Abstract: An air traffic control transponder includes a synchronous digital delay line decoder for providing mode selection signals in response to pulse space coded transmissions from the ground station. The decoder comprises a shift register delay line having a plurality of delay taps along which a first received pulse is shifted. A continuously running high frequency clock pulse generator via a frequency divider provides the shifting clock to the delay line. The first received pulse synchronizes the shifting clock via the frequency divider. A plurality of monostable multivibrators connected to selected taps of the delay line provide pulse detection windows in response to the first received pulse being shifted through the stages of the shift register to which the window generating monostable multivibrators are connected. The outputs from the window generators are applied as inputs to respective pulse coincidence detection devices, the second inputs thereof being connected to receive the incoming pulses.