New PDP-8/E acquisition

[Mike Katz]

I have recently acquired a running (I haven't received it yet so I don't
know for sure) PDP-8/E and what looks to be a PC05 (I haven't actually
received the system yet so I'm not sure).

The PDP-8/E comes with only one 20 slot H919 Omnibus back plane.  I have
previously acquired a separate H919 back plane which I would like to add
to the PDP-8/E case.

The assembly drawing for the PDP-8/E calls for some kind of foam to be
placed under the back plane.  Does anyone know what I should use?  
Should it be conductive or just something to keep the pins from shorting
to the case itself.

Does anyone know where I can buy, beg, borrow or steal 😁 a pair of M935
Omnibus Bridge sets?

The PCP-8/E has the following boards already installed:
    M8330    Timing Board
    M8310    Major Register Control
    M8300    Major Register
    M837      Extended Memory/Time Share Control
    M8650    Asynchronous Data Control
    W966      Wire Wrap Board (unknown circuit)
    M840      High Speed paper tape reader/punch control
    M849      RFI Shield
    G104       4K Core Sense/Inhibit
    H220       4K Core Stack
    G227       4K Memory X/Y Driver
    M8320    Bus Loads/Terminator

I will be adding an M848 Power fail board that I already have.

Does anyone know of a way to interface the PC05 to the M840?

Also, I am looking to acquire the following boards to add to the system:

    More memory (core or semiconductor)
    M8340    EAE Board 1
    M8341    EAE Board 2
    M868      TD8E DECTape Control
    M8360    Data Break Interface
    M8357    RX01 Controller
    M7104 \
    M7105  |    RK05 Controller
    M7106 /

    Additional Hardware I would like to add.  Most of this is just a
dream because of lack of availability of the hardware and my limited budget.
    M935 (I need a pair of these as soon as possible so I can add the
second 20 slot backplane)
    TU56 Dual DECTape Drive
    RK05 Disk Drive
    RX01 Dual Disk Drive
    TC08 DECTape Controller
    H960 Rack Cabinet w/PDP-8/E name plate preferably.

[timr...@gmail.com]

You would not want the foam to be conductive.  This would be bad.  You want to insulate all pins from the case.

As far as I know the PC05 was more designed to be used with the pdp-11.  A PC04 is for the pdp-8.  But it should be able

to be made to work.

Good luck finding M935s.  But I suppose you could make something.

[William Cattey]

Wow.  Congratulations on the 8e acquisition!  I know you've been wanting one back in your possession for some time.

It's a surprise to hear there's only one 20-slot Omnibus.  If it's an 8e, it should have two of them.  Maybe the seller said it's an 8e because of CPU, but really it's an 8f or 8m with the single 20-slot Omnibus.

[michael.9...@gmail.com]

I have two 8/e systems, one came with two 20-slot backplanes, and one came with just one 20-slot backplane.

[Steve Tockey]

For what it's worth, the stock -8/e came from the factory with just one 20-slot Omnibus backplane. The extra one was an add-on option. I suppose the number of -8/e's that DEC sold with the single Omnibus might have been one factor in deciding to market the -8/m and -8/f variants.

My -/8e came with the front 20-slot being the DEC original and the back being a cheap after-market clone built by Douglas Electronics, IIRC. Somewhere along the lines I acquired an -8/e expander box (same chassis, no front panel) with 2 DEC original 20-slot backplanes. So I swapped one out from that to my -8/e.

As far as the foam under the backplane, definitely non-conductive if you put any in at all. I'm wondering if it is 100% required if the chassis will be mounted in a stable platform. I mean, it might have been put there on the chance that the -8/e got installed in a place with lots of vibration. The biggest concern would be that the pins on the bottom of the Omnibus never come in contact with the chassis.

Regarding the PC04/PC05, I had this conversation with someone (off list) about a year ago. I don't remember all of the gory details but there are some differences between the PC04 and the PC05 in terms of the electronics inside the unit itself. I think it may have had to do with -8 vs. -11 handling of the Reader Run signal and maybe something with interrupts. You can reconfigure a PC05 into a PC04 but it involves some wire-wrap changes in the backplane inside the PC05 unit. You can find the engineering drawings for the PC04/PC05 here:

http://bitsavers.org/pdf/dec/papertape/PC04_PC05_engrDrws_Mar73.pdf

Everything you need to know about how to reconfigure is in there, sorry but I don't remember the details off the top of my head.

[Mike Katz]

Bill,
Thank you.

The front panel says PDP-8/E on it so I guess that's an 8/E.  The front panel has incandescent lights on it also indicating it's an 8/E.

               Mike

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[Steve Tockey]

One more thing, in case you are interested.  A friend passed along this link a week or so ago.

https://ethw.org/First-Hand:PDP-8/E_OMNIBUS_Ride

It's the inside story on the development of the -8/e and the Omnibus.

[Mike Katz]

Steve,

Thank you.

           Mike

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[Neil Higgins]

I’m curious. I don’t own a PDP-8 (PiDP-8 excluded) and indeed I have never seen one in the flesh. 2 x 20 slot backplanes seems a lot! Imagine an early PC with 20 expansion slots. Does the CPU occupy some of these slots, or are they all available for peripherals? We’re talking Unibus slots? Granted, memory boards didn’t have much memory then, and serial lines came (probably) one per slot. Even so …

[Michael Thompson]

You need 8 slots for a full CPU, 12 slots for 32k of core using 8k stacks, and more if you use 4K stacks. 3 slots for a disk controller, 1 slot each for serial ports, 1 for DECtape, 1 for a printer, 1 for a cassette, 3 for graphics, 1 for bootstrap, 1 or more for a clock, so it is easy to fill 40 slots. If you fill the chassis you can always connect a second 40 slot chassis with flex print cables.

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[Clem Cole]

On Wed, Sep 8, 2021 at 4:45 AM Neil Higgins <1955...@gmail.com> wrote:

Imagine an early PC with 20 expansion slots. 

Neil - remember a PC is developed with LSI and VLSI chips.  The original PDP-8 used DEC 'Flip Chips' which were boards that had discrete transistors, resistors and caps that made up the logic modules, not even SSI logic like early TI7400 series logic.  The PDP-11/20 was the first full MSI TTL machine DEC designed, and the 8/E project was a cost reduction follow-on after the PDP 11/20 [IIRC, the original 8/E prototype used a hacked 16 bit PDP-11 memory board in the lab bring up/debug while it's memory was being developed].   Furthermore, the backplace was custom and only certain boards could not be places in certain slots (i.e. not all slots were equal).    So 20 slots in a PDP-8 (or an early PDP-11 for that matter) is not that same as 20, slots in an ISA, EISA or PCI bus.

To get a better feel for how all this worked, take a look at the documents for the PDP-16 'Register Transfer Modules' system.  The full docs are on bitsavers.  The idea was that the logic was collected together in Flip-Chips that had macro functions, and were connected together in a backplane that would be custom wired to the specific function.   In fact, the granddaddy to today's VLSI HW description languages such as Verilog and VHDL was a system CMU created (and used at DEC and a number of Gov agencies) that was called ISPL and later ISPS - which originally spit out RTM modules and their connections.

BTW: The 'OMNIBUS' was the PDP-8 bus, the UNIBUS was the 11's.

ᐧ

[Mike Katz]

The CPU takes 3 slots, the front panel takes another slot, each core memory board takes 3 slots, the RK05 Disk Controller takes 3 slots.

This is similar to current PC high end video boards that take up 2 or more slots on your PC.

Here is a pic of the boards in the system.  The green connectors are inter-board signal connectors for multiple board sets.



As you can see from the table below there are only 7 slots available.  If I want to add more memory (at 3 slots each) or other multiple board controllers i will run out of slots very quickly.

The system is populated at follows:

Slot        Contents
 1            Front Panel
 2            M8330 Timing Board (CPU Board #1)
 3            M8300 Major Register Control Board (CPUI Board # 2)
 4            M8310 Major Register Board (CPU Board #3)
 5            M837 Extended Memory/Time Share Control (Required for more than 4K of memory)
 6            M8650 Asynchronous Data Control (to communicate with external terminal)
 7            Home brew I/O board
 8            M840 High Speed Paper Tape Reader/Punch Interface
 9
10
11
12
13
14           M849 RFI Shield (To protect the main boards from the magnetic fields of core memory)
15           G104 4K Core Memory Sense/Inhibit board
16           H220 4K Core Memory Stack (actual 4096 x 12 bit core plane)
17           G227 4K Core Memory X/Y Driver                                                                        
18
19           M8320 Bus Terminator
20          

[Mike Katz]

I would like to make one correction to what you said.

All PDP-8 models prior to the PDP-8/E had dedicated slots with a wire wrapped back plane.  The PDP-8/E introduced the Omnibus, with 4 connectors per slot, where any board would work in any slot.  The PDP-8/A used an expanded Omnibus with 6 connectors per slot but was almost fully backwards compatible.

The PDP-11/20 used a similar but incompatible bus called the Unibus.  Both the PDP-11/20 and the PDP-8/E were introduced in 1970.

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[Neil Higgins]

I get it. Hence all of that lovely wire wrap wiring.