Examining the Core Memory Module inside a vintage IBM 1401 Mainframe

Physical layout of the core module

The core module consists of 16 frames in a stack – 14 core planes and two terminal frames. The upper 8 frames hold the character data planes and the lower 8 frames are the I/O frames. The following table shows the usage of each frame. The terminal frames do not contain cores, but provide connections for the large wire bundles from the reader brushes and the print hammers.

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The picture below shows the large amount of wiring required by the core module. Frame 16 (YL) is at the left and frame 1 (bit 8) is at the right. The two matrix switches are on the front of the module: the 8×10 switch for the Y select lines is at top, and the 5×10 switch for the X select lines is at the bottom.

The yellow wires at the left and right connect the Y select lines on frame 16 and frame 1 to the 8×10 matrix switch. Two bundles of wires connect to I/O planes near the middle of the module. One connects the brushes in the card reader and the printer hammer drivers to terminals on frame 11. The other bundle connects read brushes and punch check brushes to terminals on frame 9. The horizontal wire bundle across the middle of the planes connects the inhibit lines of each plane.

The photo below provides another view, focusing on the data plane wiring. At front is frame 1, the core plane for data bit 8, with the gray cores visible on red wires. The other 15 frames are layered behind frame 1. The two matrix switches are on top. The 8×10 matrix switch is connected to the Y select lines on the top and the 5×10 matrix switch is connected to the X select lines on the left.

The detailed block diagram below shows how the components are connected in the 1401’s core memory system. This diagram shows the physical arrangement of the 16 frames in the core memory module, along with the driver circuitry. The inhibit drivers are at the upper left, feeding each core plane. The sense amplifiers are at the upper right. The 5×10 X matrix switch is in the lower left, and the 8×10 Y matrix switch is in the lower right. Note the read brushes, punch brushes, and print hammer drivers are wired directly into the core module through the terminal frames. The diagram also shows the timing of the read and write pulses, and how they have opposite polarity, writing 0 and 1 respectively.