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Anita Model 1011 Desk Calculator



Anita 1011

Anita 1000 Series, Model 1011, S/N M2971
Functions: ASMD, constant, percent, 1 memory
Technology: Discrete-component DTL, 7 IC chips
Display: 10 digits, Nixie tubes
Dimensions: 265W x 335D x 175H
Weight: 5.9kg
Manufactured: Bell Punch Company, England, January 1970


The Bell Punch Company of England is generally credited with building the first production electronic calculator - the ANITA - in 1962. The first ANITA was a full-keyboard machine using vacuum tube technology, modelled on the existing Sumlock mechanical comptometers.

The "1000 Series" of solid-state 10-key machines was introduced towards the end of 1969. The basic machine was a 10-digit four-function desk calculator, with the different models in the series offering different combinations of features - constants, square roots, percentages, or a single memory register. The circuitry was implemented primarily with discrete-component diode and transistor logic, with a small number of MOS integrated circuits to provide the storage registers. The circuit design and the keyboard interface used the "postfix" or "reverse Polish" logic system.

In 1971 the functions of the 1000-series machines were redesigned into less than a fifth of the volume by using the new MOS-LSI integrated circuit technology. The 1000-LSI machines used only 5 custom-built IC chips and about 40 transistors to replace the previous thousands of discrete components.

This page gives a brief overview of the construction, circuitry, and operation of the discrete-component Anita Model 1011. A similar page describes the workings of the equivalent IC-based version, the Model 1011-LSI. The descriptions are based primarily on my own observations and interpretations, supplemented by reference to the 1011-LSI instruction manual and some of the US patent documents. Corrections, comments, or further information are welcome via the Enquiry Form.



1011Internal.jpg (30kb) Internal view

The workings of the Anita 1000 Series are assembled between two plastic side plates with moulded-in card guides. A horizontal "base board" and a vertical backplane support the twenty circuit boards. An extension at the front of the side plates supports the metal frame of the keyboard, and the plastic front escutcheon. The mains power supply, complete with 3-pin input connector and a domestic push-button power switch, is mounted at the bottom rear of the frame. The whole assembly slides into a substantial plastic case which is moulded in a single piece.

KeyboardUnder.jpg (30kb) The keyboard mechanism

The Anita keyboard (seen here from underneath) is built up from four long metal-framed switch modules which are mounted crossways at the front of the machine. The modules have provision for ten change-over switches, and are similar to those used on the earlier full-keyboard Anitas. The four modules are connected to the base board via individual flying leads and quick-connect lugs.



The logic circuitry of the Anita is built primarily from discrete-component diode and transistor logic, with the storage registers being provided by MOS integrated circuits.

The power supply produces +22V, +12V, and -12V for the logic circuits, and +200V for the Nixie-style display tubes (Mullard ZM1080s). Most of the rated 35VA is consumed by the display tubes rather than by the electronics.

The operations are controlled by a master oscillator which produces a complementary square-wave clock at 100kHz, along with a number of synchronising and scanning signals.

The physical circuitry of the Anita 1011 is contained on a total of 20 single-sided circuit boards, each 9" wide and 2, 3, or 4" high. A detailed listing of the boards is provided on a separate page. Three typical examples are shown below.

AddSubBoard.jpg (27kb) Diode logic boards

The ten 2-inch boards mounted horizontally at the rear of the card cage consist primarily of discrete-component diode logic circuits. This "Add/Subtract Logic" board has only 4 transistors, but over 100 diodes of several different types.


BufferBoard.jpg (37kb) Transistor logic boards

This "Buffer" board is typical of the 3" and 4" boards mounted vertically in the card cage. It contains discrete-component diode-transistor logic, with 34 transistors and about 60 diodes.

This board has two additional components installed in a rather crude manner in the white-painted area at the bottom right. The components have pencilled numbers (TR37 and R106), suggesting that they were an official field-service modification.


RegisterBoard.jpg (27kb) The Register board

The register board is one of four 3" boards mounted under the display tube panel at the front of the machine. This board contains six integrated circuits in 12-pin metal-can packages, mounted upside-down in holes in the board. The white-painted area at the left-hand end of the board contains six sets of (presumably) assembler's or inspector's initials.


RegBoardUnder.jpg (27kb) Register board IC detail

The integrated circuits in the Anita were made by Marconi-Elliott Electronics (ME). There are only two types, numbered MA09 and MA15. The date codes show that they were made in weeks 45 and 48 (November) of 1969.

This Register board has three pairs of chips, presumably to hold the contents of the internal working register, the display register, and the (optional) storage register. The small number of connections to the chips (and to the board itself) suggests that the contents are probably single-bit serial shift registers. There is another MA09 chip on the Function Counter board.



KeyboardAndDisplay.jpg (27kb)

The calculator has a bright Nixie-tube display which always shows the full ten digits, left-aligned with trailing zeros. The floating decimal point uses separate neon indicator tubes between each of the Nixies. There are two separate neons at the left of the display for Minus and Store indicators.

The calculator operates according to the "reverse Polish" logic system. For the four basic functions:

Chain calculations can be continued in similar fashion.

The "C" (for Constant) key is a latching switch which copies the display to the internal register and locks the contents for use in a series of calculations. It is also useful with the Percent key. For example, to calculate 50 + 5%:

Strange results will follow if the C switch is accidentally left down during the next calculation.

A third "Store" or memory register is accessed through the "EntS" and "Store" keys. EntS copies the current display into the store register, overwriting the previous contents. The Store key recalls the contents of the register and lights the Store indicator until one of the adjacent ASMD keys is pressed. The Add and Subtract keys return their results to the store to provide an accumulation function, while Multiply and Divide leave the store unchanged. For example:

The blue key marked with the arch or semicircle rounds the display to two figures after the decimal point.

Resources for further information

Original text and images Copyright © John Wolff 2005-10.
Use at own risk; beware of errors; suggestions for improvement welcome.
Last Updated: 28 February 2010

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