Un-wirewrapping (and re-wirewrapping) a PDP-16

From: Richard Erlacher <richard_at_idcomm.com>
Date: Sat Jul 22 18:06:39 2000

They're not that costly, Tony. The last one I bought (for someone else)
cost about $5.

Unfortunately they only unwrap wires that were wrapped clockwise, and many
automatic wirewrapping machines wrap the opposing ends of a wire in opposite
directions. Another limitation is that they were designed for #30 wire, and
many backplanes, etc, were done with #26 or #28. #30 is the most common,

Please see my embedded remarks re: cut-strip-n-wrap bits below. These were
bought up by Advance Electric Co. which also makes the "Ad-Strip" Kynar
wire without which they work poorly.


----- Original Message -----
From: Tony Duell <ard_at_p850ug1.demon.co.uk>
To: <classiccmp_at_classiccmp.org>
Sent: Saturday, July 22, 2000 3:34 PM
Subject: Re: Un-wirewrapping (and re-wirewrapping) a PDP-16

> > Does anyone have any suggestions or pointers for tools and techniques
> > un-wirewrapping and subsequently wrapping new projects?
> You can get a little 3-in-1 tool -- one end of it is for wire-wrapping,
> the other end is for unwrapping, and there's a wire stripper blade in the
> middle. It costs about $20-$30 IIRC.
> You put the unwrapping end on the connection and turn counterclockwise.
> This loosens the wrap, and you can pull the wire off. Repeat until all
> the connections are undone.
> There are electrically-powered wrapping tools (either mains powered, or
> with internal rechargeable batteries). They are expensive, and I've never
> really seen the point for hobbyist use. The wrapping end of the 3-in-1
> tool works fine with a little practice. The only problem is that it's
> slower to use.
> Anyway, to wrap a connection you put the stripped end of the wire (about
> 1" stripped) into the small hole at the side of the wrapping end. Put the
> larger hole over the pin and turn clockwise. Don't press down on the tool
> -- you'll get the wire wrapped over itself rather then wrapping round the
> pin. Don't pull up on the tool either. This takes a bit of practice to
> get right, but generally the weight of the tool is such that it applies
> about the right downward force. So put the backplane (or whatever) pin
> side up to start with.
> I've found the built-in stripper on these tools to be useless, however.
> Get a proper wire-wrap wire stripper. Or buy pre-stripped wires, at least
> in the common lengths. I find the wrapping to be a fairly restful
> process, which is more than I can say for stripping the wires.
> Don't use the hand slit-n-warp tools. These claim to use unstripped wire,
> and to remove the insulation as you wrap. My experience, and that of
> others that I know is that they don't work. You get 50% good connections
> if you're lucky. The self-stripping bits for the electric wire-wrap tools
> are great, and do work fine, but then the bit alone is about $500 over
These bits are VERY expensive, about $250 US as opposed to $75 for the
"normal" bit that requires you cut and strip the wires yourself. I've been
using them since the late '70's, and have never found a reason to cuss them.

I've found that this type of bit, when used with the appropriate wire
(extruded one extra time after the insulation is applied, to break the
insulation loose from the wire so it comes off automatically when prompted
to do so) si worth its weight in gold. I have a couple of them and not only
do they last MUCH longer than the standard bits, but they make the job go
much faster. I wrap small circuits from memory rather than from a
schematic, though as I age I find the circuits I can remember have shrunk
from about 100 IC's to about 10. That means I can really go, since I don't
have to pick out the wire length or cut it to size. It's not unusual to
place four or five wires a minute when the wires follow a discernible
pattern. Once the board's full and there are wires all over the place, it's
a mite harder and slower going.

They do sometimes break the wire instead of just the insulation requiring
the wire be removed, including the broken-off end, but that seldom happens
if the right wire is being used. The make *wonderful* connections and the
tool lends itself very well to direct-line (strung fairly tight) wiring
which allows for randomizing the noise/crosstalk better than bundling wires
together. I've had a few cases where the wire breaks after handling, though
the insulation doesn't. It makes visual detection of the "open" impossible.
Fortunately I've only been embarassed a few times by this occurrence.

> One hint that I'll give you. You'll probably want to bus some connections
> together. Most people start and the first pin and wrap a wire to the
> second. Then wrap from the second to the third, then the third to the
> fourth, and so on.
I locate all my busses and wire them first, since they go fastest and work
most easily on a relatively empty board. Normally my circuits are designed
in blocks and I wire and subsequently check one block at the time, except
for the busses.

It should be kept in mind that it is NEVER necessary to use more than two
levels of wire. For that reason it's important to use the shortest
available pins, provided that they extend far enough through the board to
allow two levels to be wired.
> This is a bad idea, because if you make a mistake (and you will!), you
> may have to to take off _all_ the wires past the mistake. The unwrapped
> end of the 'trapped' wire from (say) the 3rd to 4th pin (trapped because
> you can't get it off the 4th pin without taking off the wire from the 4th
> to the 5th, etc) may well not be long enough to restrip and connect to
> the right pin.
> Instead. wrap 1st to 2nd, 3rd to 4th, 5th to 6th, etc. Then go back and
> do 2nd-3rd, 4th-5th, etc. If you make a mistake you only have to take off
> 3 wires and replace them, not the entire bus.
A little thought given to which pin-to-pin connections should be made on
which levels makes the job easier. It's important that signals on the same
busses be of about the same length and have about the same number of loads.
I try to wrap all the level one (closest to the board) wires first, starting
with the busses. That makes it easy to continuity-check them and know
they're done. It is pretty important to check the level-1 wiring before
going on to level-2, as it helps save wires-taken-off if you foul up a pin
count or something. This is all particularly important where connectors are
concerned. it's best if connectors are wired on level-1, since they
frequently lie at one end of a bus or another.
> -tony
Received on Sat Jul 22 2000 - 18:06:39 BST

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