Glenn’s Computer Museum

IBM Magnetostrictive Memory (from IBM 2848)

Two general types of memory were used in computers before magnetic core memory was used: CRT or Williams Tube memory, and Delay Line memory. We have an example of CRT memory in the Components section of the museum. Delay line memories differed in the type of delay medium, but all used the same general principle of representing bits as mechanical "waves" in some media, creating them at one end of a "wire" reading them as them arrive at the other end, and recirculating the data back to the start of the wire. The stored data thus consists of bits (as mechanical waves) flowing around and around a loop containing a medium for propagating the bits. The length of the loop and the frequency of the mechanical waves determine the storage capacity.

Mercury tubes were first used for delay lines; for example the UNIVAC I used mercury delay lines. A major improvement over mercury tubes was Magnetostrictive memory. The museum has a great example: it is the memory from an IBM 2848 control unit which fed video data to several IBM 2260 Display stations, the first video display terminals that IBM made (shipped in the mid-1960's)

Magnetostriction means that a material (a metal wire in our case) changes shape in the presence of a magnetic field. In our case, the wire expands or contracts , thus creating a mechanical pulse that propagates along the wire. Our wire was made of a special nickel-titanium-iron allow that had the right properties for propagating the mechanical impulse along the wire.

Figure 1 shows a memory module from the 2848 control unit. Each module held 11,520 bits, and there were up to four modules in a 2848 control unit, which could service eight 2260 terminals. A 2260 screen was 12 rows of 80 characters. Each character was represented by a six-bit code, so a 2260 needed 5,760 bits to refresh its display.

The memory worked by generating "bit" waves using a magnetic transducer at one end of the storage wire. Figure 3 shows the transducer and start of the wire, and Figure 4 shows the end of the wire, the sensor, and a fine-tuning adjustment. Figure 2 shows a close-up of the wire coils of wire, as well as the circuit board which used discrete germanium transistors. The access time for this memory was about several hundred microseconds, the time for a wave to travel along the complete wire. Both ends of the wire are dampened to prevent feedback of reflected waves.