G_DeMark

Managed Switches

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I have always used a simple Red Lion N-Tron un-managed switch for my PLC controls application (ie: 108TX).  recently i have been trying to understand if there are any advantages in going with the more expensive managed switches.  I understand that the managed switches give you more control over ports and subnet access.  However, I have not had a problem with the un-managed switches so far.  Could anyone give me recommendations or offer more specific examples of what could go wrong when using an un-managed switch from the industrial controls perspective?  I have attached a picture of my simple controls topology for the purpose of discussion.  Thanks.

Capture123.JPG.6355d32d39e753b152162dcaa

 

Edited by G_DeMark

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if you have no problem with un-managed switch, you should stick with it. However, In your current setup using un-managed switch, your PLC will  broadcast data to all channel/ports.

 If your network grows bigger with more nodes/connection or if you start to experience with dropping data or bad connection from time to time ,managed switch could help. The biggest advantage would be you can set up directly communication  between certain devices vs broadcasting to every channel in a unmanged switch case (for example if you use managed  switch your PLC can send  and/or receive only data to and from HMI  by change the setting of the individual ports that PLC and HMI are plugged in).

By doing this, you can reduce data traffic between all devices connecting to the switch and thus more robust network.

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Mikey makes a good point that A-B PLCs are notorious for a lot of broadcast traffic.  To limit this traffic's effect on the rest of the network, managed switches can be used to manage (or limit) the flow of broadcast traffic.

In some cases, a machine that communicates properly on the build floor can suffer network connectivity access problems once the machine is connected to the customer's network.  Or worse yet, intermittent connectivity problems, such as the plant finance department rectifying databases during the late night hours.

Also, for machine builders who like to use the same code over and over, or use the same Ethernet IP addresses (but the customer wants you to renumber to adapt to their network), an OEM builder can keep their addressing scheme by utilizing a managed switch with Port Aliasing.

As your machine design grows, or as controllers are upgraded or standard devices are upgraded, it is very possible that communication latency time (duration between packets of information) can increase and lead to operational problems or data losses.  For instance, if your machine is doing coordinated motion, such as servo motor control, network communication can affect machine performance or product quality.  Until you control YOUR network by adapting in a managed switch.  I see a Cognex vision system and a spindle tool controller.

The attached is an Ethernet-Ethernet IP primer I wrote some years ago.  There are a number of references to managed switches, as well as other Ethernet info, that should be useful in this case.

Primer - Ethernet and Ethernet-IP.doc

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We have a LOT of machines in our facility that have Ethernet communications.  About half are Rockwell's Ethernet/IP, the other half are Profinet.  We don't use managed switches because of our corporate IT requirements (if it's a managed switch, corporate IT in Michigan MUST have full and exclusive control over its configuration).  So far, I've only seen one issue with network traffic being a problem and that was on a machine with a 1756-L55M12 processor talking to a 1794-AENT Flex IO chassis.  It worked great until we plugged it into the corporate network for remote access, at which point it lost comms with the Flex chassis.  We just added a second 1756-ENBT module to the ControlLogix chassis for remote access.

Our biggest networked system is a 1756-L71 with the following:

ENBT #1 connects to the corporate network, through which it's communicating with a data collection PC.

ENBT #2 connects to an unmanaged switch that's connected to everything else:

BL1_NetworkArchitecture.thumb.PNG.307354

We've had zero connectivity issues with this setup, even though we half expected to.  Most of the drives are just being queried for status information but the PLC is doing a lot of calculations and PID control of the Bridle Drives.  If we ever have issues, we will have to get permission from IT to install a managed switch.  We MAY be able to pull it off if we promise (and take concrete steps to ensure) that the managed switch never connects to the IT network.

 

We have another machine that has a 1769-L33ERM doing motion control of a Kinetix 6500 over Ethernet/IP.  For that machine, the same unmanaged switch has the PLC, servo drive, an SMC valve/IO bank, 2 PowerFlex 525 drives, and a Red Lion G308 HMI.  All with zero issues over several  years.

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I have seen people go overboard with managed switches but I generally have one on each significant system I install.  The biggest networking problems I have ever had seem to be with drive systems.  They usually handle a lesser number of packets so swamping the network takes them out first.  Your system is pretty small, but the cost of a managed switch is pretty small compared to the cost of the freaky problems that can occur without one.  When you have a problem with a network it can be very time consuming (ie, downtime).  Time is always expensive. 

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