You might notice that many of my writings start with “Back in the day”. Not wanting to disappoint I will say that back in the day we used to use wire wrap technology when we needed a somewhat solid, somewhat reliably assembly. Given a readable schematic a good tech could return a working or near-working unit in a day or two depending on the completeness and accuracy of the schematic.
Properly done a wire wrap assembly is capable of fairly high speed and acceptable noise when the alternative option of creating a custom PCB would take too long or not allow enough experimentation. Wire wrap is also used in several types of production, from telco to NASA, but I am all about the engineer’s point of view on this.
My first wire wrap tool and wire wrap wire came from Radio Shack in the mid 1970’s. I still have the wire, because frankly its kind of cheap wire and I use it when it’s the only thing I can reach quickly when I need to make a jumper on a PCB. The tool is still around also, given the fact that I can’t find it at the moment the one shown here is my new wire wrap tool which is good for low quantity wrapping, unwrapping and stripping.
The skinny little wrap tool is okay for hobbyist as the wraps are fine with a little practice. But I do recommend investing in high-quality wire. A common wire available is Kynar® coated, a fluorinated vinyl that performs well as an insulator.
Before I go too much further, here’s the video walkthrough of wire wrap, its uses, and several demonstration. But make sure you also join me after the break where I cover the rest of the information you need to start on the road to wire wrap master.
Another important You will also hear me harp on good power and ground layout; “stubs or lengths of power supply wire that terminate in dead ends should be kept to a minimum and the power and ground wires are routed in close proximity to each other. Inductively coupling the supply and return together reduces impedance, hence noise. Obviously there is a lot more to it than that but that’s for another video.
A good wrap has 3-4 turns of the exposed wire on the square post and a turn of insulated wire which improves resistance to vibration and vibration based failure. The drawing here is from the NASA standards part of their website showing an acceptable wrap.
Wire wrap sockets are still available and I keep wire wrap SIP terminal strips as a custom socket footprint can be created as needed.
For more complex wire wrap components such as connectors you might have to find other sources such as electronic junk-yards. Hackaday’s [Brian Benchoff] was able to get a 64 pin “Hershey Bar” wire wrap socket from Apex Electronics in Sun Valley CA with an assist from [Todd Black].
Little slip on tags are available to help keep track of “reversed” pin numbering as seen from the bottom of the board.
Wrapping a wire on a post is accomplished by first stripping back the insulation on the wire wrap wire to 1″. The bare wire is then inserted into the hole on the wire wrap tool end (that is not in the center), and then the wire is pushed in until it is stopped by the tool. Next the tool and wire is placed over the specific square post being sure that the post goes into the hole in the center of the tool. For a manual router the tool is rotated with slight downward force to keep the wire wrap tight until the wire is completely wrapped, between 5-6 turns.
With the hand operated or power tool the same procedure is performed with inserting wire into the tool, then placing tool and wire onto the post, and finally the tool wraps the number of turns it is set up for.
There are different techniques for organizing the wires on a wire wrap board; sometimes the wires are routed together down channels and sometimes the individual wires connect straight across the board in as little length as possible. Both techniques have pros and cons as crosstalk and impedance are affected by wire placement.
Discrete components can be soldered to headers that then insert into standard wire wrap sockets as shown above with the 3.3K resistors.
And finally a technician named [Jeff] in the old MOS/Commodore R&D lab showed me how his boards never seemed to have much slack in them. He would take one of the skinny wire wrap tools and get on a socket pin/tail, then rotate the pin itself taking up the slack. While this looks better what he didn’t really address was the fact he was making a more pronounced inductor at the end of some wires. The answer to this technique as well as the channel routing technique (the techs would actually lace up the little bundles some times) was that my boards were to be “jungle routed”, I.E. more or less straight connections between pins, and slack was to be dealt with by adding some convolutions in the wire between the two pins by looping over other pins, similar to what you see on a high speed PCB where a trace will take a few extra turns to control the length/propagation time.
I should tell you that [Jeff] threw away my coffee cup one day because of what I had growing in it. From that day until I left Commodore I would make at least one trip a day to the R&D lab to throw his coffee cup away.
[Wire wrap diagram is from the nasa.gov website on Discrete Wiring]