General Notes on Wire-Wrap Construction.
First, let’s discuss tools. There are inexpensive manual wire-wrap tools available, but in my view they are of little value for anything but minor repair work. Think about it; on a project of any size, you’ll be making literally thousands of connections! You must recognize that wire-wrap is a relatively expensive technique, and that you must invest in some expensive tools…
I use a line-operated wire-wrap gun from Gardner-Denver (a division of Cooper Tools), the originators of the technology. OK Industries manufactures similar guns. They cost around $300 including the “bit” and sleeve. Mine has a standard 30 AWG bit that produces a “modified wrap” (a couple of turns of insulation at the bottom of the wrap).
You will also require a good stripping tool; I prefer the OK Industries ST-100-30. To remove a wire, I use the OK UW-093R unwrapping tool.
I bought a selection of pre-cut Kynar 30 AWG wrap-wire in lengths ranging from 3 inches (including the 1-inch pre-stripped ends) to 8 inches, and a big spool for wires longer than 8 inches. All of my wire is red, which gives good visual contrast with the common board materials, but many constructors prefer to “color-code” different portions of their circuit or to use different colors to denote differing revision levels, et cetera.
After using the pre-cut wire for a short while, you will get a feel for selecting the proper length for each connection.
There are special all-in-one “cut, strip, and wrap” bits available for the wire-wrap gun. These ought to be able to eliminate the manual stripping step when not using pre-cut wire, but I have had poor luck with them and they are *very* expensive.
How to Build?
For small projects (smaller than 30 IC’s, say), I just install wire-wrap sockets on a perforated board. Boards having power and ground planes are preferred from an electrical standpoint. Vector Electronic Co. made a nice series of such boards that they called “Any-Dip Plugbords;” unfortunately, most of these have apparently been discontinued and they have vanished from the catalogues. They may still be available in old stocks.
Use glass-epoxy boards. Phenolic boards are cheaper, but their electrical and mechanical properties are much inferior and they smell.
If you need more than ten or so boards of the same type, consider laying it out yourself and having them made for you by a small PC-board house. You can have power and ground planes, edge connectors, specific pads for capacitors, or whatever you need; you might even beat Vector on price!
Decent wire-wrap IC sockets are costly (in general, they will cost more than the IC’s they support!), but they are essential for reliability. Troubleshooting and maintaining a circuit built around cheap, junky sockets is an utterly miserable task.
I prefer machined-pin sockets. If you buy sockets having “side-wipe” contacts, be sure that they wipe both sides of the IC pin. I have had good results with side-wipe sockets from Robinson-Nugent, and with machined-pin sockets from Augat, Mill-Max, Aries, and Assmann.
You’ll probably need other wire-wrap pins in the board, primarily for mounting discrete components such as your bypass capacitors. I use Vector T-44 pins, driven in using an A13 insertion tool or with Vector’s hard-to-get (and very expensive) spring-loaded snap-action insertion tool.
For really big projects, such as the D16/M CPU, the “perf-board” approach becomes less and less practical. For these, it makes more sense to use large pre-prepared wire-wrap boards of the sort made by Augat and Mupac. Such boards are expensive new, but they are widely available as surplus.
Wire-Wrapping Tips and Techniques.
When wire-wrapping, try not to put too much tension on the wires. Leaving a little slack will prevent pins from getting bent, and you’ll be less likely to cut the insulation when going around other pins. Always keep the wire-wrap bit perpendicular to the board. Avoid kinking the wires.
Never try to re-use a wire or an end of a wire. After you unwrap a wire, its end is work-hardened, scraped, bent up, and fatigued to the point where it could never be relied upon for a good connection again. Cut off and re-strip the end if the wire is long enough, or replace the wire outright.
A good modified wrap has at least one full turn of insulation at the bottom, with all of the non-insulated turns tight and compressed. Should you get a bad wrap, take it off and re-do it; don’t be tempted to leave a sub-standard connection in your circuit.
Should you have chronic difficulty getting quality wraps, there may be a problem with your tool. --Did you drop it on the floor? If the bit is bent, you’ll have to replace it—I am not aware of such damage ever having been satisfactorily repaired.
There are two basic approaches for routing the wires. One involves orienting the wires at right angles (or on a fixed-angle diagonal); the other involves routing each wire for the shortest path. The first approach looks much neater, but it can have problems with cross-talk and noise when high-speed signals run parallel for a considerable distance. The second approach is probably better electrically, but it can look like an absolute “rat’s nest” on the board! There are vocal adherents to each way of doing this; you must make a choice based on your sense of esthetics and your own engineering judgment.
When wire-wrapping large circuits, I recommend preparing a wire-list and working from that, rather than working directly from the schematic diagram. It might seem like extra work to create the list, but while working on the D16/M, I found that it facilitated the construction task considerably. It also made it easier to plan, make, and keep track of changes, because the wire list uniquely specifies each connection on the board.
However you plan your circuit, you should employ a two-level wire-wrapping scheme (that is, you should put no more than two wires on any pin). This way, breaking any "net" requires the removal of no more than three wires. Planning the two-level connections is easy when preparing the wire-list; while wiring a complex net (such as an on-board bus line) directly from the schematic is difficult and error-prone.
Even if your board is small and access to both sides while working on it is easy, it’s a good idea to have some means of identifying socket pins from the back of the board. I used OK Industries Wrap-ID® tags on my boards; they are entirely too expensive (around 50 cents apiece in small quantities) but they saved me much time and trouble.
After the wiring is finished, conduct an “open pin” inspection. Look at every socket on the board, and if you find a pin with no wires on it, check the schematic to verify that the pin should indeed be open (an unused gate output, say). Of the four wiring mistakes that remained after I finished wrapping the D16/M processor board, I found two of them this way.