Managed Switches

[PvillePiper 0]

I need some help with this. A customer has come to us and decided to upgrade some machines that we sold him. These machines use Impact drives to control some of the devices on the machine and Bulletin 160s to control others. These two drives have been discontinued by Allen-Bradley. The current setup of the machines includes SLC5/04 or SLC5/05 processors, DeviceNet comms for the drives, and PanelView 1000 Standard displays. As a technology update the customer wishes to replace the SCL500 with Compact Logic (our current standard), the PanelView standards with PanelView Plus (our current standard) and the B160 to PowerFlex 40(our current standard). The monkey in the works is that he wants to change the drives to AB PowerFlex 755 instead of our current standard of Rexroth Servos and the control them with Ethernet. While I think the servo is a much better product than the PowerFlex 755 drives I don’t have a major issue with this. The main issue is with the Ethernet comms. We have done very little with this and I have seen others do it very poorly which resulted in lots of performance issues. Be that as it may, we may have to go with the Ethernet controls. My main issue is segregating the drives which have time critical information from a plant wide system. Looking into the issue I have come up with 3 basic control setups: 1. Use a Compact Logix L43 processor with 2 Ethernet cards. This would be very expensive as the controller costs us more and the controller has no onboard Ethernet port hence the need for the 2 cards. It does fit the need very well though and the techs would have no issues with installing these in the field. 2. Use a Compact Logix L32E processor with a 1761 NET-ENI to do the plant comms and connect the onboard Ethernet port to an unmanaged switch to handle drive communications, HMI and programming. This has an advantage of being a setup that my techs are familiar with. The disadvantage is the plant tech may just plug into an available port on the switch and bypass the ENI module and it ties up the serial port. 3. Use a Compact Logix L32E processor with a managed switch to handle everything. This is what I am leaning towards as it appears to have many pluses. The downside is that I don’t consider myself very Ethernet savvy and my techs are positively ignorant of that. So a big learning curve here. Oh, and did I mention that it will all have a short lead time as well… SOP around here. So with that all being said I have a number of questions about managed switches. 1. Startup in the plant may not include hooking into the plant wide data collection system if any exists. Do I need to setup plant communications with a VLAN and is there “default” way to do this so the customer can hookup at a later date? 2. If my initial setup of the drives, touchscreen and controller is say 192.168.1.XXX with a subnet mask of 255.255.255.0 can a plant wide system access the controller using a 10.10.1.xxx with a subnet mask of 255.255.255.0 using a managed switch? 3. Can I setup VLANs on managed switches to prioritize certain nodes and would there be any real benefit to that? For example: The PowerFlex 70 drives receive the highest priority, PowerFlex 40 is next and HMI/Plant comms is lowest. 4. I will have an average of 6 PowerFlex 40s, would there be any advantage/disadvantage to putting on their own unmanaged switch which is connected to a port on the managed switch or should I source a managed switch big enough to run all of the devices. The typical setup would be 6 to 8 PowerFlex 40 drives, 3 PowerFlex 755 drives, controller, 2 programming ports, 1 port for plant wide access and an HMI port. 5. How difficult will setup be for an average tech who programs PLCs, setups drives and DeviceNet

[Ken Roach 194]

I'm glad you're examining the issue in detail; I can't tell you how many systems I've seen where the Ethernet network was designed by digging around the server closet for old cables and desktop switches. You're right to discard the Net-ENI/CompactLogix variation, and to discard the two-Ethernet-module option. But take a step back and consider those PowerFlex 755's and how they work versus a PowerFlex 40 or an old DeviceNet drive. The PowerFlex 40, by the way, can't update any faster than about 35 milliseconds on any network because of the "DSI" RS-485 bottleneck. I set mine at 100 ms RPI at the fastest and never try to use them for rapid control over Ethernet. The PowerFlex 755 has a DPI interface that can run the ordinary "Command / Reference / Status / Feedback" connection fast, and respond to changes in about 10 ms (maybe faster with the onboard port). This is comparable to the PowerFlex 70 and 700 drives. Any CompactLogix or ControlLogix with EtherNet/IP can use the drive in this way. But the 755 can also be a ControlLogix motion axis, using "CIP Motion" over Ethernet protocol, and using the coordinated motion axis definitions and instruction set in the ControlLogix. This is the successor to SERCOS for multi-axis motion control on the Logix platform. If this is what you expect to do, you need ControlLogix 1756-series CPU and the 1756-EN2T modules. The CompactLogix L2x and L3x controllers don't support motion instructions at all, and the CompactLogix L4x (which do support the motion instruction set) don't support motion control over Ethernet. You're also going to want to consult RA on switch selection for that mode. If you are only using the drives ordinary I/O module Command/Status interface, just use a 1769-L32E or 1769-L35E. I strongly recommend an ordinary managed switch and a port-forwarding firewall to keep the automation system separate from the rest of the world. I prefer N-Tron 508TX-A or 708TX switches for their trucklike robustness. The Phoenix Contact "Lean Managed" products work nicely too and are price competitive. I just don't like dealing with the uncertainty and complexity of enterprise networks when I don't have to. If I put in a port-forwarding firewall, I can turn by back on the enterprise network and not worry about it. My favorite port-forwarding firewall is actually the old linksys BEFSR41 home router. You can get them for ten bucks used, and if they've lasted this long they will last forever. As long as you put in a proper managed switch with IGMP Snooping and Querying (another vote for the 508TX-A and 708TX, or Phoenix LM) they will never see the fast multicast data and will run fine. If you put them on an unmanaged network they will see that multicast data and bog down to the point of crashing immediately. There are DIN-rail mounted, small industrial firewalls too. Hirschmann makes one, Spectrum Controls sells the eWon thing... Comtrol might have one. I actually prefer to buy little router boxes and install the open-source m0n0wall firewall firmware on them. The handful that I have deployed have run for years without being touched, and the web-based console is very simple for even the most rank beginner to set up.

[PvillePiper 0]

Thanks Ken, I am still a little confused Realistically the PowerFlex 40 drives are not time critical and are used only in non-demanding applications. The PowerFlex 755 drives will be semi-time critical and will not need to use a full blown Control Logix setup to do their work. I would be interested to see if the Control Logix setup will make the drives more servo like, AB says this but they say a lot of things. Again, the main issue is the desire to segregate out the drives from a noisy, unplanned and ad hoc network that will have a great deal of useless traffic broadcasting and potentially interfering with drive communications. All the drives will be in the same enclosure as the controller and we have very good wiring practices that were implemented to keep electrical noise to an absolute minimum in the panel. This noise reduction made a huge difference in the reliability of the DeviceNet network and I feel it will keep us from having issues with a local Ethernet network. I have done a bit of research and most of what I can find is not tailored to machine level control so it is a little confusing. From my research it seems like I only need a managed switch if my understanding is correct: 1. Connections beyond the plant floor will be out of my purview. This is a machine base network the will be connected to a plant wide system on the floor level so things like routers and firewalls are not needed. 2. By creating a VLAN network for the drives, another for HMI and one for plant wide communications it would appear that I can segregate out the traffic. 3. By turning on IGMP snooping6 I can keep the traffic from flooding each VLAN and keep down the traffic to the processor. 4. By using QoS I can give a higher priority to the drive traffic. If these assumptions are correct would I want to place the drives onto an unmanaged switch to better facilitate the VLAN? Would there be any gain by putting the 755 on one switch and the 40 on another? Is QoS even necessary? If a drive fails what steps are required to bring that device back onto a managed network? Do you know of a good machine based Ethernet white paper or guide that might help?

[PvillePiper 0]

Okay, got some questions answered. N-TRON had some good info for beginners like me. The PowerFlex 40 does not support unicast and should not be placed on an unmanaged switch. The PowerFlex 755 does support unicast and can be placed on an unmanaged switch. As usual my AB tech has recommended the Stratix 6000 and these can be linked together so that I can get enough ports to accomplish what it need which would likely be a maximum of 14 ports. But here’s the kicker… He wants me to put a Statix 8300 in between the plant comms and drives so that the drives can be placed on a different subnet than plant network. One small problem… The 8300 has a boot time of 3 minutes! WTF! Who thinks that is a good idea. One of the things that the plant comms will have is the interlocks for stopping and starting the line feeding my machine. Now I will have to bring in an outside power supply to keep the dang thing hot so it doesn’t have to be powered down. Just what I needed… More complexity. Getting customers to provide such things is always problematic. I have always tried to design our machines to install with as little interfacing with the customer as possible and minimize the outside factors that can cause performance degradation. Things like dedicated phone lines for a modem in the panel rarely appear even if the customer has requested the device in the panel. I am starting to think that Ethernet controls will require a lot of interfacing with the customer, many of whom have very little knowledge of what is truly needed for a good clean install. The need to protect my machine from outside influences is obvious and I am getting very frustrated with this whole thing. On top of that my boss wants to skip that whole thing, put in a managed switch such as the N-TRON 516TX-A and use the Net-ENI to communicate with the plant. I am not sure this is a good thing. Wonderful… just got the quote for the Stratix parts and they are double whet he quoted me for comparable N-Tron pieces… Edited 13 Oct 2011 by PvillePiper

[rkukl 0]

Hi, I have been using Stratix 8000 switches and also do not like the long boot time. Don’t believe 3 minutes it more like 5+. I discussed this my IT guy he was so what’s the big deal? We use HP switches/routers and that’s normal. If you don’t like the long boot then I’ll get you a UPS he says. So I now have my 8000s fed from two sources so they stay up when the panel power is killed. And then a UPS on the dedicated power circuit feed. Now I know it sounds like I am complaining well, cause I am. But, I do like the switches and they have performed well. We have set up 4 ports on our Plant machinery VLAN and the other are on the machine subnet. As I am the ultimate end user a few bucks up front isn’t as big a deal so I use all pre-made Panduit Cat 6E shielded cables and run all communications in conduit. When in the panels I keep the Enet separated as best as possible and use the metal dividers that Panduit supplies to keep separation. Never had any problems or lost comms has been very robust. As for 755s I have use 3 of them to control some servo motors on a piece of OEM equipment took a bit of time to get all the set up but they work fine. They are only running rolls in a stack so no motion instructions just typical rotary stuff. Being able to keep the servos was a big, big plus. To get the equivalent AC motors would have required much larger physical size and changing gearboxes. So for me they worked well. Also have about 12 753s and those have performed well. Have mainly chose to got the 750series for the Safe off capabilities. Hope this helps?

[Tim Williams 1]

For what it is worth, while I can't say I like the 3 minute boot time, I don't really see it as a problem. I've been running our Stratix 8300's for nearly a year without once having it reboot because of flaky power. No UPS and only one power supply connected. It even ran through a bunch of brown outs that caused PCs around the building to reboot.