Tuesday, February 12, 2019

Preparing to add text decals and glue up the DSKY display, still working on keyboard

EXAMINING REAL DSKY AND CHECKING ANOTHER ERASABLE MEMORY MODULE

We were given access by the Computer History Museum to the Apollo Guidance Computer and DSKY that they have on public display. The computer is the first block II built, used at MIT to check out each change they made to hardware. The DSKY appeared to be an early block II DSKY, since the indicator panel had three lights (AUTO, HOLD and FREE) that were removed before the versions that were placed in all the spacecraft.

The plan for the DSKY was to record the serial numbers from the plate on the back, which was not accessible while the DSKY was mounted on the wall at the museum. The number matches a slightly later DSKY design, which should not have had the three removed indications I discussed above. Mike will be investigating all the paperwork to try to understand how this hybrid version came about.

I had the chance to work the keyboard, validating the amount of movement and the resistance of the keys when pushed. The actual DSKY feels indistinguishable from the keyboard action I achieved in my substitute, which made me happy.

Seeing the DSKY reinforced how deep is the real item. The enclosure I am fitting into is less than a third as deep. In some ways, that is not a problem because I can use higher density PCBs and components instead of the banks of crystal can sized relays in the real item. However, my interior is pretty crowded anyhow.

We removed the erasable memory module (2K words of traditional core memory) to test it electrically with a modern resistance meter (low applied voltage and current). We were able to confirm that all the addressing, inhibit and sense lines behaved as expected, with all the diodes operating properly in one direction.

That does confirm that our module from the AGC we are restoring does have an open inhibit line for bit 16. It is a final proof for what was already quite overwhelming evidence. There is a note on the memory module from the museum's AGC that indicates that bit 13 of a specific word address drops bits.

Thus, even if this otherwise perfect memory module were to be inserted into the AGC, we would have to carefully check all the software we want to run (e.g. Luminary) to understand the impact of bit 13 of that word becoming zero. At this time there is no expectation that we would be able to plug in the module, but we will do the advance research just to be prepared.

We then sat down to plan out means to test the memory drive electronics (current drivers, switches and sense amp modules) before we insert our memory module into the tray. We will design and build a PCB, populate it with some of the recreated connectors that are being made for us by a sponsor, and use it as a testing bed.

DSKY SUBSTITUTE FOR THE AGC

Keyboard

Marc and I worked out a method to glue cardboard shims into the honeycomb after the plungers are inserted in each cell. This lessens wobble and erratic movement of the key as it is moved up and down by user presses. He did a test on one key by hand cutting and maneuvering cardboard pieces, but to complete the other 18 keys would be too slow and labor intensive.

Thus, I designed the cardboard pieces to be laser-cut at Ponoko.com as well as refining my cardboard masks for the top of the displays. The parts arrived on Tuesday Feb 12th and I did a test fit. Now to do the careful gluing for all 19 keys. .

When the honeycomb is completed, with its shims, I will need to etch out some space on the bottom edge of the honeycomb to clear small features on the surface of the PCB, before I install the coil springs and join the board to the honeycomb assembly.

Display

I refined my ideas for the light masks that sit under the acrylic panels on the DSKY. The new designs were shipped off to be laser-cut; these also arrived on Tuesday. After I satisfied myself with how everything fits together and looks while lit up, I can start gluing up the displays.

For the EL panel side, that involves gluing down the electroluminescent wires, gluing the sign digit light dam, gluing on the diffuser sheet, gluing down the main light dam, then gluing on the light mask at the top.

Quick and dirty test of stack, but still have mylar protective fil which is blurring the output slightly.
I had frosted the back of my acrylic panes, which I tested quickly on the board to see how the light dams, masks and frosting worked out. I was satisfied with the results - they are not perfect but good enough for me to complete the EL panel side. The indicator side is not sufficiently diffused to hide the two discrete LEDs that light up the space. While this is not great, I will proceed anyway.

Illumination behind the PROG indicator space - still have mylar film over acrylic panel

Friday, February 8, 2019

Paint, light blocking materials and other cosmetic work on DSKY substitue

DSKY SUBSTITUTE FOR THE AGC

Display

My cardboard light masks came back from the laser cutting service. I had to make a few additions but they do fit properly. I am now ready to begin gluing up the sandwich of materials on the PCB face. First are the electroluminescent wires, then the black light dam. Next is the cutout for the plus/minus signs. Third would be the diffuser paper over the signs. Finally, the light masks are glued down.

I have also frosted the rear of both acrylic panels. White for the indicator panel and light gray for the EL panel side. These are now ready to have the labeling installed on the front with dry rub-on decals.

Frosted acrylic on left, plus cardboard light dam
I primed and painted the plastic covers that will hold the acrylic panels in place in the faceplate. Once the labeling is complete I can glue the covers over the panels and install them. The covers look fine, as does the case.

Case and faceplate

I cleaned, primed and painted the case and faceplate in the light gray I chose. I did not try to exactly match the Apollo spacecraft color, simply to produce a plausible gray that the brain will associate with pictures of the installed DSKYs.
Painted enclosure and panel covers
I still need to cut openings for cables that will run between the DSKY, Apollo Guidance Computer and external power supplies. Decision time is near - cables through holes or connector installed on side of case?

Sunday, February 3, 2019

Creating masks and text for DSKY substitute display panels

DSKY SUBSTITUTE FOR THE AGC

Display

I created the mirror image files for the DSKY display and used the printer at CHM to produce several copies of the dry rub transfer decals which I will use to layer on the acrylic panels. The indicator panel is simply black text which will be backed with some frosted white paint. The EL panel has six white dots, a blue line, and black text to apply, requiring three sets of decals for the various colors. It will then be backed with frosted gray paint.

Three color layers for decals on EL acrylic panel
Multiple copies of text for indicator acrylic panel
The system I use to create the decals has the color layer on the top of the text/image, which means that it is not facing the surface where you adhere the decal. This posed a challenge for my original intent to build the panel by applying decals to the underside of an acrylic panel, since only the underside of the decal will be visible by the user.

I have found that black mylar is good enough even when viewed from the underside of the decal, but I was concerned about how the blue line and white dots would appear. I realized I needed to place these on the top surface of the acrylic, instead of underneath.

The only downsides of placing decals on the top is that they are susceptible to damage and might be visible as raised features. I can spray clear acrylic paint to form a protective barrier over the top. The frosting paint (white or gray) can still be applied to the underside.

I chose to use one sheet for each panel, although I was able to make four copies of the indicator panel on a single sheet so there is redundancy to handle any errors while building decals. The EL panel does not give me the same redundancy, but I printed several copies instead.

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Another cosmetic detail to build is the framework around the indicator light sections on the indicator panel. I can laser cut them from thin cardboard stock of the correct color, apply to the top of the panel once I have it constructed, then cover all with clear acrylic spray paint. I did have to check to verify whether I have enough margin around the text to build the framework, but that worked out fine after I shrank the font size slightly.

Framework from a real DSKY indicator panel
After designing the framework, I decided to use the remainder of the 7" x 10" sheet of material, the minimum size from the lasercutting service (Ponoko) to make a mask to fit under the EL panel. My idea here is to narrow the size of the illuminated line coming from the EL wire below, plus to constrain the size of the lighted area behind the PROG, NOUN, VERB and COMP ACTY legends to match a real DSKY panel.

This would sit atop the displays, with cutouts for the lighted legend areas, the three lines, the 21 display digits and the sign area, but block all other light from reaching the panel. I measured the spacing carefully and drew up the cut lines with Inkscape.

The lasercutter requires blue lines that are 0.01mm wide to control the laser for a full cut-through of the cardboard stock. Works fine for the cutter but makes the image impossible to print successfully or view on the screen. I made a copy where I could thicken the lines and make them black, solely for the purposes of printing the design.

I printed the design, cut it out and verified that it exposed and blocked all the portions that I intended of the LEDs, 7-segment displays and EL wires on the PCB. That verified, the order was placed with Ponoko and should be back before the end of the week.