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What is the physical size of a PDP-8? The photographs I found seem to be evenly divided between showing it as a compact unit that looks maybe one foot high, and as a tall cabinet maybe five or six feet high.

Conjecture: the different sizes correspond to different models (maybe the smaller one is the PDP-8S?)

Conjecture: the computer itself is always the smaller box, but the tall cabinet contains that plus peripherals such as disk and tape drives. (In that case, is the core memory included in the small box?)

Which of these is correct?

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    Which model are you asking about? The Straight-8 is much larger than a Harris 6120. – Wilson Mar 7 '18 at 15:42
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    Plus many models were designed to fit in a 19" rack, the same kind we use today for servers. So the CPU itself would be only a small part of the whole system which would also contain things like tape drives, extra memory and whatever. – Wilson Mar 7 '18 at 15:44
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    The PDP-8 Models and Options FAQ is worth reading for this kind of information. It gives sizes for the base models (mostly 19" wide as mentioned by Wilson; the exception is the PDP-8/I which was available in table-top and pedestal variants). – Stephen Kitt Mar 7 '18 at 15:49
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There were different models of the PDP-8, but most models (say, PDP-8/E) consisted of a number of rackmounted boxes, one of which you could call the CPU (or, compared to a modern PC, maybe the motherboard is a better comparison).

Here and here are some good pictures.

The unit shown with the front panel in the typical orange/yellow colors containing the switches and lights, which were more or less directly connected to the data and address bus, contains a number of connectors on a backplane, which can be populated with cards about the size of a modern motherboard. You can see part of the backplane in the second link.

The first card controls the front panel, there are several cards that make up the CPU, and again several cards for the memory (a controller and the core memory itself). After that come cards for peripherals, and it was also possible to extend the backplane into a second module for even more peripherals IIRC.

Some peripherals did have a considerable amount of logic in their own units.

There was also a (quite large, on the 8/E) unit for the power supply.

If I should call something the "PDP-8 computer", I'd have used that name for the whole rack, including the peripherals in the rack, excluding the teletypes and printers that were connected with cables to it.

Later, there were also desktop versions of the PDP-8 (the DECMates), when the whole CPU had been miniaturized to a single chip, and it could be integrated into a video terminal.

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The original (straight) 8 is a fancy shaped box of about 2 ft wide, 3 ft height and 2 ft deep.

Later models where mostly standard 19" rackmount boxes, roughly 2x1x3 ft^3 until the last ones made that like 'half height' of ~6 inches.

So yes, different sizes correspond to different models over time.

And yes again, the racks usually shown are whole machines, including peripherals like Disks, Tapes and so on in addition to the CPU. After all, the open modular concept was one of the main advantages of the PDP-8 and -11 series.

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    Don't forget the desktops like the VT78 and DECMates, these had PDP-8 put on a single chip – Wilson Mar 7 '18 at 16:16
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The photographs I found seem to be evenly divided between showing it as a compact unit that looks maybe one foot high

It is most likely, based on sales numbers, that you're describing a PDP-8/e or PDP-8/I. What you're seeing is just the front panel. The PDP-8/e front panel is 10.5 inches high (PDF!), or 6 rack units. The PDP-8/I front panel is a bit shorter, 5U high, or 8.75 inches. Another way to come to these values is by inference from the fact that DEC used standard 19 inch wide racks, that being the width of a panel mounted across the front of the rack. You can get an accurate aspect ratio from an undistorted photograph, which lets you determine panel height.

The front panel on the earliest PDP-8s was a separate unit, only a few inches deep, since it's only the switches, indicators, enclosure, and direct support electronics.¹ Indeed, some PDP-8s offered a choice of different front panels, such as a version with very few switches and indicators for embedded applications.

Early rack-mounted PDP-8s were made of many separate components, interconnected with ribbon cables and wiring harnesses, so the layout within the rack cabinet was flexible. The front panel was connected to the CPU proper and the power supply in this way.

Let's get concrete and focus on the PDP-8/I, the model I'm most familiar with.² Here's a dimension diagram from the PDP-8/I Maintenance Manual, volume 1, page 2-2:

PDP-8/I rack layout diagram

The "console" in the diagram's upper left corner is what I've been calling the "front panel," and it's often all that's shown when you search for a picture of a PDP-8/I, even though it's really only one small-ish part of the whole computer. It's the sexy bit.³

As you can see in the diagram, some of the space left behind the front panel is taken up by the top end of the CPU proper, which was roughly 2 feet high by two feet deep and took up about half the rack width. The other half of that space was taken by the power supply.⁴ You can see that layout in these photographs. The diagram doesn't show the core memory as a separate component, since it was mounted within the processor chassis.⁵

Over their two decade span of development (1965-1985) PDP-8s moved from discrete transistors to ICs, then to denser and denser ICs, so they became smaller and more power-efficient. By the time of the PDP-8/e (1970), the CPU, core memory, power supply, and expansion slots were entirely behind the front panel. Its close relatives the PDP-8/f and 8/m used shallower chassis, but were the same basic idea.⁶ These systems were only self-contained within that 6U space if your application didn't require any sort of durable storage or I/O peripherals; add that in, and you're still talking about taking over a large portion of a full-size rack, at least.

The front panel would usually be mounted about halfway up the rack, since that's a convenient height for a seated person, who would normally sit at a Teletype Model 33 ASR, the standard terminal for DEC computers of the time. Since this is an essential component in many PDP-8 systems, it should also be considered part of the size of the computer: it's roughly the same width and depth as a rack cabinet, and about half the height. (Source)

Whereas the CPU proper is normally mounted below the front panel on the PDP-8/I and behind it on the PDP-8/e, it was mounted above the front panel on the original PDP-8. The original PDP-8 front panel + CPU + core + power supply assembly was about the same size as the PDP-8/I discussed above.

The rack cabinets usually used for PDP-8s were 5 or 6 feet high. The rest of the space in the CPU rack would be used for peripherals: TU56 DECtape drives, RK05 removable platter hard disks, etc.

It was not uncommon for a PDP-8 computer of the era to require a second rack for more peripherals: A/D conversion hardware, paper tape punch/reader units, more tape and disk storage, etc.

Eventually, the PDP-8 escaped the rack with the DECmate series, which packed a single-IC PDP-8 CPU, memory and a dual floppy drive into a large video terminal cabinet, not unlike early consumer-focused microcomputers.


Digressions

  1. The front panel electronics consisted of simple circuits like lamp drivers so that the power to drive the lamp didn't have to come from the CPU, only the low-current signal to turn it on.

  2. I'm the current maintainer of the PiDP-8/I software project.

  3. That's one reason the PiDP-8/I DIY project is basically just the front panel. Cloning the other parts hidden away by the rack isn't of general interest.

  4. According to the same page in the manual where I found the above rack layout diagram, the power supply for a PDP-8/I is rated to provide about 1.7 kW of power, but another source says it only provides 380 W. I assume the latter is a measurement of steady-state current draw, and that the computer could surge into the kW range in some conditions. Other sources I've read say that a complete PDP-8 computer would draw roughly 1 kW, depending on the type and number of peripherals.

    You may be reading this on a computer with a several hundred watt power supply which is much smaller than all of this and may be wondering at the discrepancy. There are many factors:

    • These early PDP-8 power supplies are inefficient linear-regulated power supplies rather than the switching-regulated power supplies that became universal for computers after this early PDP-8 era. They throw off more heat per unit power delivered to the point of load, so they need bigger heat sinks and more space for cooling air to circulate.

    • Modern power supplies are much more densely-packed than the DEC 704A.

    • These PDP-8 power supplies relied more on big transformers and big capacitors to provide clean DC output power than on clever electronics.

    • They were designed with discrete transistors rather than integrated circuit regulators, which were brand new technology at the time. IC regulators weren't capable of delivering the loads required by such computers.

  5. In the PDP-8/I, the first field (4 kWords) of core memory is mounted close to the CPU proper because the timing of the CPU was based on the speed of core memory access. Indeed, the early PDP-8 CPUs weren't rated in MHz, but in core memory cycle times, with instruction times given as the number of core cycles they took. Memory reference instructions took more processing time than purely in-CPU operations. Core memory on a PDP-8 was like L1 cache in a modern CPU: tightly integrated and the fundamental limit on CPU processing speed.

    The PDP-8/I then allowed a second 4 kWord field of core to be mounted on the CPU backplane along with a memory controller, the assembly being called the MC8I. This would then control rack-mounted MM8I core memory modules, each adding another 4 kWord field, for a maximum of 8 fields total, or 32 kWords.

    The above details differ across the 20-year commercial history of the PDP-8. For example, the later PDP-8/a had the MS8C option which provided 16 kWords of solid-state static RAM in the form of an expansion bus card. The PDP-8/a was also the first to provide a way to get past the 32 kWord memory limit of all earlier PDP-8 models.

  6. The PDP-8/f and 8/m were the first PDP-8s based on a switching power supply, rather than the linear-regulated supply of the PDP-8/e and earlier.

5

Before the PDP-8/e, two other models of the PDP-8 had their electronics all fitting entirely beind the front panel. The PDP-8/S, which arrived shortly after the original PDP-8, was in this category; however, because it used a serial arithmetic unit, it was slow and of limited popularity. The PDP-8/L, a computer using the same technology as the PDP-8/I, but with fewer available options, also had its logic behind the front panel.

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