An FMC (FPGA Mezzanine Card) Loopback Test Module is a specialized daughterboard designed to test, validate, and debug FPGA carrier boards that feature FMC connectors (governed by the ANSI/VITA 57.1 and VITA 57.4 FMC+ standards).
Instead of connecting to an external peripheral or sensor, a loopback card routes outgoing signals directly back into incoming channels on the same connector. This lets engineers run self-tests on the host FPGA board without needing complex target hardware setup.
Please find the detailed product information here:
VITA 57.1 FMC Loopback Test Module