Memory weaved by hand – THe LVDC

The Launch Vehicle Digital Computer a.k.a LVDC is one of the critical components of the Saturn V launch. LVDC is the computer that provided the autopilot for Saturn V from launch to Earth Orbit Insertion.

LVDC is designed and manufactured by IBM’s Electronics System Center. It is considered as one of the technologically advanced memory modules during the mid-1960s.

LVDC is capable of executing 12190 instructions per second. ( A 2012 up can achieve 12 billion instructions per second. The core memory module stores 4K words of 26 data bits and 2 parity bits.

The core memory was built from tiny ferrite rings called cores, that would store one bit in each core by magnetizing the core either clockwise or anti-clockwise. A core was magnetized by sending a pulse of current through wires threaded through the core. The magnetization could be reversed by sending a pulse in the opposite direction.

To read the value of a core, a current pulse flipped the core to the 0 state. If the core was in the 1 state previously, the changing magnetic field created a voltage in the sense wire threaded through the cores.

But if the core was already in the 0 state, the magnetic field wouldn’t change and the sense wire wouldn’t pick up a voltage. Thus, the value of the bit in the core was read by resetting the core to 0 and testing the sense wire. An important characteristic of core memory was that the process of reading a core destroyed its value, so it needed to be rewritten.

By using a method called coincident–current, engineers during that time created a grid of wires to select a core.

The below plane has 128 X wires(vertically) and 64 Y wires(horizontally). This has 8192 locations, each storing a single bit. The LVDC memory used a stack of 14 core planes (below), storing a 13-bit “syllable” along with a parity bit.

Closeup of an IBM 360 Model 50 core plane. The LVDC and Model 50 used the same type of cores, known as 19-32 because their inner diameter was 19 mils and their outer diameter was 32 mils (0.8 mm). While this photo shows three wires through each core, the LVDC used four wires.

The image you see above is a segment of the plane. They are literally the bits. Physical bits. They literally weaved the circuit by hand.

The Saturn V has an instrumentation ring that would have computers that were digital on it.

In each plane, there are 8192 locations/nodes on a single plane. 14 planes are stacked up to create a module. LVDC has 2 sets of 4 of these modules, i.e ~16000 words of memory. During the Saturn V flight, both of these computers execute the same program parallelly and check the similarity of the results generated by the computers. If there are different results, they would call their subroutines and check approximate values at a particular state of execution during the flight and execute the results that are closest to it.

The details and description of LVDC are explained beautifully in the article mentioned below. Please do read to know more about LVDC.