An optional ``shutter'' can be enabled to synchronize the acquisition window to a signal, such as the spill signal from a beam line.\\
When the shutter is ``closed'' triggers are vetoed and no triggers are sent. When the shutter is ``open'' triggers can be generated and sent to active \gls{dut}s.\\
When the shutter is ``closed'' triggers are vetoed and no triggers are issued from the \gls{tlu}. When the shutter is ``open'' triggers can be generated and sent to active \gls{dut}s.\\
The shutter cycle can either be started by an external signal or synchronized by a counter clocked by the system clock (i.e. internally-generated shutter, which can be used to debug hardware).\\
The external signal, if used, must be connected to one of the six LEMO trigger inputs.\\