AB PLC to a Real Time Controller over UDP

[MAPYN 0]

Hello Everyone, I am working on a design that has a requirement of using a AB PLC to control 9 different real time controllers (Third Party Devices) over a UDP link. The communication between the PLC and the 9 devices has to happen every 20ms (Each packet is less than 10 bytes that has to be sent to all 9 devices every 20ms). Has anyone worked on a UDP link using a AB PLC ? and can a deterministic 20ms packet communication be achieved using a PLC based system over UDP ? Any comments / insights will be appreciated -Ajay

[Ken Roach 194]

That's exactly how ordinary A-B EtherNet/IP I/O connections work. The devices have to implement the EtherNet/IP cyclic I/O connections; you can't just "send raw UDP packets" cyclically. What is the OS of the other device ? There may already be an EtherNet/IP server stack for it.

[MAPYN 0]

Hello Ken, Thanks for the prompt reply. The third party device is a RTOS based system and has its own Ethernet I/P Stack. For that RTOS application has been written that creates a few UDP sockets to which the PLC needs to talk to. Does it mean that any AB PLC having a Ethernet interface can create a UDP socket and communicate with third party device over UDP socket as I have mentioned above. OR Do we need any additional hardware to create a UDP socket to communicate with the third party device. Thanks once again for your prompt reply -Ajay

[Ken Roach 194]

In the context of ControlLogix-family programmable controllers, "EtherNet/IP" is a trademarked name for an Industrial Protocol (get it, IP ?) used by Rockwell Automation and other companies in the industrial automation business. "EtherNet/IP" is a trademark of the Open Device Vendor Association (ODVA.org). It's often confusing for people outside the industry, who assume that automation folks are talking about Ethernet and Internet Protocol (IP). EtherNet/IP uses UDP multicast or unicast as the transport protocol for cyclic data exchange between controllers. There's a lot more to an EtherNet/IP connection than the UDP packet, though; there's a connection serial number, and a data size negotiation, and a bunch of other stuff that does with creating, operating, and closing a connection between a controller and an I/O device. It is significantly more complex than "a few UDP sockets". There are A-B controllers that can open generic TCP and UDP sockets. This is referred to as the "open sockets feature", and is supported by devices like the 1756-EWEB, 1768-EWEB, and the newest CompactLogix 5370 series controllers. But those generic sockets are not cyclic I/O; they are handled by the user application and are not an automatic, real-time data exchange like an ordinary I/O connection would be. The right way to implement realtime connectivity to a Rockwell Automation controller is to build or license an EtherNet/IP stack and do conformance testing with the ODVA.

[MAPYN 0]

Hello Ken, Thanks for the email. Let me answer your last line first - We cannot implement the EtherNet/IP stack on the Third Party Device as it is a Black Box for us and cannot be changed. We have the documentation and communication protocol for that unit. Based on your reply we need to use the "1756-EWEB, 1768-EWEB, and the newest CompactLogix 5370 series controllers" for a "open socket feature". Thanks for the same. Moreover as I had mentioned initially the communication has to be deterministic (every 20 ms to 9 such similar third party devices). We shall read the datasheets of the products but if it is quickly possible for you to answer - Is such deterministic communication possible using these devices? Really appreciate the time and efforts that you are taking to reply to our query. Thanks and Regards -Ajay