Modbus cable types?

[Ausman]

Hi all,

 

I know this is a dumb question, but I am considering using Modbus on a job.  I have never had to use it before, with components normally fairly close together and direct IO being perfectly adequate.  I have read far and wide about cable requirements and need some advice from those who regularly use it.

 

To do a run of 40m max, am I going to get away with Cat5 in a fairly noise-free environment?  Using one pair as signals and another commoned as the ground, with the other two pairs perhaps being used as 24dc digital IOs or as spares.  I guess I am really asking what types of cables have people used for what distances?  V130 as controller, speed isn't that important but it has to be reliable.

 

Cheers from Mr Modbus-Know-Nothing-but-Learning-Fast!

[Paul Rylatt]

For running the V130 as a MODBUS RTU slave over RS-485, my preferred method is shielded-twisted-pair cables with termination resistors at each end and the shield grounded at only 1 end, usually the Master end.  These cables must be rated for the maximum voltage present in the installation.  You will need sufficient conductors in the cable to provide all the required signals that can be found listed on the V130 technical documents.  I would not recommend using this cable for anything else.

 

If you are running the V130 as a MODBUS TCP/IP slave over Ethernet, use a CAT 5e or CAT 6 cable.  This should be either straight through or crossover, depending upon the hub/switch you are using.  Most modern hubs and switches can detect either and self-configure for this.  Again, only use this cable for your communications.

[Ausman]

Thanks Paul,

 

I realise now I should have been more specific, sorry.  It's Modbus RTU on 485 with the 130 as a master getting info from various modules by other makers.

 

Call me cheap!, but it's just that I've been a bit amazed at the prices of 'proper' 485 STP 1.5 cabling and was wondering if I really needed it for the runs I might be doing, or might be able to get away with a cheaper alternative.  I've read of lots of people using some of the twisted pairs on ethernet cables quite successfully.

 

thanks for the help,

 

Aus

[Gabriel Franco]

I do not recommend UPT, even in a noise-free environment as you mentioned. I´ve used UTP in the past, about 50 meters with a lot of trouble and loosing communications too often. I have tied all spare wires to ground to avoid antenna effect.

 

If not possible to use RS485 cable, which is preferable, you may try SFTP, but do not use spare cables for another use than communication, as Paul already wrote.

[Emil]

Hi,

First - Modbus is communication protocol. The question "Which cable to use with Modbus" seems to me like "Which kind of fuel to fill in my horse?"

Let's assume Modbus is running on RS485. In this case the question needs to be "Which cable to use for RS485"

... and the answer is:

Shielded twisted pair with impedance between 100 and 120 ohms.

Some producers offer special cables for RS485, which, of course is the best.

Some recommended cables in attachment.

 

RS485 recommended cables.bmp

[Joe Tauser]

Don't use regular CAT5 - Gabriel is correct.  I had an RS485 installation dumped on me where the customer pulled 100m or so of unshielded CAT5 between nodes and we've never been able to get reliable communication, even using RS485 repeater/isolators from B&B.

 

I realize that the wire vendors are very proud of their RS485 cable.  If you can get your hands on a 500' spool of CAT5e (which is shielded) you can do some testing with that length in your shop before you go to the field.  I'd highly recommend getting familiar with the nuances of RS485 Modbus before I went to startup a system.

 

Joe T.

[Ausman]

Hi all,

 

thanks very much to everyone for their input.  I love your comment, Joe, about wire vendors being "proud".

 

Also, about "getting familiar" with modbus, I am doing lots of workbench mockups for my knowledge increase, before committing it's use to the project.  If I decide to go the bus way I'll bite the bullet and invest in some proper cable for the final install as it looks to be the only surefire thing.

 

Cheers from Aus,