIVozmVbHuGitJrDb51AabSmVZF8z2-0MiyQPIB-Aw2ufk26SWxC6BzooR-ON9gdwMjRtwF8kedC3mqU3NRO6luhOEghOn5JLutS3iBNiBZGTH1C1_Dm7bXCW6u04ip7l2PR-JjdnvfX7rzqgZSFHOD1PrnxrsygGPtrW8W8.Ht7DFa3EWIMLXCilJ_GcgT9UB6FJKdfCdNcrrNrdHWs&dib_tag=se&hvadid=83906731234106&hvbmt=be&hvdev=c&hvlocphy=99543&hvnetw=o&hvqmt=e&hvtargid=kwd-83907225559816:loc-190&hydadcr=18013_13444571&keywords=rj45+connector+with+screw+terminals&qid=1727823821&sr=8-9

-Hal

Thanks, Hal! I was just wondering if there was such a thing. Now I'll have to pull that 4 conductor belden back through. Arrrgh

 

[LarryFine]

Then what will you use as the shield? Remember, it's not an EGC.

It should be terminated at only one end, clipped at the jacket at the other end.

 

[petersonra]

Plenty of ri45 din rail mount breakout adaptors available.

 

[TwoBlocked]

Then what will you use as the shield? Remember, it's not an EGC.

It should be terminated at only one end, clipped at the jacket at the other end.

What I have been doing is using 4 conductor "Belden" (my generic name for instrumentation cable). Red for positive, black for negative, green for 0 volt reference (or ground if there is no actual 0 volt ref), drain wire for shield, and white for spare. Green gets landed at both ends when possible. Drain wire at the controller, often along with the ground.

 

[LarryFine]

What I have been doing is using 4 conductor "Belden" (my generic name for instrumentation cable). Red for positive, black for negative, green for 0 volt reference (or ground if there is no actual 0 volt ref), drain wire for shield, and white for spare. Green gets landed at both ends when possible. Drain wire at the controller, often along with the ground.

I didn't know your cable was shielded.

I recommend omitting the green connections. Ground loops.

You're defeating the purpose of bonding the shield at only one end.

 

[TwoBlocked]

I didn't know your cable was shielded.

I recommend omitting the green connections. Ground loops.

You're defeating the purpose of bonding the shield at only one end.

I know what you mean. But when I read about how RS-485 is "supposed" to be wired and work, there needs to be a 0 volt reference. A terminal specifically for that is often designated. In those cases I do use it. That is when: "Green gets landed at both ends when possible."

 

[tallgirl]

I know what you mean. But when I read about how RS-485 is "supposed" to be wired and work, there needs to be a 0 volt reference. A terminal specifically for that is often designated. In those cases I do use it. That is when: "Green gets landed at both ends when possible."

There absolutely has to be a common for RS-485.

 

[petersonra]

There absolutely has to be a common for RS-485.

There is supposed to be but a lot of implementations don't bother.

 

[tallgirl]

There is supposed to be but a lot of implementations don't bother.

Well … it’s likely they are using the power supply common / negative / neutral for that, or that it just gets used anyway. The electrons gotta go somewhere.

 

[rlundsrud]

You should use a low capacitance cable (<15pF) for rs-485. I also recommend using 9600 baud for modbus communication. FYI, We typically don't use terminating resistors for modbus. This would be more practical for your application as there wouldn't be an easy way to put one on the end with the RJ-45 connector and you must use them on both ends if you do use them.

 

[tallgirl]

You should use a low capacitance cable (<15pF) for rs-485. I also recommend using 9600 baud for modbus communication. FYI, We typically don't use terminating resistors for modbus. This would be more practical for your application as there wouldn't be an easy way to put one on the end with the RJ-45 connector and you must use them on both ends if you do use them.

9600 baud is for wimps. I've not worked on microcontrollers that can go over 430kBaud, but I have written software, and tested it out with actual cables which can go that fast!

Terminating resistors aren't as important at 9600 baud, because the 3.5 character timer is so god-awful long (3.6 milliseconds), but if you are running it at more reasonable speeds like 38.4 or 115.2 kBaud you absolutely must terminate and absolute must use low capacitance cables.