Friday, July 11, 2008

Lower Cost Cellular to Rockwell AB PLC

I have several customers now working through how to manage cost-effective cellular access to Rockwell PLC such as ControlLogix, CompactLogix, Micrologix 1100 and so on. Unfortunately the most straight forward way to link using Ethernet/IP is fairly costly.

First, a personal recommendation from me – a free tool which I find very useful and think you will too. Today, you can buy 2GB USB flash drives for $15 – if you're old like me, you remember when an entire Windows computer only had 0.020GB of hard drive space! Did you know you can literally install and run many Windows applications from these portable USB drives? This means any Windows computer you plug this USB drive into has your applications, your settings, and your data files. I've used one of these for over a year and it is invaluable - all free open source code too! You can run OpenOffice (which can read/write MSOffice 2003 files and is much faster than the MSOffice 2007 we use at Digi), Firefox web browser, plus a dozen other tools. Take a special look at KeePass, which I use daily from my USB drive to securely hold all of my hundred-plus account names and passwords. - What is a Portable App?

Okay, back to work.

Periodic PLC Access from RSLogix

Customers who want to peek into a single PLC at a remote site for an hour or two can use RSLinx to connect to either an IP or DNS name, then see the PLC via cellular. The catch is RSLinx will create from 12MB to 200MB of background traffic per month. So you need to create a new Ethernet Driver (not Ethernet/IP!) JUST for this one-time use, configure in your details, connect and do your work. When you are done you need to turn browsing off, then delete the comm driver. Why not just delete the IP or DNS name? Unfortunately once RSLinx has seen a device, it can be like a bad rash to get rid of it.

Data polling – at Central Office

Customers with an OPC server speaking DF1, CSPv4, or even Ethernet/IP can poll PCCC-type data through a Digi One IAP, which converts the polls into DF1 Radio Modem under UDP. Tests have show using DF1 Radio Modem every few minutes accomplishes the same data movement as Ethernet/IP with only 5% the data cost (or Ethernet/IP uses 2000% more data bytes). One unit of Digi One IAP can poll up to 60 remote IP or DNS names. If your OPC server can encapsulate DF1 Radio Modem directly into UDP/IP, then you won't need the Digi One IAP to act as your host.

Data Polling – at Remote Site

If you have an AB PLC which speaks DF1 Radio Modem directly, then any Digi cellular router can be configured for UDP Sockets, with shuttles UDP data received to the serial port. Make sure you use the latest Digi firmware so it can just return UDP responses to last sender without explicit address configuration.

If your AB PLC doesn't speak DF1 Radio Modem, or you want to use an Ethernet link, then using a Digi cellular router with Python support allows a simple script which accepts DF1 requests and uses a local Ethernet/IP session to query responses from the PLC's PCCC Object. This Python code even runs on a PC under Windows or Linux. As soon as I have a link or web page explaining how to get and use this code, I'll edit this post to add it here.

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