Core Board Assembly
How to assemble the Core64 Core Board.
Last updated
How to assemble the Core64 Core Board.
Last updated
This is a challenging assembly process - by far the most challenging part of the kit. The first several steps are relatively easy, and the last steps are relatively easy. Somewhere in the middle is the delicate process of weaving. Just take it slow - do not rush it. I hope the techniques I provide will make the weaving go smoothly for you. I'm going to remind you to go slowly and be gentle about 10 more times before we're done. If you get frustrated at any point, stop. Take a break and let me know what step you are struggling with via Discord.
At some point you'll notice the Core Board has 8 pins which are labeled, but not used or connected to anything. They are the top and bottom pairs of holes on both the left and right edges of the board. From left-to-right, top-to-bottom: LSIG/USR1, 5V0/GND, USR2/USR3, +BATT/5VUSB. These pins are active in the Logic Board and LED Matrix, but unsused on the Core Board. They are labeled on the Core Board for reference/expansion if you come up with something clever.
Whether this project looks daunting, easy, or somewhere in between, these tips and techniques will help you be successful. Start with a clean and bright work area so you can find the cores when you drop them... Set up your PCB holder and soldering related tools.
Now is a great time to tune into some good music or a podcast!
In the kit you will find 1x Core Board, 70±2 Ferrite Cores, 100 inches (2.5 meters) of magnet wire (insulated annealed copper 0.2mm / .008in / 32AWG), 4 plastic rivets, 2 or 3 screen protectors, a tall 36-pin header, and in some kits the SAO 2x3 Socket is included here (otherwise it is in the Logic Board kit). The top of the core board is shown below. Wire and board color may be different than shown.
Be careful not to kink the wire when handling it. Any little kink or bump or bend in the wire makes it difficult to thread through the cores. The Y-wires are a little more forgiving, but the X-wires need to be S-M-O-O-T-H. The sense wire is much easier to pull if it is smooth as well.
To configure this core board to work as a single plane, 8 solder jumpers require soldering. They are JP9-16 on the backside, near the upper left and right:
The other solder jumpers and unpopulated components are designed to allow a stack of up to 8 core planes. I have not tested that yet, so it's not supported. There are more notes in the schematic if you are curious. https://github.com/ageppert/Core64/blob/master/Schematics/Core64%20CB%20v0.5%202021-02-28.pdf
Straighten and clean the wire with isopropyl alcohol and a lint free wipe. Making sure the wire is clean and mostly straight will make the weaving process easier. I pinch the damp cloth and wire between two fingers and pull the wire through gently, adjusting pressure of my fingers so the wire comes out mostly straight. If the ends are damaged, cut off 1-2cm so the end is clean and straight.
Cut a total of 16 pieces of wire each 8-9cm long. The Core Board is 8.5cm square for reference, so you can cut the wires to the same length as the Core Board is wide. This will given you 8 X-wires and 8 Y-wires.
This leaves the remaining long piece of wire for the final sense wire weaving steps after X and Y wires are woven.
Place the board upside down. Put a touch of flux on all 34 X-Y-Sense wire pads. Apply the flux sparingly so it doesn't wick through the pad hole. If flux gets onto the wire and the cores, it makes them sticky and a bit more difficult to work with.
Place a gentle curved 90° bend about 1cm from the end of 8 wires. Don't make it a sharp bend which will weaken the wire. Insert the wires from the backside through YL0 to YL7 so they hang down and are stopped by the bend. Skip the S1 and S2 positions - those are for the sense wire which is installed after the X-Y weave is complete. Solder the 8 wires in place on the back side of the board. You do not need to dwell on the solder joints at this time. In later steps we will come back and heat the joints and apply gentle tension which will soften the enamel and burn it out, leaving a good connection between the wire and the pad.
Flip the board top-up and arrange all of the wires so they are standing straight up and are not twisted against each other. This makes it easier to slide the cores on.
There are 70±2 cores included so you can loose only a few... be careful! If one disappears there is a 99% chance you will never see it again. I use a tweezers to pick them up and place them. Make sure you have a balanced grip on each side of the core so that your gentle tweezer pressure doesn't cause them to fly out of the tweezers.
Pouring the cores out of the bag onto a soft surface is helpful to prevent them from scattering. A paper towel or microfiber cloth is a good choice.
Using a tweezers, pick up the cores and slide 8 of them down onto each of the 8 wires from the top side. They will stack up nicely and you can inspect that all of the stacks are the same height to confirm there are 8 on each wire.
Gently arc the wires over to the opposite side and insert them into the YR0-7 holes. Slide the cores up to the middle before you pull the wires tighter. The top of the wire arch can be mostly flat/level so the cores stay in place.
Now you can gently pull the wires through from the bottom of the YR0-9 side to flatten them out. As you do this, make sure the cores closest to the ends don't become trapped/crushed between the wire and the board. They can be close to the edge like this:
Flip the board over and solder the wires on the back side at the YR0-7 pads. You don't need to dwell long with these solder joints because we will come back to them after all of the weaving is complete. Use just enough tension to straighten the wires. Final tensioning will be done after all the weaving is completed.
Important note about tensionin: I was using a curved nose hemostat initially, and it was way too easy to apply too much tension and break the wires. I have switched to using tweezers with smooth gripper faces, which is effective at being an automatic tension limiter.
Congratulations! Your abacus is complete!
The second and last batch of 8 wires is used next. Clean and straight, with a tiny little bend at the end is my preference. The bend is subtle, but SOOO helpful! Here is what it looks like, shown as the bend passes through the final core in the first column.
Check out this video showing and explaining the "bent wire" technique: https://youtu.be/PfxXD_jm978
To begin weaving the first column, slide 7 cores on each Y wire out of your way, and leave one column of cores to work with. All of the cores need to be installed with alternating 45° offsets from the wires, from top to bottom, and left to right. This pattern enables optimizations in the wiring and driving circuitry - it was figured out decades ago. The outer corners will all end up having their axis-of-revolution point inward to the center of the core plane. This alternating pattern also makes it easy to thread the sense wire diagonally. If this isn't making sense, here is a visual reminder of how all the cores are arranged:
Notice how all of the cores have alternating arrangements, which sets up to allow the single continuous sense wire to wind back and forth through all 64 cores later in this process.
It's way too easy to get focused on getting the wire through the core... and then fail to notice the orientation of the core. Keep double checking your work so you don't end up getting the column woven, only to find some cores in the wrong orientation.
Start at the lower left corner, and follow the pattern shown above. Gently nudge the wire through the cores from the bottom row, proceeding upward. Twist the wire back and forth 180° to make it easier to enter the next core in the column, depending on the orientation that core needs to be at. You don't have to be concerned with whether or not the vertical/horizontal wires are on the top/bottom of each other when they go through a core. I try to keep my weaving consistent "just cuz" but it really doesn't matter.
The further you get into the weaving process, the more challenging it can be to keep things lined up squarely. Be careful, try not to stretch the wires. They can handle a little bit, but you'll have much better results if you gently push and/or pull the wires during the weaving. Sometimes a core will bind and lock against the wires because it is at a 45° angle to both wires. When this happens, use a tweezers with each leg straddling the wires and nudge the core in the right direction to unlock the intersection joint.
After you complete a column, leave the wire flat on top of the board, roughly centered vertically. Check to make sure your alternating core orientations are correct.
Take a break. This is the most challenging part of the process. It's tedious, and if you rush it, things can go bad in a hurry. Work through this vertical wire weaving carefully 7 more times and you'll be through the worst of it.
Once you have the weave together and aligned, you can carefully insert the ends of each vertical wire into the associated column through-hole. Try not to create a sharp bend or kink in the wire between the array and the through-hole. Slow. Careful. Gentle. You get the idea... Here's the technique I use: https://youtu.be/vPc71MGQkeA
Next, flip the board upside down again, and gently pull the vertical wires straight. Solder all of the vertical wires into place. No need to dwell on the solder joint just yet.
Now we will tension and align the array. Once you have confirmed the core pattern is correct, you can dwell a little longer on each solder joint to make sure the enamel is burned away. Apply gentle tension to the wire with a tweezers while heating the connection. Dwell for 4-5 seconds, and remove the soldering tip - but leave the tension for 5 more seconds so the solder joint can solidify before you remove the tension. Go around the entire board perimeter and treat all 32 solder connections like this at least once to get good continuity at the solder joint. Here is a video of the techniques I use: https://youtu.be/shIGhJ2QlZo
The array should be coming together nicely and squaring up like this:
Do NOT cut the wires yet, even though they are shown as cut in the next image.
If you see solder joints where the solder is not wetting out and flowing on to the wire, you probably don't have a good solder joint. On the backside of the Core Board, where all of your wire weaving solder joints are, you ultimately want to see each joint look like this, where the solder flows up on the wire.
Now is a great time for a continuity check. Video of the technique: https://youtu.be/t-OxUjbSRk8
Use a multimeter to ensure continuity is solid. Measure each wire by putting the multimeter probe points into the PCB PAD at the end of each wire. In other words, make sure you are measuring for continuity through this entire circuit path: Solder Pad -> Solder Joint -> Wire -> Solder Joint -> Solder Pad. If any of the wires do not show good continuity, gently tension the wire and reflow the solder (add a touch of flux first) at each end of the wire to make a good connection through the enamel.
A nice taught (not tight, just taught) array will not only look nice, but it will help making threading the sense wire easier. Do NOT cut the excess off the X/Y wires yet.
At this point, you are through the worst of the weaving process! I think the sense wire is easier to manage. You are on the home stretch!
Using the third and final piece of wire you cut at the beginning of this process, start by inserting the sense wire from outside the bounds of the plane into the core in the upper right corner. You will want the tip as straight as possible. Gently push and pull the wire through the diagonal cores toward the lower left corner. When you get the tip of the wire to the lower left corner, insert it through the S1 pad hole, letting it extend about 3/4" or 15 mm through the board out the bottom side. Don't solder it right now.
Here is a video where I start pulling the wire from the lower left. The technique varies slightly but ends up the same. https://youtu.be/LnRh11oBx6Q (ignore the weaving fixture, it wasn't that useful).
Be careful handling this wire, especially the long floppy part that is outside of the plane while you are focused on weaving the tip. You don't want to let it snag on something and get a kink in it.
The next step is to take the free end that is extended out of the upper right corner and follow the path taking shape here:
It is helpful to use a tweezers, and be slow and careful. When you turn the corners, it helps to have a small blunt tip object to arc the wire around and get it to form the right shape. You want to avoid kinking the wire or putting too much strain on the X-Y weave that is already in place. Here is an example using the outside edges of the tweezers (with it's flat faces squeezed together against each other) in order to guide the wire in a gentle curve while I am gently pulling on the wire from the upper left (out of view):
The sequence for the sense wire 1 to 15 as shown here:
Now solder the sense wire on the back side at the S1 and S2 pads. Verify good continuity with a multimeter. You can put the probes on the SEN1 and SEN2 pads in the middle of the left side to ensure the connections are good. Another video of these last steps: https://youtu.be/SAAkIWg5GDQ
During the sense wire weaving process, you might have bumped the X/Y wires out of place. You can gently nudge them back. If needed, you can go around the perimeter of the board and reflow and retention the wires.
When you are happy with the wire alignment, and have double-checked the continuity of all the wire solder joints, you can finally trim all of the excess wire off the back side. A few more details in the video: https://youtu.be/g-dSgPB2wDE
Admire your work!
The left and right edges of the core board have 20 solder pads, but you will only be soldering headers into the middle 16 pads of each side. The top and bottom pairs, on both the left and right, are not used in the core board. Cut the 36-pin long header into two 16 pin headers, leaving 4 extra pins to set aside for optional use later. This is how the Core Board will look with the long pins of the headers on the bottom side of the board.
Using the LED Matrix as a fixture (do not solder to it in this step), sandwich the headers in place against the Core Board. In this photo, the top of the Core Board is facing right against the spool of solder, and the LED Matrix is on the left, with the LEDS facing right, under the Core Board.
Cut the excess header pins off that are sticking up above the top surface of the Core Board. Proceed to solder the headers in place. Keep them parallel to each other and perpendicular to the Core Board. If you solder each pin once, and let the solder flow in and fill just barely to the top of the pad, you can come back around a second time and touch a small bit of solder to the top of each pad and get a rounded top on the solder joint. This will prevent scratches on the screen protector.
Here are the headers poking out the backside of the LED Matrix board. The Core Board is underneath, face down, and not visible:
After all of this soldering, clean off any flux with isopropyl alcohol. Be careful around the wire weave.
Congratulations! Core Board assembly is complete. Time for screen protector.
