Need help on choosing cable to connect rs485 devices to PLC

JJ1234

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I am going to daisy chain a couple Modbus devices off of a Click plc using rs 485. There will be three slave devices daisy chained off one another about every 100 to 150 feet. The field devices also require 24V power.

I have a question about the wiring. From all I've read you have to use special low impedance, twisted pair for the communications.

However, I've noticed some of our automated equipment has this bus or junction blocks they use for communications like these from AD:
https://www.automationdirect.com/ad...ion_blocks/m12_socket_(6-port)/zp-jbh65-00-fw

Can I use these to run my rs485 lines?

These have the voltage and signal lines together and recommend pur/pvc cable or the basic SJW stuff.

Cant really find this out from google.

Thanks.
 
Those are essentially breakout boxes for digial and analog sensors/devices. I wouldn't use them for RS-485 comms.


How much 24V power do your field devices require? Unless it is over one amp or so, you should be fine with just daisy chaining the devices with a standard 2 pair cable like this one.


You might have to go up to 600V insulation depending on your installation.
 
Thank you for the reply.
Field devices are under 100 milliamps.
So I don't need the low impedance stuff?
Can I use that cable you recommended to run power and the communications in the same line? It won't interfere with signal lines?
The devices have m5 connectors so I think i need at least a junction box to daisy chain.
 
Welcome to the forum.The m5 connectors you mention look like DeviceNet M12 connectors with 5 pins. What is the baud rate for this Modbus network? I would recommend low impedance cables such as those used in DeviceNet or Profibus unless the baud rate is really slow, like less than 100Kbaud, say. I think you can use DeviceNet media and you can use T-connectors for daisy-chain. For your network being RS485 I do recommend you to use terminator resistors. In case of RS485 standard the resistor value is 150 OHM.
I put quickly some pictures that can give you an idea of what I mean below. You need to check the pinout. In case of DeviceNet since it is based CAN the value of the resistor is 120 OHM. The problem is I am not sure you can find 150 OHM M12-5 pin terminators for the pinout and key hole shown in the diagrams you linked. Since the distance is not too long and if the baud rate is not above 1 MBaud perhaps the DeviceNet resistors could be OK in spite of the difference. It would be too geeky but a scope trace to see if the signal looks healthy would be a plus, especially for higher baud rates.

Profibus DP, which is based on RS485 so the terminator resistors have the required value for RS485, does have an option for power lines, but the M12 connector key hole is different for Profinet and DeviceNet.

2021-05-11_M12_Connector.png
 
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For RS485 daisy chain is is recommended that the termination is done directly at the device not a terminal (or connector) preceding it.

rs485 termination.png
 
I think you will need the "T" connectors. With M12-5 pin terminators connected to the T connector and connecting the T connector directly to the device I think you can achieve a correct wiring. How fast does the process require to update the data. I would recommend you use the slowest baud rate that your process allows. The number of nodes is 4 (including the Modbus RTU master) and the overall length is 300 ft. The fastest baud rate is 115200 baud. To be on the safe side I would recommend you start testing with slower baud rates.
 
I am going to daisy chain a couple Modbus devices off of a Click plc using rs 485. There will be three slave devices daisy chained off one another about every 100 to 150 feet. The field devices also require 24V power.

I have a question about the wiring. From all I've read you have to use special low impedance, twisted pair for the communications.

However, I've noticed some of our automated equipment has this bus or junction blocks they use for communications like these from AD:
https://www.automationdirect.com/ad...ion_blocks/m12_socket_(6-port)/zp-jbh65-00-fw

Can I use these to run my rs485 lines?

These have the voltage and signal lines together and recommend pur/pvc cable or the basic SJW stuff.

Cant really find this out from google.

Thanks.

Ror RS485 Belden 9841 or 3106A or 9842 depending on your slaves, i would run 24V seperate
 
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Ror RS485 Belden 9841 or 3106A or 9842 depending on your slaves, i would run 24V seperate
Why would you not use 24 VDC on the same 5-line cable? DeviceNet has been around for almost 30 years using power and signal over standard DeviceNet media. The installations that I have had to help customers fix were in most cases not using the correct DeviceNet media.
 
Why would you not use 24 VDC on the same 5-line cable? DeviceNet has been around for almost 30 years using power and signal over standard DeviceNet media. The installations that I have had to help customers fix were in most cases not using the correct DeviceNet media.

Depends on the application and architecture and the slave 24V power requirements really, a load of slaves all with 500mA requirements over 300 feet of cable isn't going to work very well down a Belden....
 
In the case of DeviceNet, without drops it is possible to support up to 8 Amps for a line. There are only three devices, hence way below the recommendation of a networks such as DeviceNet.
 

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