Over the last decade, the continuing decline in the cost of digital computing technology has brought about a dramatic transformation in how digital instrumentation for radio astronomy is developed and operated. In most cases, it is now possible to interface consumer computing hardware, e.g. inexpensive graphics processing units and storage devices, directly to the raw data streams produced by radio telescopes. Such systems bring with them myriad benefits: straightforward upgrade paths, cost savings through leveraging an economy of scale, and a lowered barrier to entry for scientists and engineers seeking to add new instrument capabilities. Additionally, the typical data-interconnect technology used with general-purpose computing hardware -- Ethernet -- naturally permits multiple subscribers to a single raw data stream. This allows multiple science programs to be conducted in parallel. When combined with broad bandwidths and wide primary fields of view, radio telescopes become capable of achieving many science goals simultaneously. Moreover, because many science programs are not strongly dependent on observing cadence and direction (e.g. searches for extraterrestrial intelligence and radio transient surveys), these so-called commensal observing programs can dramatically increase the scientific productivity and discovery potential of an observatory. In this whitepaper, we detail a project to add an Ethernet-based commensal observing mode to the Jansky Very Large Array (VLA), and discuss how this mode could be leveraged to conduct a powerful program to constrain the distribution of advanced life in the universe through a search for radio emission indicative of technology. We also discuss other potential science use-cases for the system, and how the system could be used for technology development towards next-generation processing systems for the Next Generation VLA.
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