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PDP-8 Summary of Models and Options (posted every other month)
Section - What is a PDP-8?

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Date of introduction:  1965 (Unveiled March 22, in New York).
Date of withdrawal:    1968.
Total production run:  1450.
Also known as:
	Classic PDP-8 (to point out lack of a model suffix)
	Straight-8 (Again, points out the lack of a model suffix)
	PCP-88, an OEM label, used by Foxboro Corporation.
        AN/GYK-6, (Army-Navy Ground-based (Y)data-processing Komputer 6)
Price: $18,000

Technology:  Mostly standard DEC R-series logic modules; these were
	originally discrete component transistor logic, but around the
	time the PDP-8 was introduced, DEC introduced the Flip Chip, a
	hybrid diode/resistor "integrated circuit" on a ceramic substrate.
	These could directly replace some of the discrete components on
	some logic modules, and DEC quickly began to refer to all R-series
	modules as flip-chip modules; they even advertised the PDP-8 as
	an integrated circuit computer.  A typical flip-chip module, the
	R111, had three 2-input nand gates and cost $14, with no price
	change from 1965 to 1970.  Some special dual height R-series
	modules were designed specifically for the PDP-8.

	S and B-series logic modules were also used; these are similar
	to their R-series cousins, but with different speed/fanout
	tradeoffs in their design.  Some logic modules have trimmers
	that must be tuned to the context, making replacement of such
	modules more complex than simply swapping boards.

	As with the system modules used in the PDP-5, the supply
	voltages were +10 and -15 volts and the logic levels were -3
	(logic 1) and 0 (logic 0).  Logic was packaged on boards that
	were 2.5 inches wide by 5 inches long.  The card edge connector
	had 18 contacts on 1/8 inch centers.  Some double height cards
	were used; these had two card edge connectors and were 5 1/8
	inches high.  Machine wrapped wire-wrap technology was used on
	the backplane using 24-gauge wire.

	The "negibus" or negative logic I/O bus used -3 and 0 volt logic
	levels in 92 ohm coaxial cable, with 9 coaxial cables bundled
	per connector card and 6 bundles making up the basic bus.  5
	(later 4) more bundles were required to support data-break (DMA)
	transfers.  The total bus length was limited to 50 feet, and bus
	termination was generally kluged in with 100 ohm resistors
	clipped or wrapped into the backplane, although a bus terminator
	card was sometimes used.  Some time after the first year of
	production, flat ribbon cable made of multiple coaxial cables
	was used, and later still, shielded flat stripline cable was used
	(but this cut the allowed bus length by a factor of two).

	Core memory was used, originally made by FERROXCUBE, with a 1.5
	microsecond cycle time, giving the machine an add time of 3
	microseconds.  4K of core occupied an aluminum box 6 inches on a
	side and needed numerous auxiliary flip-chips and for support,
	as well as an array of boards from the core vendor.  It is worth
	noting that the PDP-8 was about as fast as was practical with the
	logic technology used; only by using tricks like memory
	interleaving or pipelining could the machine have been made much

Reason for introduction:  This machine was inspired by the success of
	the PDP-5 and by the realization that, with their new Flip-Chip
	technology, DEC could make a table-top computer that could be
	powered by a single standard wall outlet; of course, adding any
	peripherals quickly increased the power requirement!

Reason for withdrawal:  The PDP-8/I was less expensive, and after
	initial production difficulties, it equalled the performance of
	the PDP-8.

Compatability:  This machine defines the core of the PDP-8 instruction
	set, but with restrictions that were lifted on later machines.
	The Group 1 OPR instruction IAC cannot be combined with any of
	the rotate instructions.  If RAR and RAL or RTR and RTL are
	combined, the results are unpredictable (simultaneous set and
	reset of bits of AC results in metastable behavior).  The IOT 0
	instruction was used for the internal type 189 ADC, and not for
	the later CAF (clear all flags) instruction.  As a result, if
	the ADC option was not present, IOT 6004 (or microcoded
	variants) would hang the machine.

	The SWP instruction (exchange AC and MQ) never works, even if
	the extended arithmetic element is present.  This works on later
	models when the EAE is present, although it was only documented
	with the introduction of the PDP-8/E.  Finally, the EAE lacks
	the SCL (shift count load) instruction that is present on later

	On machines with 8K or more, an attempt to change the data field
	to a non-existant field caused a bizarre double-indirect and
	skip instruction execution that must be accounted for in memory

Standard configuration:  The PDP-8 was sold as a CPU with 4K of memory,
	a 110 baud current loop teletype interface and an ASR 33 Teletype.
	In addition, the standard in-cabinet logic includes support for
	the full negibus interface, including data-break (DMA) transfers.

	Both a rack-mount model with rosewood trim and an elegant
	plexiglass enclosed table-top configuration were standard.  Under
	the skin, the basic machine occupies a volume 33 inches high by
	19 inches wide by 22 inches deep.  The two halves of the backplane
	are mounted vertically, like the covers of a book, with the
	spine in back and circuit modules inserted from the two sides.
	Sliding the CPU out of the relay rack or removing the plexiglass
	covers allows the backplane to swung open to access the wires-wrap.

Expandability:  In-cabinet options include the type 182 extended
	arithmetic element (EAE) ($3,500), the type 183 memory extension
	control subsystem ($3000), and the type 189 low performance
	analog to digital converter ($1450).  Prewired backplane slots
	were reserved for all of these, as well as the optional type 129
	data channel multiplexor ($2700).

	Expansion beyond 4K of memory requires rack mounting space (at
	$690 per CAB-8 rack).  Each type 184 memory module adds a 4K
	field of memory ($10,000), seven modules may be added.  The
	rack-mount CPU occupied a large part of one rack, allowing room
	for a single memory expansion module below the CPU; generally,
	a second rack was needed for added peripherals or memory.

	At the end of the production run, some PDP-8 systems were sold
	with PDP-8/I memory, allowing room for an additional 4K without
	need for an expansion chassis.  These nonstandard machines were
	very difficult to maintain!

Peripherals:  At the time of introduction, the following negibus
	peripherals were offered.

	-- Type 750 high speed paper tape reader and control ($3500).
	-- Type 75A high speed paper tape punch and control ($4000).
	-- Type 138 analog to digital converter ($4500).
	-- Type 139 analog multiplexor ($3300).
	-- Type 30N precision CRT display ($13,400).
	-- Type 34B oscilloscope display ($3600).
	-- Type 370 high speed light pen ($1625)
	-- Type 350 incremental (CalComp) plotter and control ($8,900 up).
	-- Type 451 card reader and control ($14,900).
	-- Type 451B fast card reader and control ($25,600).
	-- Type 450 card punch control for IBM Type 523 punch.
	-- Type 64 (later 645) Mohawk line printer and control ($28,900).
	-- Type 250 (RM08) serial magnetic drum (256K words for $43,600).
	-- Type 552 DECtape control (for type 555 DECtape drives, $9500).
	-- Type 555 dual DECtape transport, $7400).
	-- Type 57A magnetic tape control with IBM type 729 drive ($15,200).
	-- Type 580 magnetic tape system with one transport ($19,700).

	By 1966, the following peripherals had been added to the line:

	-- Type AA01A three-channel digital to analog converter.
	-- Type CR01C card reader control.
	-- Type TC01 DECtape control for up to 8 TU55 transports.
	-- Type 251 drum (8-256 tracks, 8 sectors/track, 128 words/sector).
	-- Type 645 line printer control.
	-- Type 680 data communications system (allows 64 teletypes).

	By 1967, the following peripherals had been added to the line:

	-- Type AF01 analog to digital converter and multiplexor.
	-- Type AX08 parallel digital input port.
	-- Type 338 Programmed Buffered Display (vector graphics).

	By 1968, the following new peripheral had been added:

	-- Type DF32 fixed head disk system (32K to 256K words).
	-- Type BE01 OEM version of the TC01 (no blinking lights).
	-- Type BE03 dual TU55 drive for the TC01 or BE01.

	Finally, as DEC abandoned the negibus, they introduced the
	DW08B negibus to posibus converter so newer posibus
	peripherals could be used on older negibus machines, and the
	DW08A posibus to negibus converter to allow use of old
	peripherals on new machines.

Survival:  Many classic PDP-8 systems survive to this day in working

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Top Document: PDP-8 Summary of Models and Options (posted every other month)
Previous Document: What is a PDP-5?
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