Rod memory is one of the many variations on

magnetic core memory In computing, magnetic-core memory is a form of random-access memory. It predominated for roughly 20 years between 1955 and 1975, and is often just called core memory, or, informally, core. Core memory uses toroids (rings) of a hard magneti ...

that attempts to lower costs by automating its manufacturing. It was introduced by NCR in 1964 as part of the

NCR 315 The NCR 315 Data Processing System, released in January 1962 by NCR, is a second-generation computer. All printed circuit boards use resistor–transistor logic (RTL) to create the various logic elements. It uses 12-bit ''slab'' memory struct ...

RMC computer, RMC for "rod memory computer". It was also used in their

Century A century is a period of 100 years or 10 decades. Centuries are numbered ordinally in English and many other languages. The word ''century'' comes from the Latin ''centum'', meaning ''one hundred''. ''Century'' is sometimes abbreviated as c. ...

line. Like many similar concepts, notably

twistor memory Twistor memory is a form of computer memory formed by wrapping magnetic tape around a current-carrying wire. Operationally, twistor was very similar to core memory. Twistor could also be used to make ROM memories, including a re-programmable fo ...

and

thin film memory Thin-film memory is a high-speed alternative to magnetic-core memory developed by Sperry Rand in a government-funded research project. Instead of threading individual ferrite cores on wires, thin-film memory consisted of 4-micrometer thick do ...

, rod memory was competing for the role of taking over from core when the first

semiconductor memory Semiconductor memory is a digital electronic semiconductor device used for digital data storage, such as computer memory. It typically refers to devices in which data is stored within metal–oxide–semiconductor (MOS) memory cells on a si ...

systems wiped out the entire market in 1970.

Description

Instead of magnetic rings, the ''cores'', and rod memory uses magnetic bars surrounded by a metal coil. Instead of the electrical current running linearly and the magnetic field around the core, in rod memory the current runs around the solenoid and the magnetic field is linear in the rod. The end result is essentially the same, however, with the bits being represented by the direction of the magnetic field in the rod. To do this, the rods are placed in the middle of tiny

solenoid upright=1.20, An illustration of a solenoid upright=1.20, Magnetic field created by a seven-loop solenoid (cross-sectional view) described using field lines A solenoid () is a type of electromagnet formed by a helix, helical coil of wire whos ...

s, which produce a linear field in one direction or the other depending on which direction the current is flowing. The sense/inhibit line is constructed by winding the rod itself in a separate wire coil. This ''two-wire linear select mode'' of operation has inherent drawbacks in the way memory can be accessed, which is generally solved with slightly more complex input/output circuitry. The bars were formed by electroplating a 97% iron/3% nickel plate onto beryllium-copper wire 10

thousandth of an inch A thousandth of an inch is a derived unit of length in a system of units using inches. Equal to of an inch, a thousandth is commonly called a thou (used for both singular and plural) or, particularly in North America, a mil (plural mils). Th ...

in diameter (10 mils), coating it in

polyurethane Polyurethane (; often abbreviated PUR and PU) is a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane term ...

, and curing it in an oven. The long length of coated wire is then wound with thin copper ribbon, 10 mil wide and 2.5 mil thick, coated in urethane again, and then cut into 6-inch long bars. Separately, another machine is fed copper wire and periodically creates small coils of ten windings forming a

. The assembly machinery then left a short section of the wire unbent, before winding another solenoid into the wire, and so on. The result is a string of coils on the wire, which from a distance looked like a series of knots evenly spaced along a rope. A number of these ropes were then laid parallel to each other and a second set of straight wires ran parallel to the ropes so they lay under one end of the solenoids. The assembly is then completed by inserting one of the rods through a series of solenoids and of the potting the entire assembly. The result is a single plane of memory, which is then assembled into a larger frame to form a complete memory with multiple bit planes.

Assembly

For assembly, the rods were inserted into a plastic alignment sheet which was wound with read, write, and sense wire coils arranged in columns and rows. To get the rods to stand up straight on the sheet (so that they would drop into the coils for assembly) a large electro-magnet was turned on and made the rods stand up and "dance" into the individual holes. The economy of machine assembly was augmented by selling rod memory without paying patent royalties on core memory to NCR's competitor, IBM.

References

Citations

Bibliography

* * {{refend Computer memory