ControlNet cabling

[Bobshep]

Good evening gents, can anyone tell me whether it is necessary to use A-B controlnet taps rather than simply running the trunk cable thro a standard BNC T-connector at each drop?

 

[rdrast]

Necessary?
Not if you don't care about flaky comms issues randomly in the middle of the night.
Not if you absolutely guarantee that the BNC T's do not contact ground at any time ever.
Not if you ever have trouble (with any connected device) that requires a call to Rockwell or a service call from them.

/shrug. Taking shortcuts on a live system is always a random shot in the dark. Sure, they are not inexpensive, but compared to the hardware they connect to they add only a small fraction of the cost to a system.

 

[Ken Roach]

The ControlNet taps aren't just copper and plastic; there's a little inductor and capacitor inside that "tap off" the ControlNet signal so that it will be at maximum amplitude precisely 1 meter from the junction.

A ControlNet network built with incorrect co-ax or without actual ControlNet taps may function correctly if it is very short, but RA will definitely not support it.

 

[kbpatton]

A few years back, when the AB Control-Net Flex I/O was popular, we had a bunch of presses installed with this I/O. For cost savings, all of the cheaper taps were used and we had a lot of issues. They were not all the fault of the cheaper taps but they definitely contributed. All of them were eventually changed to to AB's.

 

[dmroeder]

Another vote for buying the taps. Troubleshooting comms issues = no fun.

 

[MannyM]

One golden rule in PLC hardware, don't mix non-tested with tested known hardware. It doesn't work in the long run.

 

[muusic_man]

I am in Taiwan right now starting up a new plant. There is a lot of ControlNet. Believe me when I tell you that you want all AB stuff and make sure the cable is double shielded and the BNC ends are properly installed. My biggest headaches to date have came from ControlNet.

 

[Bobshep]

ControlNet Taps

I am indebted to all of you for your overwhelming vote on this matter, particularly the less sarcastic ones. So A-B taps it is. Many thanks to all

BobShep

 

[rdrast]

/shrug. I answered your question, which in one form or another has been asked dozens if not hundreds of times.

It matters not what communications topology you are using, RS-485, ControlNet, DeviceNet, EtherNet, Profibus, even SneakerNet. If the specification or manufacturer recommends specific cabling, hardware, connectors, USE IT. If there is not spec, then use the absolute best quality componants you can, and be smart. Do not crimp solid wire into an RJ-45 for example, plug it into a typical 'Office Store' type switch or hub, and expect it to work. Sure, it probably will. For a week, a month, maybe even a year, before you begin to get very strange, intermittant errors.

I've seen more installations be labeled as "S**tty jobs, s**tty company" simply because someone decided to save a couple bucks and use the shielded cable they had laying on the shelf, rather then the specified cable.

Not only for communications either. Not taking a hard look at encoder cables can also get you into trouble. Encoder type, driving capacity, cable capacitance, frequency, length, and load can all affect if an encoder is going to work or not, or again, if it is going to seem like it works mostly.

1000 perfect successes are always forgotten. It's the one, singular screwup that you are remembered for.

If someone deviates from specifications, especially if it is from pressure from a client or management, make sure that someone signs a piece of paper acknowledging that they understand that performance, operation, and or safety might be compromised by "XYZ" deviation from specification.

Later on, you at least have some evidence.

 

[MannyM]

Could not agree more with rdrast.