Saturday, June 06, 2009

Cellular to Redundant Rockwell ControlLogix

Can RSLinx access a redundant pair via cellular?

A Pair of Rockwell Automation ControlLogix racks with SRM Module and dual ENBT's will share a pair of IP addresses. One IP address is "the primary", and the other IP is "the backup" (if/when it is online). When the ENBTs switch role, they will issue the requisite Gratuitous ARPs to cause other local Ethernet devices (like a Digi cellular gateway) to update their ARP cache, thus comprehending that the IP-to-MAC address mapping has changed. Thus a NAT/Router forwarding to the primary IP should handle the fail-over with only modest bumps.

Config Details
A user desiring RSLinx (and RSLogix or RSView etc) to access a remote Rockwell ControlLogix (any RA/AB PLC) will be doing what the industry calls "Mobile-Terminated" access. The user needs to arrange a cell plan which offers either a fixed IP address to target, or at least a Dynamic DNS name to target (like or This is NOT what you obtain with an iPhone or personal air-card data plan. Those will have private IPs which only permit outgoing connections - called "Mobile-Originated". That was a buzzword lesson - expect to be asked about those two terms when you ask about cellular data plans!

So once you can arrange your targetable IP or DNS name, you need a cellular router such as one of the Digi Connect WAN family products. My favorite model today is the ConnectPort X4, but that has large memory for Python programming, wireless mesh and other goodies you won't need to link up your Rockwell PLC (Hey, I said it was MY favorite - doesn't mean it has to be yours!)

Note that contrary to folklore or urban legend, all cellular devices need certification to work on a system - even GSM devices. Many small suppliers get around this by including fine-print that say the device buyer is responsible to arrange such legalities, and since you (the buyer) don't read such fine print the salesperson will just say "Heck, it's GSM - so it is allowed everywhere world wide!" Deal with this issue as you see fit, but Digi has more formal certs in more countries worldwide than any of the other industrial players.

But back on subject, when the gateway comes up, it is assigned your known IP or DNS name. This is exactly how your home or business DSL/T-line line works. Yet when RSLinx tries to talk to the gateway on Ethernet/IP's well-known TCP port of 44818, the gateway will reject the connection as a weird attempt at hacking. You need to instruct the gateway:
  1. to not reject the Ethernet/IP traffic on TCP port 44818
  1. to instead forward it to a local IP on the Ethernet - which would be the IP of your ControlLogix ENBT (or the primary IP of the redundant pair)
The details of how this man-in-the-middle fake-out works is fascinating (to me), but quite a pile of text. If you are interested, this older blog entry goes through the NAT/Router details blow by blow. But bottom-line, your ENBT receives the RSLinx packet and needs to have its own Gateway IP set to the Digi gateway's local Ethernet IP address. Free hint: 9 out of 10 guys who call saying "Why can't RSLinx see my PLC through your Digi gateway?" have failed to set the correct Gateway IP in the PLC/ENBT.

So assuming your gateway and PLC are setup correctly, then targeting the RSLinx "Ethernet Devices" driver (with timeouts slowed down to 30 seconds) will cause your PLC's little icon to show up. With RSLinx running, you will create up to 200MB of billable cell traffic per month doing absolutely nothing - so don't leave it active. Note that the "Ethernet/IP Driver" won't work as it requires UDP broadcast, which can't be routed over the Internet.

At this point you'll say "Cool, now can I see my backup ControlLogix or a second PLC?", and the simple answer is "No." One of the realities of RSLinx and AB PLC is that the Ethernet/IP protocol is hard fixed to only the TCP/IP port 44818, and the NAT/Router can only forward TCP port 44818 to a single local PLC. The easy fix would be for Rockwell to change RSLinx to enable adding both an IP/DNS name and TCP port number - then the NAT/Router could forward TCP port 44818 as 44818 to the primary ENBT, TCP 44819 fixed-up to 44818 to the secondary ENBT, TCP 17256 (a random number :-]) fixed up to 44818 to an RSView panel and so on. Because the NAT/Router can restore all traffic to 44818 on the local Ethernet, RSLinx is the only tool needing to change.

Will Rockwell ever do this? It would take a programmer half a day to do - then a few weeks to test - then a few months to document and forestall support headaches. So who knows. They might. They might not.

But bottomline is a simple cellular NAT/Router can be used to talk to a pair of ControlLogix running in a redundant configuration - you will just be limited to seeing only the primary ENBT and the primary IP address.

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