Each board in the CTR2 ecosystem connects to the other boards using a 10-conductor ribbon cable. I don’t supply pre-built cables because the length of each cable depends on the stacking order of the boards.

By far the trickiest part of building CTR2 is constructing the ribbon cables. It’s not that they are difficult do build, it’s getting the connectors installed in the correct orientation that’s the challenge. Because of this, I highly suggest that you purchase extra ribbon cable and connectors because you’ll probably need to redo a cable or two.

Ribbon cable connectors are referred to as IDC types because they use a process called Insulation Displacement to make contact between the wire and the connector. Small, pointed split connectors are pushed into the ribbon cable during the crimp operation. They cut through the insulation and pinch the wire in the connector’s split. If all goes well they form a tight connection. If all doesn’t go well… you redo the connector 🙂

Because of the tight tolerances involved if you crimp a connector on crooked or the split connectors are not perfectly aligned (slightly off-center), cut wires and short-circuits will occur. I always recommend ohming out a cable after you build it. Check for both end to end continuity and for shorts to adjacent wires.

The example in this post describes the process of building the Radio I/O cable that connects the HMI board to the Auxiliary board. All other ribbon cables are built using the same techniques.

The lead photo shows a completed cable

I use old ribbon cables from junk PCs to build my cables. They’re usually more than 10 conductors so I just strip off a piece with 10 conductors in it.

First, cut the cable to length. The finished cable needs a little over 25mm (1″) between the faces of the connectors that plug into the boards (the boards are 25mm apart due to the standoffs used). I use 65mm for the unfinished length. This allows the cable to wrap around the strain-reliefs on each connector.

While you’re measuring, use the end of the ruler to square off each end of the ribbon. A square end is critical when aligning the cable in the connector prior to crimping.

Measure the cable and square off the ends

Before installing the first connector, take some time to plan out the route the ribbon cable needs to take. There are a couple of things to consider.

#1 Pin

First, locate the #1 pin on the connector. The photo below shows its location. There’s a similar triangle mark on the socket on the board. The #1 pin must line up with the #1 wire in the ribbon. Your ribbon cable should have a tracer (a colored stripe) on one of the outside wires. If you’re using a salvaged cable from a PC with no tracer, use a permanent marker to mark a wire on either side of the cable like I did in the photo. This is your #1 wire.

Determine the Routing

This is the most important step. Determine how you want the ribbon cable to route between connectors. This will determine which side the ribbon must exit the connector’s strain relief. For this cable I wanted the ribbon to exit toward the outside of the board from both connectors (i.e. from the even pin# side).

In the photo above, the ribbon was inserted into the connector from the odd pin side. It was then crimped and the ribbon was folded back over the top before the strain-relief was installed. The ribbon now exits on the even pin side.

Prepare the First Crimp

Insert the ribbon cable into the connector with its squared off end pointing the direction you want the ribbon cable to go once it wraps around the strain-relief. The photo below shows the squared off end facing the camera on the even pin side. The ribbon cable extends rearward on the odd pin side. Note the position of the IDC split connectors. They are positioned such that the splits are directly in line with the conductors in the cable.

NOTE: The squared end of the ribbon cable should be flush with the edge of the connector’s body.

I use a pair of slip-lock pliers to compress the two halves of the connector together. I work slowly and make sure the connector compresses evenly. You can use anything that puts a even load across the connector. A vice works well and there are connector tool kits available that make the job a lot easier.

When the crimp is complete the locking tabs latch to secure the two sides. Note how the square notches in the top half line up with the wires in ribbon cable. This connector is properly crimped.

Next, fold the ribbon cable over the connector and insert the strain-relief clip. It will lock into place. The finished connector will look like this.

The second connector goes on the same way. Align the ribbon cable so that the #1 conductor lines up with the #1 pin. The square cut end should face the same direction as it did in the first connector (on the even pin side in this example). Crimp the connector, fold the cable around the body and secure the strain-relief clip. The finished cable will look like the one in the lead photo.

The last photo shows the ribbon cable installed, linking the HMI‘s Radio I/O connector to the Radio I/O connector on the Auxiliary board.