Recently, I built a cable with pin 0.1 inch header connectors so that I could modify my Macintosh SE to locate the clock battery off the motherboard, and to provide adaptability in what kind of battery I have connected. Cool. Unfortunately, there were a few obvious design oversights that made the cable a bit more difficult to build.
I bought standard pin headers and sockets that are designed to be soldered on to circuit boards. I just take these and solder them onto the wire, easy, right? Not so fast. Before getting too far along, the potential problem I’ve observed was that this method provided for no strain relief mechanism. Any stress when pulling the wire, whether accidental or intentional, would transfer all strain to the solder joints, which runs the risk of breaking them. And once your solder joints are broken, all heck is out. Not only does your primary cable connection not work anymore, but the exposed wires may violate your power wiring conventions of always having a power source connected through sockets, never pins or plugs. Once you’ve got a power source with stray pins sticking out, that could cause a short through other components and damage them, effectively ruining your vintage computer. That is definitely a no-go.
Typically, insulation displacement and crimp-on connectors provide strain relief by default. That is, after all, how they are designed to work. At the wire to connector connection, much of the insulation is left on the wire and the connector physically grips onto the insulation. This essentially gives you a degree of strain relief for free. Solder-on connectors such as DE-9 connectors, by contrast, must provide an additional separate mechanism for strain relief. You will note this by the outer shell of such connectors having a gripping end at the entrance/exit of the main cable bundle. Likewise, some punch-down connectors like RJ45 Ethernet will also have such a similar additional clamp at the cable entrance/exit for strain relief.
On Digi-Key, “Rectangular Connectors - Free Hanging, Panel Mount” is where you should look for these kind of connectors. Insulation displacement connector (IDC). Not “pin headers” as those are what go on boards. Wire to board, panel mount is what you are looking for.
20191207/https://en.wikipedia.org/wiki/Insulation-displacement_connector
Another issue with soldering pin headers to wires is that, well, they are not designed for that kind of soldering. In the case of solder-on DE-9 connectors, there is a concave connector protruding from the D-shell connector where you can place wire leads in and get plenty a good solder connection secured. But pin headers on their own come without such a structure. The intent is that they are to be soldered into the holes of a circuit board, and that will provide the adequate structure. Also, a circuit board provides insulation between the separate pins, which direct soldering will not provide for you, giving you the risk of a short.
Therefore, to resolve both of the problems of my soldered-on connectors, (1) lack of strain relief and (2) risk of shorts between soldered connections, I cut out a small strip of paper, inserted it between the soldered pin connections, and wrapped it around the outside. This isolates the soldered connections from shorting with each other or any external components. Finally, I taped down outside of the paper to both the plastic part of the pin header and the wire insulation itself. This completes the design of a strain-relief connector by distributing force between the plastic edge of the pin header and wire insulation. The solder joints, by virtue of the paper being wrapped around them, are held in a steady location where they will not be subject to unnecessary stress, thereby reducing the risk of breaking a solder joint. And even if a solder connection does break, it is much less likely for it to come loose in a way that causes a short due to the strength and closure of the strain relief structure.
The final problem that was left with my pin header design was lack of polarized connectors. Although I had Molex PicoBlade connectors that were polarized, and some of which were also crimp-on connectors, I wanted to use the pin header connectors instead because theye seemed to be more consistent with the vintage technology of the Macintosh SE. But, this is where I fell short. Almost every single pin header connector inside the Macintosh SE was a polarized design. Had I thought a few steps ahead and bought the right parts, I could have also used polarized two position pin headers similar to the sound connector on the main logic board. And that also would have provided me with strain relief on the connectors.
Because I am doing a retrofit modification, chances are that reversed polarity will be very unforgiving, possibly causing damage to the circuit. Make no mistakes when inserting the pin headers. The safest way is to disconnect all batteries, insert the pin headers, double-check for correct polarity, and insert the battery last. Likewise, it is safest to remove the clock battery first thing when opening the computer for servicing, but this comes at the expense of loosing all your PRAM.
But, alas, these are all shortcoming that I’ve made, and regardless, I am fortunate that the whole system works nicely!
Also, one last note on polarized versus non-polarized pin header connectors. After this discussion, you might think that polarized pin header connectors are always superior to non-polarized connectors, so why use the non-polarized connectors at all if you are using a 2-pin header pair? There are, of course, some uses where the connected device does not need a polarized connection, such as a connection for a tactile switch. Or, if you are expecting alternating current like for a speaker wire connection.
ANOTHER note on polarized connectors in the Macintosh SE. You can rest assured that my statements of reversed polarity causing damage. Why? Because the clock circuit’s power system is designed with diodes so that the chip can be powered either from the motherboard power source OR the battery source. A diode-OR circuit is configured, which is also necessary to ensure that the motherboard power does not attempt to send power to the battery like “recharging.” As a nice effect of this design, it also means that the clock chip is protected from reverse polarity damage. Also, it’s worth noting, that if “diode D3” is not working, your clock chip will not work at all due to lack of power.
Another comment on strain relief connectors. Soldering directly to a circuit board with through-hole components is great because it causes the circuit board to act as a pretty good strain relief connection. If you’ve got a great solder connection, it’s not just the solder joints bearing the load, the force is well transferred to the through hole areas of the circuit board itself, thereby ensuring that the solder joints don’t need to take all the blunt of the force. Through-hole perfboard also works very well, even if you are using a sheet of cardboard for your circuit board.