seeking advice on moving PDP 11 and related gear

tony duell ard at
Thu Jul 2 10:08:04 CDT 2015

> Ive been trying to get a pdp 11 for quite a few years now, I recently found
> someone selling a 11/34 with related gear in a couple of racks Here in my
> state of florida. I jumped at the chance and bought it, i have not found
> anything for sale this close to me before, yet alone in my state.

The 11/34 is a pretty good 'starter' Unibus PDP11 IMHO. It has memory management
and 18 bit addressing (so a maximum of 124kW of RAM [1]). But it is simple enough
to be able to understand and repair easily.

[1] A PDP11 address addresses a byte in memory. So 18 address lines gives an
address space of 256k bytes or 128k words. But the top 4k words are reserved for 
I/O devices, boot ROMs, etc. So 124k words of RAM. Of course it is a 16 bit machine
and will read or write 2 bytes in one go in case you are worries.

> The plan is to go get it all later this week. It is a 11/34 in a rack with
> a bunch of related Rl drives and a couple of big reel to reel tape drives.
> Not sure the specifics anymore, after I paid him he took down the ad, and i
> did not write down the details. What I want to ask is is there anything i
> should look out for, any precautions I can take to make sure this all gets
> back in one piece? Especially in relation to the RL drives, ive never dealt
> with any of this equipment before and am not sure what to expect.

Things to take with you : 

A good hand tool kit. You will need assorted flat blade screwdrivers, 
#1 and #2 Phillips. A set of inch-size nutdrivers and allen keys is very 
useful too. And a pair of sidecutters ('DIkes'). You do not cut cables, of
course, but you may need to cut cable ties. 

A notebook and pen. And a digital camera if you have one and if the 
seller doesn't mind you taking photos (if you are getting it from a 
company they may, quite reasonably, prohibit any photography 
in their buildings). Even if you take photos before dispmantling, you
should still make notes as to where cables go, in case it is not clear.

Tie-on labels. You may want to tag cables when you disconnect them.

Boxes for screws, cable clips, cables, etc. You will have loose parts.

A friend. The RL's and 11/34 CPU are not hard for 2 people to lift and
carry. But I wouldn't want to do it on my own. RL's maybe, but if I had
to move an 11/34 single-handed, I would take the PSU off first which is
more work. 

Then you need to disconnect things. You will find the AC mains cables
from everything are plugged into a 'power controller' box at the back
side of the rack (normally). This will have 2 classes of socket outlets, 
'switched' and 'unswitched'. The CPU normally plugs into an unswitched
one, everything else into switched. But check and make notes. Then
unplug the lot and free the cables from any clips or ties so they are all

Still on the power controller you will see a little 3 wire cable. Unplug
it. The other end goes to a similar socket on the back of the CPU. 
Unplug that end too. If the cable is free, remove it, and store it 

On the RL drives there are 2 connectors one above the other on
the back. The plugs have a little quarter-turn locking peg at the end
the cable doesn't come out of. Turn that counterclockwise to free
them. And unscrew the P clips that hold the cables to the drive. 
You should have a terminator (looks like a plug with no cable)
in the last drive, then cables between the drives, and a
cable back to the CPU. This might go to a bulkhead bracket 
screwed to the rack. Unplug it there in the same way. 

Now for the RLs themselves. You _MUST_ make sure there are
no disks in them, which means opening them up. The problem
is that there are interlocks which mean they will only open if powered
up and connected to a working controller. Not a good idea at this
point. Pull them out of the rack one at a time. They will come part way
out, Now look at the sliders, you will see a little rocking lever on them. 
Frob this and the drive comes even further out revealing the top rear
cover held on by 4 screws. Undo these, lift the cover, unhook it from
the rear edge of the top door, then lift the front edge first so that
it hinges on the cables, and hang it on the back of the drive. Now lift the 
rear edge of the door slightly, slide the catch and lift the door right off. 
There are a couple of spring loaded arms that will snap over and scare
you, but nothing to worry about. Look inside the drive. If you see a disk
pack (white thing) inside, go very carefully. Lift up the read/write module
(middle of the rear section of the drive) and check the heads are fully
retracted. If not, they may be toast anyway, but carefully move them 
back (this is a darn bad thing to do to disk and heads, but since the
heads have already hit the disk, it's not going to do any more damage!)
Then remove the disk pack as described in the manual. Now, there
is a locking clamp for the RL head and it's shown in the manual. You can
move it to the locked position before puttiing the covers back, but my 
experience is that if you transport them back end downwards the heads
will not move. So if you can't figure out the clamp, do that. Put the covers
back on the RLs

Now look at the slides again. You will see one clearly visible screw holding the 
slide to the drive. Remove it. Then press in the leaf springs that are preventing 
the drive from going back in the rack, and slide it back until another screw is 
visible through the hole in the slide. Take that out too. With all the screws out
pull the drive fully out again and lift the catch on the bracket on top of the slide 
(on the drive unit). Slide the drive a little further forwards and lift it off the
slides. Stand in on its back. Push the slides back in and if possible tape them so 
they don't slide out at the wrong moment and hit your shins...

Now for the CPU. Again pull it all the way out of the rack. Loosen the screw each side 
of the top cover (on the side of the CPU box) and slide the cover to free it. Lift it off.
There is a cable clamp strip at the back on top of the PSU held down by 3 screws. 
Remove it to free the cables and either store it or put it back under the cables for the
moment. Now, for each cable you have a choice. If it's free you may want to leave it and
move the CPU with it dangling off . If you don't, or if it's fixed to something else, remove
the PCB that it connects to, make a note of how it is connected (which connector, which way 
up, etc) and unplug it. Put the PCB back in the right slot. With all the ccables removed, 
put the top cover back on the CPU to protect it, and then press the buttons on the sliders.
With a friend helping, pull the CPU right out of the rack and put it down.

If there are any Unibus expansion boxes then most likely they will be like the CPU, Deal with
them in the same way.

I can't comment on the tape units as I don't know what you have.


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