EPICS process variables in different subnetworks and different IOCs without the use of the CaGateway

This technical document describes the comparison of the EPICS PV gateway and a new solution based on relay of UDP packets using the UDP-HELPER switch feature, iptables and a C program. The solution can be applied on environments that contain multiple sub-networks and a number of IOC’s on the same host or multiple IOC’s on the same sub-network. Experimental Physics and Industrial Control System (EPICS) is a set of software tools and applications that provide a suite for the conception of distributed control systems and the operation of the particle accelerator and beamlines, and this tool can also be used for any purpose that requires automation and control. These distributed control systems normally comprise tens or even hundreds of computers connected in a network to permit communication among them and to provide control and feedback to various parts of the device from a central control room or remotely through the internet. EPICS uses Client/Server and Publish/Subscribe techniques for communication among various computers. The majority of the servers (called Input/Output Controllers or IOCs) executes I/O tasks both externally as well as locally and publishes a series of variables with information that the system administrator may find relevant to the clients using the Channel Access (CA) network protocol. Channel Access uses the IP protocol for the publication, search and transfer of data between the various IOCs. There are two current methods for searching and publishing information from other IOCs: the monitored and spot methods. In the first method, the client that requires frequently updated information broadcasts in his subnetwork, searching for the IOC that answers the variable in question. The IOC responds with the IP to which the client should connect to seek the information, and the client is registered in a list of clients interested in the variable who will receive the information whenever it is altered. This system is also used by the “camonitor” command line program. For the spot search/publication, the “caget” and “caput” programs are used; thus, a network broadcast is sent whenever the client needs information or wants to publish a new value for the variable.