<- Virtual Exhibitions in Informatics

Delay line memory

Mercury memory of UNIVAC (1951)

The basic concept of a delay-line memory consists of inserting an information pattern into a path which contains delay. If the end of the delay path is connected back to the beginning through amplifying and timing circuits, a closed loop is formed allowing for recirculation of the information pattern. A delay-line memory resembles the human device of repeating a telephone number to one's self from the time it is found in the directory until it has been dialed. The delay medium should slow the propagation rate of the information sufficiently so that the size of the storage equipment for a large number of pulses is within reason.

The first such systems consisted of a column of mercury with piezo crystal transducers (a combination of speaker and microphone) at either end. Data from the computer was sent to the piezo at one end of the tube, and the piezo would pulse and generate a small wave in the mercury. The wave would quickly travel to the far end of the tube, where it would be read back out by the other piezo and sent back to the computer.
To form a memory, additional circuity was added at the receiving end to send the signal back to the input. In this way the pattern of waves sent into the system by the computer could be kept circulating as long as the power was applied. The computer would count the pulses by comparing to a master clock to find the particular bit it was looking for.
Mercury was used because the acoustic impedance of mercury is almost exactly the same as that of the piezoelectric quartz crystals; this minimized the energy loss and the echoes when the signal was transmitted from crystal to medium and back again. The high speed of sound in mercury (1450 m/s) meant that the time needed to wait for a pulse to arrive at the receiving end was less than it would have been a slower medium, such as air, but also that the total number of pulses that could be stored in any reasonably sized column of mercury was limited. Other technical drawbacks of mercury included its weight, its cost, and its toxicity. Moreover, to get the acoustic impedances to match as closely as possible, the mercury had to be kept at a temperature of 40 degrees celsius, which made servicing the tubes hot and uncomfortable work.

EDSAC, the first practical stored-program digital computer, began operation with 512 35-bit words of memory, stored in 32 delay lines holding 576 bits each (a 36th bit was added to every word as a start/stop indicator). In the UNIVAC I this was reduced somewhat, each column stored 120 bits (although the term "bit" was not in popular use at the time), requiring seven large memory units with 18 columns each to make up a 1000-word store. Combined with their support circuitry and amplifiers, the memory subsystem formed its own walk-in room. The average access time was about 222 microseconds, which was considerably faster than the mechanical systems used on earlier computers.