Moving to the open source open62541 library for PLC communication at the European Southern Observatory's Very Large Telescope

In the last ten years the European Southern Observatory's (ESO) Very Large Telescope (VLT) Instrumentation Framework has begun moving its low level interface to instrument functions away from VME-based Local Control Units (LCU's) to Commercial Off The Shelf (COTS) components connected with industry standard fieldbuses. This move has resulted in the adoption of PC-based Programmable Logical Controllers (PLC's) that directly control the instrument devices, connected via Ethernet to Linux workstations that provide high level coordination and user-interfaces. To enable this shift to COTS components, a new "fieldbus-aware" Instrument Control System Base (IC0FB) was developed which utilizes, among others, the Open Platform Communications Unified Architecture (OPC UA) standard for communication between the workstations and the PLC's. The initial implementation of IC0FB used closed source libraries for this OPC-UA-based communication, however, licensing restrictions made compiling and distributing difficult throughout the VLT project. This has prompted the recent re-implementation of the OPC UA IC0FB Communication Interface using the open source library open625411 and the adoption of open62541 for ESO's new Extreme Large Telescope. In this paper, we discuss the lessons learned in moving to open source implementation of an industry standard. We compare the performance of the open62541 implementation and the implementation based on the commercially licensed Softing Automation SDK2 and show that the performance of the open source solution is comparable to the closed source implementation.