VFAT Scan Mode
Since the VFAT modules have all logic implemented in triplicate to protect against single event upsets (SEU), a chip with a stuck flip-flop may be difficult to detect. When "ScanMode" is enabled all the flip-flops are cascaded, and any serial pattern put into ScanIn should be passed through to ScanOut. Any stuck flip-flops will interfere with this signal and be detectable.
The breakout box should probably have a way to enable "Scan Mode" on all of the VFAT boards at the same time. If the ScanOut from one VFAT is connected to the ScanIn of the next, they can be chained together and tested at once. Thus only one ScanIn and one ScanOut should be needed per breakout box. However, disconnected VFATs would stop the signal unless some bypass mechanism is in place.
The number of SEUs is also stored in a UpsetReg counter that can be read over I2C, so any stuck flip-flops should be easily detected while in operation by a high value in UpsetReg without a separate test mode. The full "Scan Mode" test will probably be used for troubleshooting or pre-experiment validation while the UpsetReg will be monitored while an experiment is running.