Tuesday, July 19, 2011

PLC ControlLogix Ethernet Interface

The programmable logic controllers (PLCs) Allen-Bradley ControlLogix line presents some interfaces: RS-232, DeviceNet, Ethernet, ControlNet, and others. The interface module 1756-ENET of ControlLogix Ethernet applies EtherNet/IP, the protocol of ControlNet, summarized in packages of Ethernet, with specific service codes. The Industrial Control System (EPICS) and Experimental driver Physics has been built up that uses this protocol of EtherNet/IP for controllers to run the vxWorks RTOS as well as a Unix/Linux and Win32 test program.

Some Spallation Neutron Source project (SNS) subsystems use ControlLogix PLCs of Allen-Bradley. The input/output controllers (IOCs) of EPICS need interpret and write contact to the PLC data to incorporate these into the EPICS-based of accelerator control system. As the IOCs, their Win32 or Unix boot hosts as well as roughly every PC which is employed to program the PLC is already provided with an interface of Ethernet, it is advantageous to apply the same technology for the PLC data transferring. Accessible support and knowledge for network hardware, cabling, maintenance and configuration can therefore be utilized.

ControlNet is a system of deterministic serial communication, its requirement expands from the physical to the layer of application the model of seven layer ISO OSI. ControlNet Release 2.0 initiated the data packages TCP/IP encapsulation, replace the Data Link and Physical layer with IP/UDP/TCP of Ethernet respectively. The outcome was recognized as “ControlNet over Ethernet” and is currently popular as an EtherNet/IP. After linking to a target of EtherNet/IP, by default on port 0xAF12 of TCP/IP, and creating a session ID via the protocol of encapsulation, messages can be replaced. They are identified in the object oriented of Control and Information Protocol (CIP), the ControlNet specification part. One still decides “connected” and “unconnected” CIP messages in reference to the transport layer of ControlNet. They are summarized another way but can both be transferred via TCP, which is always linked. The PLC of ControlLogix employs ControlNet to communicate with the boards of local I/O over the back plane, the Ethernet module of 1756-ENET supports EtherNet/IP.

Following the specification of EtherNet/IP, one can apply the SendRRData encapsulation instruction and transfer this unconnected interface of CIP message: The object model of CIP also comprises “Discrete Input Group” (0x1D) and “Analog Input Point” (0x0A) objects, but so far the efforts to apply these for contacting the analog of ControlLogix or modules of digital input have failed. There is no typical CIP object that recommends usability for contacting tag names on the PLC.

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