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

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Date of introduction:  1970 (during or before August).
Date of withdrawal:    1978.
Also known as:
	PDP-8/OEM
	Industrial-8 (with a red color scheme)
	LAB-8/E (with a green color scheme)
Price: $6,500

Technology: SSI and MSI TTL logic were used on these boards, and the
	entire CPU fit on 3 boards.  Nominally, these were DEC M-series
	flip Chip modules, but in a new large format, quad-high (10.5
	inch), extended-length (9 inch, including card-edge connector,
	excluding handles).  The terms used for board height and length
	are based on the original working assumption that all flip-chips
	were plugged horizontally into a vertially mounted card-edge
	connector.  On the PDP-8/E, the cards were plugged vertically
	down into a horizontally mounted connector, so many users
	incorrectly refer to these boards as quad-wide double-high.

	Interconnection between boards was through a new bus, the OMNIBUS.
	This eliminated the need for a wire-wrapped backplane, since all
	slots in the bus were wired identically.  A new line of peripheral
	interfaces was produced, most being single cards that could be
	plugged directly into the inside the main enclosure.  These
	included a set of posibus adapters allowing use of older
	peripherals on the new machine.

	Interboard connectors were needed for some multiboard options,
	including the CPU and memory subsystems.  These used standard
	36-pin backplane connectors on the opposite side of the board
	from the backplane.  Some boards, notably memory boards, had a
	total of 8 connector fingers, 4 for the omnibus and 4 for
	interboard connectors.

	The core memory cycle time was 1.2 or 1.4 microseconds, depending
	on whether a read-modify-write cycle was involved (a jumper would
	slow all cycles to 1.4 microseconds).  A 4K core plane was
	packaged on a single quad-wide double-high board, with most of
	the drive electronics packed onto two adjacent boards.  Soon after
	the machine was introduced, an 8K core plane was released in the
	same format.

Reason for introduction:  The cost of the PDP-8/I and PDP-8/L was
	dominated by the cost of the interconnect wiring, and this cost
	was high as a result of the use of small circuit boards.  By
	packing a larger number of chips per board, similar function
	could be attained in a smaller volume because less interboard
	communication was required.  The PDP-8/E exploited this to achieve
	a new low in cost while attaining a new high in performance.

Reason for withdrawal:  This machine was slowly displaced by the PDP-8/A
	as the market for large PDP-8 configurations declined in the face
	of pressure from 16 bit mini and microcomputers.

Compatability:  As with the PDP-8/I and PDP-8/L, there are no limits on
	the combination of IAC and rotate instructions.  Unlike the early
	machines, basic Group 3 OPR operations for loading and storing
	the MQ register work even if there is no extended arithmetic
	element.  Finally, a new instruction was added, BSW; this swaps
	the left and right bytes in AC, and is encoded as a Group 1 OPR
	instruction using the "double the shift count bit".

	An odd quirk of this machine is that the RAL RAR combination ands
	the AC with the op-code, and the RTR RTL combination does an
	effective address computation loading the high 5 bits of AC with
	the current page and the lower bits of AC with the address field
	of the instruction itself!

	The EAE has a new mode, mode B.  Previous EAE designs were
	single-mode.  Mode B supports a large set of 24 bit operations
	and a somewhat more rational set of shift operations than the
	standard EAE.  All prior EAE designs would hang on the microcoded
	CLA NMI (clear/normalize) instruction applied to a nonzero AC.
	This instruction is redefined to be a mode changing instruction
	on the 8/E.

Standard configuration:  A CPU with 4K of memory, plus 110 baud current
	loop teletype interface.  Both a rack-mount table-top versions
	were sold (both 9" high by 19" wide by 21" deep).  The rack mount
	version was mounted on slides for easy maintenance.  The OMNIBUS
	backplane was on the bottom, with boards inserted from the top.

	The PDP-8/OEM had a turn-key front panel, no core, 256 words of
	ROM and 256 words of RAM, and was priced at $2800 in lots of 100.

	The standard OMNIBUS backplane had 20 slots, with no fixed
	assignments, but the following conventional uses; certain board
	sets were jumpered together (shown with brackets) and therefore
	were required to be adjacent to each other:

	-- KC8E programmer's console (lights and switches)
	-- M8300 \_ KK8E CPU registers
	-- M8310 /  KK8E CPU control
	--
	--
	-- M833  - Timing board (system clock)
	-- M865  - KL8E console terminal interface.
	--
	--
	--   -- space for more peripherals
	--
	--
	-- M849  - shield to isolate memory from CPU
	-- G104  \
	-- H220   > MM8E 4K memory
	-- G227  /
	--
	--   -- space for more memory
	--
	-- M8320 - KK8E Bus terminator

	Most of the early boards with 3 digit numbers were defective
	in one way or another, and the corrected boards added a trailing
	zero.  Thus, the M833 was generally replaced with an M8330, and
	the M865 was replaced with the M8650.

Expandability:  The following are among the OMNIBUS boards that could be
	added internally:

	-- M8650 - KL8E RS232 or current loop serial interface.
	-- M8340 \_ Extended arithmetic element.
	-- M8341 /  (must be attached in two slots between CPU and M833.
	-- M8350 - KA8E posibus interface (excluding DMA transfers).
	-- M8360 - KD8E data break interface (one per DMA device).
	-- M837  - KM8E memory extension control (needed for over 4K).
	-- M840  - PC8E high speed paper tape reader-punch interface.
	-- M842  - XY8E X/Y plotter control.
	-- M843  - CR8E card reader interface.

	There were many other internal options.  There was room in the
	basic box for another 20 slot backplane; taking into account the
	2 slots occupied by the M935 bridge between the two backplanes,
	this allowed 38 slots, and a second box could be added to
	accomodate another 38 slot backplane, bridged to the first box by
	a pair of BC08H OMNIBUS extension cables.

	Given a M837 memory extension control, additional memory could be
	added in increments of 4K by adding G104, H220, G227 triplets.
	The suggested arrangement of boards on the OMNIBUS always
	maintained the M849 shield between memory other options.  The
	one exception was that the M8350 KA8E and M8360 KD8E external
	posibus interfaces were typically placed at the end of the
	OMNIBUS right before the terminator.

	The following options were introduced later, and there were many
	options offered by third party suppliers.

	-- G111  \
	-- H212   > MM8EJ 8K memory
	-- G233  /
	-- M8357 -- RX8E interface to RX01/02 8" diskette drives.
	-- M7104 \
	-- M7105  > RK8E RK05 Disk Interface
	-- M7106 /
	-- M8321 \
	-- M8322  \ TM8E Magtape control for 9 track tape.
	-- M8323  /
	-- M8327 /

Survival:  It is still fairly common to find PDP-8/E systems on the
	surplus market, recently removed from service and in working
	condition or very close to it.  A modest number are still in
	service doing their orignal jobs, and there is still a limited
	amount of commercial support from both DEC and third-party vendors.

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Last Update March 27 2014 @ 02:11 PM