Cracking the Code of the Apollo Saturn V LVDC Logic Devices

(February 2013) I spent many late nights in squinty-eyed examination of the X-rays we took of my LVDC board, and plowed through hundreds of pages of publicly available documents about the IBM System/360 looking for clues that would help unravel the convoluted maze of puzzle pieces and reveal the nature of the LVDC logic devices. IBM developed the System/360 and the LVDC in tandem, and I expected that there would be much crossover of the two systems. In the end I found that these systems had surprisingly little in common in their gate designs, and some surprising facts emerged. IBM introduced the System/360 in 1964 with much fanfare. They proudly flaunted the new DTL solid state logic devices which they called Solid Logic Technology (SLT) and published photos and descriptions of their new "Hybrid" devices; ceramic based micro packages with thick film deposited resistors and semiconductor chips (both transistors and dual diodes) called "Flat Chips" which were solder ball reflowed upside down directly onto tiny tinned pads going to lead traces. The DTL gates contained one common anode dual diode chip, a dual series diode chip, and a transistor. Rather than having a full circuit completed in the traces inside of the device, the circuit was divided into modular components, and depending on which pins were selected or jumped to which other pins, could configure the DTL device to be a NAND or a NOR, and to have a choice of possible output configurations.

This System/360 board has 20 DTL logic packages - 40 dual diodes and 20 transistors in total. Each SLT package was just under 1/2 inch square. Note the similarities to the LVDC board, with the matrix type layout and edge connector, but much larger and less dense than the LVDC. The LVDC logic devices are almost 1/4 this size, and containing either 3 or 4 transistors or dual diodes each, which if fully populated a single LVDC board could contain up to 140 semiconductors in the same board size. My first approach was to try to translate this System/360 DTL technology to what could be seen in the LVDC, but despite weeks of postulating dozens of possibilities there were no complete fits with what was visible in the X-rays. One thing that made the LVDC logic devices so hard to figure out was that one of my initial assumptions about the LVDC PCB was wrong - the board's device mounting locations were not universal as I had thought. In fact, each mounting location was dedicated and wired for a specific type of device package, though there seems to be no interconnecting between gates within the board, again suggesting that the gate array would be created within the backplane wiring. Also, the pinouts on the two types of logic devices on my board were not the same, and there was no universal Vcc pin, or ground pin. For example, an input pin on one device would be a jumper point on another. The whole LVDC system may have operated on many different types of gate packages, but my board contains only AND gates and NOT (inverter) gates. The X-rays revealed that jumpers inside of the PCB layers were critical to completing the gate circuits, and these critical connections seem to have been intentionally designed out of the logic devices themselves. But why? The answer is most likely that this technology was highly classified at the time, and probably exclusive for military or government use only. The complete absence of any documentation about the LVDC hardware would seem to corroborate this assumption. In the 1960's IBM made the computers for every branch of the military, and this technology was the future - for use in ICBM's, guidance, and satellites. They would have expected some espionage - that some of these logic devices would fall into enemy hands - the Soviets, or the competition. But if you were to have possession of these logic device packages and broke them open you would not see anything special in them - just semiconductors and resistors in broken circuits. Without the primer of jumper connections needed to make the device active the secrets of how it operated would be safe. Another big difference in this applied DTL System/360 vs. LVDC technology is the scale. The individual System/360 SLT device packages were much larger (about 12mm square) than the LVDC device packages (5mm X 7mm) and the SLT film deposited resistors were also larger and deposited on the top side of the ceramic base along with the semiconductor traces. The LVDC device packages had the film resistors deposited on the bottom side of the ceramic base, connected in circuit to certain pins with the top side, making the total package much more compact. One thing about deposited thick film resistors is their inherent sloppiness for value - up to ±30-40% variation in tolerance - but what was revealed in the SLT devices was that the film was deposited between two conductive rails set some distance apart, with an unusual shape to the resistors which seemed to suggested that the resistor film had been either mechanically or chemically trimmed after being deposited to tune its specific resistance. There must have been a process to do this accurizing on a smaller scale, since the resistive elements in the LVDC devices are very small. My disclaimer about this project is that without using destructive methods it is impossible to know exactly how these LVDC devices were constructed or how they operate, and not much more can be deciphered through what was visible in the X-rays alone.

An empty slot - note that there are no internal connecting traces. This was the proof that the mounting locations were wired for a specific device.

LVDC AND gate package - note the many interconnections in the PCB layers under the device as compared to an empty slot.

LVDC NOT gate package - note that two of the underside resistors are faintly visible on the outside edges of the package, seen as thicker vertical lines over the outer via pads.