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

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Date of introduction:  1974 (Announced in May for December delivery)
Date of withdrawal:    1984
Also known as:
	KIT-8/A (CPU plus 1K RAM)
        CLASSIC (CLASSroom Interactive Computer)
	DECDataSystem310 (an 8/A 500 sold as a word-processor)
Price: $1,835

Technology:  This machine used the OMNIBUS with a new single-board CPU.
	The backplane was reoriented so that boards plugged into it from
	the front, with the board held horizontally.  The new omnibus
	allowed a board format half again as wide as the original
	(formally, this was called hex height), but the extra 2 groups
	of contact fingers added to each wide board was largely unused.
	(the 6th contact group was not connected on most backplane slots;
	the 5th was unsupported on 8 of the 12 or 20 backplane slots, and
	was used primarily for additional power and ground distribution).

Reason for introduction:  Using TTL MSI and LSI components, DEC was able
	to reduce the PDP-8 CPU to a single oversize board (formally, hex
	height, double width).  Similarly, they were able to make an 4K
	core memory board, and later, an 8K board in this format, and they
	were able to introduce a static RAM card using semiconductor
	memory.  The minimum system was thus reduced to 3 boards.  The
	relatively expensive lights and toggle switches on the front panel
	of the PDP-8/E were replaced with an octal membrane keypad and
	4-digit 7-segment LED display.

	The market for the PDP-8 was dominated by small systems, with
	fewer and fewer customers needing large-scale expandability.
	Thus, the 20 slot backplane of the early OMNIBUS machines was too
	big; with the new single board CPU and memory, a 12 slot backplane
	was enough, allowing further cost reductions.

Reason for withdrawal:  The market for the PDP-8 family was shrinking in
	the face of pressure from larger minicomputers and the new
	monolithic microcomputers.  After 1975, many PDP-8 sales were to
	captive customers who had sufficient software investments that
	they could not afford to move.  Only the word-processing and
	small business markets remained strong for first-time PDP-8
	sales, and in these, the specialized DEC VT-78 and DECmate
	machines were more cost effective than the open architecture
	OMNIBUS machines.
		
Compatability:  The new PDP-8/A CPU was largely compatable with the
	PDP-8/E CPU, except that the combination of RTR and RTL (Group 1
	OPR instructions) loaded the next address.  The power-fail
	auto-restart option included the standard skip on power low
	instruction, but also a new skip on battery empty instruction to
	test the battery used for back-up power on the new solid state
	memory.

	The standard parallel port on the M8316 was not software
	compatable with the earlier line-printer interfaces used with
	device code 66.

Standard configurations:  The PDP-8/A was sold with a new short OMNIBUS
	backplane, mounted on its side above a power supply and a
	battery to back up the solid state memory.  The minimum
	configuration included a limited function control panel and the
	following components on the bus:

	-- M8315 -- KK8A CPU board
	-- M???? -- MS8A 1K to 4K solid state memory.
	-- M???? -- MR8A ROM companion for the MS8A.
	-- M8316 -- DKC8AA serial/parallel interface and clock.

	The M8316 board contained a remarkable but useful hodgepodge of
	commonly used peripherals, including the console terminal
	interface, a parallel port, the power/fail auto-restart logic,
	and a 100 Hz real time clock.

	The smallest PDP-8/A configuraton marketed was the KIT/8A, either
	just the KK8A and MS8A 1K boards for $572, or $1350 for a system
        that appears to have included the M8316 and a 4 slot backplane.

	The 8/A 100, was a computer system with a 10 slot backplane
	and a poor power supply.  The 8/A 400 was a better system with
	a 12 slot backplane, and the 8/A 420 had a 20 slot backplane.
	The 8/A 600 and 620 were the 8/A 400 and 420 with the KK8E
        PDP-8/E CPU set allowing added speed and the use of the 8/E EAE.

Expandability:  All PDP-8/E peripherals and options could be used with
	the PDP-8/A.  For those configurations requiring more than 20
	backplane slots, A pair of PDP-8/A backplanes could be connected
	using BC08H cables, and there was a special cable, the BC80C,
	for connecting a hex wide 8A backplane to a PDP-8/E, -8/F or
	-8/M backplane.

	By February 1975, the PDP-8/A was being sold in a workstation
	configuration, with the CPU and dual 8" diskette drives in a desk
	with a video terminal (VT52) and optional letter quality printer
	on top.  For the educational market, this configuration was
	marketed as the CLASSIC.  As an office system, such configurations
	were marketed as DECDataSystems.

	The following additional PDP-8/A (hex) boards were offered:

	-- G649  \_ MM8AA 8K Core stack (too slow for 8/E CPU!).
	-- H219A /  MM8AA 8K Core memory control.
	-- G650  \_ MM8AB 16K Core stack (ok for 8/E CPU!).
	-- H219B /  MM8AB 16K Core memory control.
	-- M8349 -- MR8F 1K ROM (quad, overlayable with core).
	-- M8317 -- KM8A memory extender (with variations).
	-- M8319 -- KL8A 4 channel RS232 or current loop serial I/O.
	-- M8433 -- RL8A controller for 1 to 4 RL01/RL02 disk drives.
	-- M8410 \_ FPP8A floating point processor control
	-- M8411 /  FPP8A floating point processor data path

	The PDP-8/A model 800 was the same as the model 600, but with the
	FPP8A floating point processor included as part of the package.

	-- M8416 -- KT8AA Memory management unit for up to 128K.
	--       -- KC8AA Programmer's Console (requires M8316)
	-- M8417 -- MSC8DJ 128K DRAM MOS Memory.

	Note that memory extension to 128K was a new PDP-8/A feature that
	was necessarily incompatable with the older PDP-8 memory expansion
	options, although the conventional PDP-8 memory expansion
	instructions still operate correctly on the first 32K.  Access to
	additional fields involved borrowing IOT instructions that were
	previously dedicated to other devices.

	The MM8A core memory options require the use of a box with a G8018
	power supply that provides +20V, while the semicondustor memory
	options require a G8016 power supply with built-in battery backup.
	Also, the use of the MSC8 DRAM memory cards require a CPU that
	supports the memory stall signal; early PDP-8/E CPUs did not.

Survival:  As with the PDP-8/E, these machines are moderately common on the
	surplus market and a modest number are still in use.

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