A readout system, comprising: a data channel for conveying data; a first plurality of detector units, each detector unit comprising: an input which is arranged to receive a pulse having a magnitude; an input which is arranged to receive a pulse having a magnitude; a storage buffer which is arranged to store an indication of the magnitude; and output circuitry which is coupled to output a request-to-read signal in response to the storage buffer receiving the pulse, and to output the indication to the data channel upon receiving a select signal in response to the request-to-read signal; a processor, which is coupled to receive the request-to-read signal and in response transmit the select signal, and which is coupled to read the indication from the data channel; and characterised by: a plurality of selectors, which are coupled as a tree of hierarchical rows comprising decreasing numbers of selectors, and which are arranged to convey the request-to-read signal from the detector unit to the processor, and to convey the select signal from the processor to the detector unit, thereby causing the processor to read the indication. a data channel for conveying data; a first plurality of detector units, each detector unit comprising: an input which is arranged to receive a pulse having a magnitude; an input which is arranged to receive a pulse having a magnitude; a storage buffer which is arranged to store an indication of the magnitude; and output circuitry which is coupled to output a request-to-read signal in response to the storage buffer receiving the pulse, and to output the indication to the data channel upon receiving a select signal in response to the request-to-read signal; a processor, which is coupled to receive the request-to-read signal and in response transmit the select signal, and which is coupled to read the indication from the data channel; and characterised by: a plurality of selectors, which are coupled as a tree of hierarchical rows comprising decreasing numbers of selectors, and which are arranged to convey the request-to-read signal from the detector unit to the processor, and to convey the select signal from the processor to the detector unit, thereby causing the processor to read the indication. an input which is arranged to receive a pulse having a magnitude; an input which is arranged to receive a pulse having a magnitude; a storage buffer which is arranged to store an indication of the magnitude; and output circuitry which is coupled to output a request-to-read signal in response to the storage buffer receiving the pulse, and to output the indication to the data channel upon receiving a select signal in response to the request-to-read signal; a processor, which is coupled to receive the request-to-read signal and in response transmit the select signal, and which is coupled to read the indication from the data channel; and characterised by: a plurality of selectors, which are coupled as a tree of hierarchical rows comprising decreasing numbers of selectors, and which are arranged to convey the request-to-read signal from the detector unit to the processor, and to convey the select signal from the processor to the detector unit, thereby causing the processor to read the indication. a plurality of selectors, which are coupled as a tree of hierarchical rows comprising decreasing numbers of selectors, and which are arranged to convey the request-to-read signal from the detector unit to the processor, and to convey the select signal from the processor to the detector unit, thereby causing the processor to read the indication.