Core memory frame from an IBM 1401

 
This is the best piece I have from the early 1960's era. It's the core memory stack from an IBM 1401 computer and associated  electronics. It was built in 1964, and provides 4000 eight bit storage locations for the machine's main memory, plus some extra and unidentified storage function. 

     What is Core Memory?

Core memory is consists of thousands of small rings (cores) made of ferrite (iron dust in a binder), strung together on a matrix of fine wires. Electric currents driven through the wires can cause each core to be magnetized either clockwise or anti-clockwise. Thus each core can store one "bit" of information, a "zero" when magnetized in one direction, and a "one" when magnetized the opposite way. The matrix of cores is called a "core plane", and several are piled together to form a "core stack". There is typically one plane for each bit in the computer's "word" (eight for the 1401), so that any particular location in the machine's memory is composed of a row of cores, each one being in a different plane in the stack, but in the same corresponding positions on each matrix. One of the quirks of core memory is that the process of reading data out of it also erases the data in the cores, and so the contents of a memory location must be immediately written back each time it is read.

     What is a 1401?  This picture (at another site) shows, from left to right: 1402 Card Reader/Punch, 1401 CPU, 729 Tape Drive and a 1403 Line printer.

The 1401 was an unusual computer in a number of respects, one was that the parity bit (used to check for errors) is included in the 8 bits of each location, whereas it is usually an extra, ninth bit. Another bit was needed to indicate "word marks", so there were only six bits bits actually usable for data in each location. 
To expand a 1401 beyond 4000 bytes, a separate 1406 core memory unit (about the size of a washing machine) could be installed, taking the total to 16,000 locations.



Click the images below to enlarge
The core memory frame. It's about 350x530x200mm (14x21x8") in size The memory electronics, consisting of 53 Standard Modular System (SMS) cards on the left. Each is 114x67mm (approx. 4.5"x2.6"). On the right is the core stack, made up of 16 planes of ferrite core memory.

The other side of the core memory block. The row of connectors below it  were used to connect to the electronics on an adjacent frame.  Most of the wiring from the core stack goes to these connectors, so there was probably even more memory electronics on the adjacent frame than on this one.

The rear view.

With the end plate removed the core plane is visible. The 8 planes at this end of the stack are sparsely loaded, having only about 320 cores instead of the 4000 of a full plane.. 

This is one of the core memory planes (with paper inserted behind), consisting of 50 rows of 80 columns (4000), with one bit per plane, eight are required to make up 4000 bytes. 

Here is a close-up of a section of the core plane, the ferrite core rings are plainly visible. There are four wires threaded through each core (X,Y,Sense and Inhibit). 
Another closeup of the cores, each is about a millimeter in diameter.
Click to really see the donuts.

Another view with the paper inserted behind. The remaining eight planes have some cores in them, but far fewer than the first eight. They may have been used as registers, or I/O buffer storage
 

There are many, many wires soldered to the core stack, and then laced into bundles that snake their way around the frame. 
(If it was my task to design these things, I would have found another more maintainable and less labour intensive method of construction) 

There are eight types of SMS cards in this frame.
The board type identifiers and the numbers used of each type are:-

AQW - 4        DKA - 4

AQU - 17      AQV - 7

WX  - 15       AQX - 4

AKB - 1        AKC - 1

Core memory requires fairly large current pulses to magnetize the cores, and therefore many of boards involve power circuitry with large heatsinks.


 
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