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Core Stack

Technical Information

Catalogue No: C1059
Category: Stores/Weapon Management
Object Type: Module/Sub-Assembly/Component
Object Name: Core Stack
Part No: P259 Issue 8
Serial No: 44-082
Manufacturer: Plessey
Division: Unknown
Year of Manufacture: circa 1973
Dimensions: Width (mm): 159
Height (mm): 67
Depth (mm): 105
Weight (g): 1,370
Location: Rack RAA03 [Main Store]

Manufactured by
GEC-Computers Limited
Core Stack
322 Purch 259
Serial No:- 44-082
P259 Issue 8
IGV 044254
Made in UK
Plessey Core Store
Type No. 73'1556'A
Serial No. 44082
Ref. No. P259 Issue 8
Towcester England


This is a low-density early stacked 8k x 18-bit core memory array, made by Plessey Components Ltd, Towcester, England. Plessey made core memories for a number of computer companies, and for example made 48-bit core memories for the Ferranti Atlas and for the English Electric KDF9. This type of stack was also used in the Elliott 903,and 920B computers in the mid 60’s.
Magnetic-core memory was the predominant form of random-access computer memory for 20 years between about 1955 and 1975. Such memory is often just called core memory, or, informally, core.

The cores in the stack However the robust construction of the unit and its colour suggests a military application, and probably from the Integrated Fire Control System (IFCS) for the Challenger Mk1Tank. The Challenger Mk1 went into service in 1983 but development of the Fire Control System was underway in the previous decade. The system activity was at Marconi Space and Defence Systems on the but the 12/12 computer and Power Supply were produced at Rochester. The gunnery control system was accurate but slow and it was replaced in the Challenger Mk2.

The stack is marked on the top:
TOWCESTER, PLESSEY COMPONENTS, ENGLAND with the Plessey ‘Oscilloscope’ mark. Another label on the top has:
Yet another label also on the top has:
Manufactured by GEC Computers Limited, CORE STACK 322 PURCH 259,
Serial No. 44082, P259 Issue B. Borehamwood England. A subsidiary of the General Electric Company.

The Core Stack dates from about 1973 and many items at that time in the 903 Computers were marked with"322 PURCH" numbers, (most transistors are 322 PURCH 100 and diodes were 101).

322 might be the divisional number of Mobile Computing, and Airborne Computing Division (ACD) had the Divisional number 363 in Borehamwood then 263 in Rochester.

The length of the two Flexi-PC interconnections are 105mm.
The length of the two wired interconnections are 125mm.

Magnetic-core memory was the predominant form of random-access computer memory for 20 years between about 1955 and 1975. Such memory is often just called core memory, or rope core memory and also core plane.

Core uses tiny magnetic toroids (rings), the cores, through which wires are threaded to write and read information. Each core represents one bitof information. The cores can be magnetized in two different ways (clockwise or counterclockwise) and the bit stored in a core is zero or one depending on that core's magnetization direction. The wires are arranged to allow for an individual core to be set to either a one or a zero and for its magnetization to be changed by sending appropriate electric current pulses through selected wires. The process of reading the core causes the core to be reset to a zero, thus erasing it. This is called destructive readout. When not being read or written, the cores maintain the last value they had, even when power is turned off. This makes them nonvolatile.

The 920 series of computers was developed at Elliotts Borehamwood Computing Division in the 1960’s and was derived from the successful 901 commercial computer which was miniaturised for airborne use. One of the primary production derivatives was the MCS 920M which was used in the
Jaguar Navigation and Weapon Aiming Sub-System NAVWASS This system comprised the MCS920M central digital computer, E3R inertial platform, projected map display and horizontal situation indicator together with the necessary power supply, electronic, interface and control units.
The MCS 920M is a microminiature general purpose digital computer using flat pack integrated circuits and. has a random access, 8192 word, 18 bit ferrite core store with a 2microsecond cycle time. Elliotts lacked access to multi-layer printed circuit board technology so used a special technique developed at the Borehamwood Computer Laboratory (see ‘Moving Targets’ by Simon Lavington). The box is unusual in that it folds out into three blocks.
In addition to performing navigation and weapon aiming calculators, the computer caters for in-flight monitoring, initial alignment and in situ first line system testing. The Computer also had an Interface Unit which enables the digital computer to receive and transmit information from and to other units in the aircraft in compatible signal form. The Interface Unit also supplied the power for the computer and Navigation Control Unit.
In its production form the MCS920M was widely used in its intended role in the Jaguar navigation attack system and eventually grew to a 64 K machine. The 920M also saw use in the ELDO rockets.
The MCS920M was also adopted for a number of naval inertial navigation applications.

Thanks to Terry Froggatt we have two pages of how a 920M is built.

The two pages shown look very like the Core Stack photos (minus a top plate) catalogued as C1059.

We assume an Elliott typo (not uncommon), for "7 epoxy-glass printed circuit boards" read "9 epoxy-glass printed circuit boards" There are 24 chips in the Core Stack and a pair of 24-way ribbon cables. Possible explanation:

“8192 words is not a square number. On a 920B/903 the store is addressed as two 4096-word stores, so there are 64 "Y" address wires running though ALL 8192 words, and 64 "X" address wires running through EACH 4096 words.

Total 192 wires, = 24 * 8, (and each wire needs 4 diodes).

So, your core store may well be a spare for a 920M, or perhaps from a 920M "extra store" module, or a 920C.


See:    http://www.computerconservationsociety.org/software/elliott903/more903/Manuals/index.htm  this gives copious links to the 920 Computer family.

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