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As a result of my X-ray analysis of my own LVDC board in February 2013, a viewer donated another LVDC page assembly board to me that had been exposed to the elements in a salvage yard for over 40 years.  They sent this priceless artifact for the specific purpose that it be disassembled and destructively reverse engineered down to the component level, so that once and for all the real technology beneath the surface could be understood, that could explain how the Apollo Launch Vehicle Digital Computer really worked.

And you can get a chance to be a part of this too!

This genuine LVDC board does not belong to me, or to any one person - It belongs to all of us.   When I complete my initial investigation, I will be passing along this Apollo Saturn V LVDC board to another engineer...  Who must then in turn must pass it on to another engineer....  and so forth... in order that this artifact will forever remain in our community, and that we may all have a chance to share and discover this piece of history for ourselves.

This is The Apollo Saturn V LVDC Project!

In the (final) conclusion for my part of the LVDC Project I went back to the workbench to find the best way of depotting these logic devices without destroying them, and if I could do that successfully then I would try to verify the circuits inside by comparing them against the available data in several documents found online. 

Here I have chiseled the top off of an AA type package.
 

I used a soft plastic toothpick this time, with a gum brush attached to one side, and under a magnifier I very delicately removed the silicone potting compound from around the tiny dies inside. 
 

Success!  I cleared out the potting of the AA type logic device package without dislodging any of the dies from the traces.

A close up view of the top side of the cleared AA logic device.  It contains 4 dies, all of them common anode dual diodes.
 

A close up view of the bottom side of the AA logic device, where I have carefully pulled back some of the silicone coating with fine tweezers to reveal the traces.  Unfortunately, exposure to the elements has taken its toll -  moisture had penetrated the seal and the resistive film inside had completely disintegrated.  I could not get a reading on the resistors.
 

An INV type logic device package decapped.
 

The INV type logic device cleared with much patience.
 

A close up view of the top side of the INV type logic device.  The die on the upper right hand side is an NPN transistor, the rest are dual common anode diodes.
 

 Here I made a working model of the AA type quad input diode logic AND gate using (4) #4148 signal diodes and a 2.2K pull up resistor.  It works very well!
 

Now I will pass the board along to another engineer in our community, and they will add their skills to deconstructing this relic, and perhaps revealing more of its secrets.  To be continued!

Chip type #321 was the first to come off the LVDC board, and I found that the coating on the underside which was covering the resistive elements was in fact a thin rubbery coating of some kind of silicone.  I successfully removed the coating by gently using a small screwdriver to distress the silicone, then brushed it off of the ceramic wafer with a medium bristle toothbrush.  Using my DMM I found the values to be very precise, and the mechanically trimmed deposited film resistors are indeed very accurate in value.

My first attempt to open one of these device packages was not elegant - I broke the thin ceramic top off of #321 in pieces, which revealed a pink putty-like silicone fill. 

Removing the potting I found that the package was empty - no traces, no semiconductors.  It seems that #321 is simply two resistors, nothing more.

This is the cleaned interior of a type #322 package - 
Possibly a bootstrapped transistor configured as a simple constant current source(?):

I used a sharp chisel to open each package by prying off the cover.

Inside each device package was a pink silicone based putty that filled the cavity.

I carefully picked away the filling to reveal the circuitry inside, but as it turned out my light touch was not light enough.  I accidentally dislodged many of the semiconductors from their mounts in the process.

Missing transistors.... Doh!  I found them in the potting I pulled out later.  Those traces are wide, but the mounting points on the semiconductors are microscopically small, just a few mils across at best.  Not much surface area for the solder joints.

Resistors on the underside of the AB type device package - I damaged the one on the far right while taking the silicone coating off.  This is much harder than it seemed to remove the potting without damaging the electronics. 

 The semiconductors are very small - here a normal pencil point is placed in view for scale.

 Close up of a transistor I inadvertently snapped off:

When chipping off the top cap of one of the device packages I dislodged a wafer slightly, and accidentally discovered that the leads on these logic device packages were actually held by spring tension to deposited metallic contacts on the bottom and top sides of the ceramic base wafer.  The leads are not soldered to the devices, but instead each making purely mechanical contact to the base wafer.  A conductive medium might be present in the clip part, but that should get further investigation by another engineer in the project. Another great reason why these page assembly boards were so very heavily conformal coated for flight - not just because of the liquid cooling throughout the LVDC, but it would also have been necessary to prevent the devices moving out of place due to the extreme vibration of the Saturn V in flight.

Close up of the spring clip leads still in place after I successfully pushed out a logic device package.

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. 

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. 

Click on the images below for full size:

This is a picture of the Apollo Saturn V LVDC board that is in my collection.  It was a spare that would have been placed into one of the later Apollo moon rockets.  It is a marvel of its time and demonstrates so many firsts that it stands as a representation of the tremendous leaps that technology took for the Apollo program.  It is the first example of applied microchip technology [update- individual transistors and diodes wired into gates on one ceramic chip], surface mount components, multi-plane circuit board construction [update- 12 layers with full ground plane], plated through-hole vias, and modular logic in one system.  The relationship between this LVDC IU board and all modern computers is staggeringly close by electronics technology perspectives, and it is a monumental leap past anything that had ever existed before it.  Digital watches, pocket calculators, home computers, and all facets of modern electronics sprung out of these technologies, developed solely for the purpose of flying this unheard of machine, the fantastic Saturn V Moon Rocket. 

The custom made integrated circuit chips were apparently made in two types (need to confirm this), to contain either two AND gates or inverters each [update- the LVDC may have contained several types of logic devices but my board (pictured) contains only two types of logic devices, AND and NOT (inverter)], with up to 35 IC's per board, and banks of these individual hard wired logic boards made up the heart of the LVDC.  I had read long ago that it was thought that AND logic was chosen because it was inherently more stable to produce. Reliability was a very big consideration in the design of the IU.  The IU also ran on core memory, again chosen for its proven reliability.

The LVDC was manufactured under contract by IBM and was housed in a ring at the top of the Saturn V rocket, behind the CSM.  The Saturn V was controlled and monitored on the pad by ground operators via an umbilical, but once the ignition sequence start was initiated the IU took over.  From that point to orbit all guidance, attitude, control, engine adjustment, telemetry, staging, and all other onboard operations were controlled entirely by the IU.  Because of the LVDC and IU the mighty Saturn V was the most sophisticated fully autonomous system then constructed.  The core memory program it ran on could not be programmed on the fly, it had to be set on the ground, so the flight path was predetermined.  As such the IU was also responsible for any course correction and compensating for any malfunction, as it did on Apollo 13 when a center engine failed. During ascent the only human control was the option to abort, and until the final staging when the CSM was free in orbit it was the IU that called all the shots.

Whenever I look at this magnificent artifact of our Apollo program sitting in my own collection I am always awestruck.  It recalls an excerpt from one of my favorite poems, Ode, written by Arthur O'Shaughnessy in 1874 –

WE are the music-makers, 
And we are the dreamers of dreams,
Wandering by lone sea-breakers, 
And sitting by desolate streams; 
World-losers and world-forsakers, 
On whom the pale moon gleams: 
Yet we are the movers and shakers 
Of the world for ever, it seems.