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BA11-L mounting brackets.


Veteran Member
Sep 30, 2013
Stockholm, Sweden
I have a small PDP-11/04 that is in the BA11-L chassis. 5,25 inch high. It is missing the rear mounting brackets. Anyone has a BA11-L rackmounted that can take som photos?
I have two such machines and neither is presently mounted in a rack, but it is not clear to me what you need photos of. The BA11-L chassis slides into a metal enclosure ("envelope") that is secured to the rack with four 10-32 screws and KEP nuts at the front and two at the rear. A chassis “extender” at the back of the BA11-L includes two latches which must be depressed to fully remove the PDP-11 from the enclosure but there are no “rear mounting brackets” on either assembly. Might you be missing the enclosure that bolts into the rack?

Perhaps you are right in that there are not supposed to be any L-shaped brackets for mounting the sheet-metal envelope. I got this suggestion from others as well.

The problem is that when I mounted it into the rack the holes (three by the way) didn't line up very well vertically with the rear rack post side holes.

Then the size of the screw is not the normal #10 pan head that DEC uses for most rack mounting. The nuts that are riveted into the envelope is #8 as far as I understand. The machine is 100km right now so I cannot check for sure, but looking at the picture and my remembrance of it gives that a #10 would not fit. It also has to a very short screw and perhaps a small shim in between the envelope and the rack post?

All this together made me think that there were to be brackets to hold the rear end of the envelope in place. Hence my question if anyone had one rack mounted already or perhaps know. The BA11-L manual doesn't give much information in this area.


However I did take another look at the drawing again and now I think I can see a screw in that area from the rear rack post into the envelope. It also mentions an item 54 (space enclosure) with a note #8 that says that there indeed is supposed to be a shim of some kind to handle the distance between the envelope and the rack post.

"8. Item #54 to be used A/R to take up any gap present between enclosure and cabinet uprights."

What is A/R?

Looking at the drawing it looks like that the screw head is on the inside of the envelope and that the nut is placed on the outer side of the rack post. But the drawing had perhaps not been updated for use of the riveted nuts?

I will make another try at this next time I get to my machines.

I have an 11/23 in a BA11 and sleeve as described above. The hand written label on my BA11 says it is a "BA11-(scribble)" so I can't be more precise about the type. The drawing above shows something similar but it looks like the "sleeve" is mounted in a cabinet of some sort instead of a rack. And it is offset to the right. Mine is not. Note in the drawing the bracket on the right is a part #4 which appears to be two "L" shaped pieces joined together with two sets of (probably #8) hardware (#12 & #13) and a pass through hole with a grommet. (#20). On the left side the bracket is a part #8 with a similar set up but the bracket itself has a somewhat more complicated shape.

Mine looks more like your picture. I can't post a picture of mine at this time due to a lack of a USB-C to USB-A cable. "For want of a nail..." and so forth. And I can't find anything similar posted on the web. So a description will have to suffice. My brackets are "L" shaped, with the "L" laying on its side. They are fabricated from 0.060" steel. Referring to your picture, the long side of the bracket measures 4.75" from right to left, 3.25" from top to bottom, and has a 0.75" flange that wraps around the rail from the inside to the outside. At the sleeve end there are two horizontal oval shaped holes to accept #8 hardware that threads into the captive nuts in the sleeve and allow some adjustment front to back to fit the rack. At the rail end, there are two horizontal slots to accept the #10 hardware to fit the threads or captive nuts on the rack and allow a small amount of adjustment for the width of the rack. In addition to the fact that you're in "Metric Land" and I'm in "Inch Land", you're probably going to need to adjust the top to bottom and front to back dimensions to suit your situation, and perhaps add provision for 3 sets of mounting hardware vs the two on my setup. I'm going to guess that your unibus 11/04 weighs quite a bit more than my qbus 11/23. In your situation, I'd be happier with 3 #8 machine screws on each side.
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My first response to your post was based upon recollection of how my enclosures had been mounted, but your photos made me wonder how accurate my memory was, so I investigated further over the weekend. I personally rescued one of the enclosures and all the mounting hardware from a DEC H9645 cabinet whose history is familiar and which had never been altered, so I am quite certain what I describe below is original to DEC.

The rear of the enclosure was fastened to the rack on each side by a single 10-32 5/8" long truss-head screw, toothed washer and KEP nut. As you already stated, the space between the enclosure and rack rail was also filled with shims. The screw (fastened from inside the enclosure) passed through the toothed washer, enclosure, shims and then nut (in that order). In my case, two shims (each about 1/16" thick) were found between the enclosure and rack rail on each side.


Most curious though is that your photos would indicate that this method of mounting could not have been used with your enclosure as it appears to be missing the large, oblong hole which accommodates the mounting screw. On my enclosures, this hole measures 1/4" x 3/8" at the widest point. The center of the hole is 13/16" from the rear edge and 1 1/2" from the bottom edge of the enclosure.



I'm thinking this discrepancy may be the result of engineering changes. You mentioned that you have a PDP-11/04 which I believe may be one of the earliest uses of a BA11-L. (The machine I rescued was a PDP-11/24 built in 1981.) In any event, it appears we still have no real evidence of how yours was originally mounted but I hope this was somehow helpful anyway.
Hi Mattis,

I've been given this PDP11/34. I was wondering if your machine also 'too long' for the case... I'm not sure if this is the original housing, but it seems that it has the same depth as your case. It is not really portable like a small 11/10 because the machine can move inside the case. If this is not original I would like to know what I need. Maybe the housing for the H960 rack differs from the later PDP11 racks?

Regards, Roland


What puzzles me even more is that the head of your screw would interfere with the BA11-L chassis. It is quite tight fit between the chassis and the shell as can be seen on my picture above. Is there are actually space for it? Looks strange but I am not nearby it right now so I cannot verify.

@Roland Huisman

Yes. Mine is also about five centimeters longer than the shell. I see that your shell also have the same hole that @KM11 describes.

Then I am still a bit curious on how mine was supposed to get mounted to the rack. Perhaps mine is an earlier model where the idea was to mount three #8 screws from from the rackposts. Then they recognized that very often the fit of the rack to the shell was not 100% and they added this elongated hole in the side for the purpose of easier fit?
Referring to the picture in post #9, just below the unit with the three fans is a BA11 that looks quite similar to mine. The mounting hardware on the right side of the picture looks identical to mine. The hardware on the left side is not clear enough to say for sure.
That is a BA11-N chassis not a BA11-L. Not sure that the sleve is the same. But I have a couple of BA11-N so I will check if they are same. I don’t think the BA11-N is as deep as the BA11-L. It extends way further. Thus it would interfere with the mounting screws for the bracket.

However that was basically how I thought it was to be mounted with a L-shaped bracket.
The cable I needed to move files from my new phone to my laptop showed up a few minutes ago. So, from the "picture is worth a thousand words" department, here are pictures of how my BA11-(?) is mounted into my home built "teacart" like rack.

The computer is a PDP-11/23. The hand written label on the BA11 box is illegible. Maybe someone on here knows what variant was used to house an 11/23.


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I'm pretty sure that we're confusing two different shells. Mattis is speaking of the upper one in the attached photo (not my photo; don't recall where I originally saw-and-saved-it). It's 23.5" deep, which is 1.5" short of the rear-column face on a DEC-standard 25" deep rack. Any applicable L-tabs will be short. However I believe that it was designed to attach to the column-sides directly, using a shim, either as identified by KM11 from inside-out (possibly with clearance issues even with a pan-head) or using _short_ 8-32 UNC bolts from the outside-in (rack to the nut-inserts in the shell). IMO the latter is the preferred mounting technique.

Other folks are speaking of the lower one in the attached photo. It's shorter and needs serious-length L-tabs added to reach *anywhere* for attachment. I've seen a variant that is separated into two halves and has no top-sheet. I have at least one set of those, but I also have one like the photo. There may have been a variant that was a full-shell.

Importantly, note the differences in ventilation slots, which corresponds to differences in the design of the chassis that goes inside. The upper-shell has ventilation only on the right-front side, specific to the power supply; the modules are ventilated side-front-back (L-path, unless the applied face includes ventilation openings of some sort -- e.g., under the lip of a protruding control panel on an 11/04), thus "Unibus style" for 6-high modules. The lower shell ventilates entirely side-to-side -- which everyone will recognize is "Qbus style" for 4-high modules :->.


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So to clarify this in my mind, it seems we're talking about (at least two) ways of mounting a PDP11. Is there a factor that can be identified that controlled which way DEC used in a particular case? Was it determined by whether it was to be mounted in a rack vs in a "cabinet": i.e. front and rear corners to the uprights of a rack vs front corners and rear sides to the walls of a cabinet? Or was it determined by the version of the PDP11 being mounted?

I admit to being a bit puzzled by the drawing in post #3. There is obvious provision for the unit to be slid and out of some kind of enclosure. Note the provision of a "service loop" in one of the cables on the left side. Yet there is no mention of being mounted in a rack. The notes in the lower right and left refer to a cabinet. As best I can recall, all of the PDP11s I've seen were mounted to a "rack" at the front and rear, even when the "rack" was a sheet metal stamping that was part of a "tea cart" like my 11/23 or a "business cabinet" like my 11/23 Plus.
WRT "mounting a PDP11", there are *many* ways ... almost as many as there are models! At the moment we're looking at "slide-in" style mounting in a 19" rack (of any height or external "finish"), and the mountings are different because the chassis are different because the bus-design (and thus supported modules) are different. Note that in later QBus systems there is the deskside pedestal mounting -- no rack (or cabinet) involved, basically just a plastic clam-shell.

In Post #3, the "service loop" is the ground wire from the chassis to the rack-frame. The diagram shows the "cab(inet) frame ref" for two posts of a rack. Cabinet and Rack are synonymous in this usage. One variant of the H960-type "rack" is commonly called the "corporate cabinet".
This topic has certainly generated more discussion than I would have expected. With respect to mounting of the BA11-L enclosure, I think you may find the following document of interest:


Although admittedly not the best drawing, page 2-8 would appear to corroborate the configuration I described earlier.


Also note that there is ample clearance between the BA11-L frame and screw head, at least in my case.