Delay line memory

• First described in the First Draft by #vonNeumann based on work by Eckert and Mauchley.

• Information is stored as acoustic waves travelling through a medium, typically mercury. As sound travels more slowly than electric charge (light), if electric signals are converted to sound, the delay time involved can be used as a type of store.

• It worked as follows:

  • Imagine we have an 8-bit number. The number is input as a sequence of pulses where a pulse is 1 and the absence of a pulse is 0.
  • The pulses are converted into sound waves (using a transducer, as with a speaker) and sent through a mercury-filled tube.
  • The length of the tube is calculated so that it takes exactly the time of one machine cycle for a pulse to travel from one end to the other
  • At the other end of the tube, another transducer converts the sound waves back to electrical pulses
  • This operates in a loop: the pulses go back into the input end of the tube and the cycle runs again.

To read the data, the computer would “listen” to the pulses coming out of the receiving end. To write new data, the computer would inject new pulses at the precise moment when it wants to change a ‘0’ to a ‘1’ or vice versa.

For an 8-bit number there would be 8 pulses for each bit and it would be necessary to keep track of 8 specific positions in the delay line where each bit would be stored.

The limitations were timing - difficult to keep track of each precise bit and when it would arrive at the output. Also it was sequential rather than random access. Not every bit could be accessed at once, you had to wait for its time slot to come around to access it.