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Anita Model 1011-LSI Desk Calculator



Anita 1011-LSI

Anita Model 1011-LSI, S/N LM 107206
Functions: ASMD, constant, percent, roundoff, 1 memory
Technology: MOS-LSI, 5 chips
Display: 10 digits, Nixie tubes
Dimensions: 225W x 140D x 90H, weight 1.0kg
Manufactured: Bell Punch Company, England, 1971-


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.

The 1000-LSI series from 1971 replicated the functions of the 1000-series machines in less than a fifth of the volume by using the new MOS-LSI integrated circuit technology. The 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 Anita Model 1011-LSI. A similar page describes the workings of the earlier discrete-component version, the Model 1011. 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.



Anita 1011-LSI external view External view

The calculator is housed in a simple 2-piece plastic casing. The two sections are held together by moulded clips at the rear and two screws from underneath at the front. The distinctive styling is dominated by the full-size Nixie-tube display and the "wrap-around windscreen" (which was popular on the cars from the same period).

Anita 1011-LSI internal view Internal view

The keyboard assembly is mounted in the upper section of the case. The power transformer and three circuit boards are in the lower section. The main board is secured by moulded clips at the rear and two screws at the front. The keyboard is attached by an 11-core cable and pin-and-socket connectors.



This section describes the physical circuitry of the machine. I have also prepared a set of reverse-engineered schematic drawings which can be made available to interested parties on request. These show all of the discrete-component circuitry, including the keyboard and display multiplexing arrangements and the communication paths between the IC's. Unfortunately neither the schematics nor the operating instructions give any information about the overall logical structure of the machine or the internals of the IC chips. The available patents describe various individual features, some in infinite detail, which may or may not apply to the early Anitas, the 10-key machines, the 1000 series, or the 1000-LSI. It appears that all the machines operate in a similar manner using a pulse-counting system, but for the moment I do not propose to investigate any further.

Power supply board The power supply board

The power supply board is mounted in the bottom of the lower section of the case. It produces a single regulated +15V DC supply, and includes a protection circuit to shut down the regulator in case of overload or short-circuit. There is no 200V DC supply for the Nixie tubes, as the necesary high voltages are generated on-the-fly by pulse transformers in the multiplexing circuit. The total current drain from the power supply is only about 250mA at 15V, or less than 4W.


Main circuit board Main circuit board

The main circuit board measures 100 x 195mm. The ten ZM1082 Nixie tubes are mounted at the rear, with individual neon lamps to form the decimal points. There are two separate neons at the left for Store and Minus indicators. The display always shows the full ten digits, left-aligned with trailing zeros.

The anodes of the ten display tubes and the two indicators are driven from the six pulse transformers and associated transistors near the centre of the board. The cathode drivers are along the right-hand side, with the controlling IC mounted upside-down through a hole in the board. The "odds and evens" multiplexing system is outlined in US Patent 366987, filed by James Drage of Sumlock in November 1970.

The main board also carries part of the keyboard pulse shaping syatem and the master clock generator. The clock generator (in the front left corner) is a simple multivibrator which produces a single-phase square wave at about 330Khz.


IC board IC board

The IC board measures 55 x 155mm. It mounts above the main board with the component side down, and attaches via the vertical pin connectors at the lower left. The keyboard cable attaches to the horizontal pins at the right. The under side of the board is visible in the Internal View above.

The ICs were custom-built by General Instrument Microelectronics (GIM) to replicate the functions of the earlier Anita 1011. The chips are housed in 12-pin metal-can packages which are mounted upside-down through the board. The IC numbers in this machine are 201C2, 261L1, 241L2P, and 161C10 on the IC board, and 121T1 as the display controller on the main board.

Apart from the power and clock signals, there are only three data lines interconnecting the five IC chips. The internal data signals are multiplexed onto these three lines and transmitted serially. The multiplexing arrangement is outlined in US Patent 3683415, filed by Lloyd and Letheren of Sumlock in November 1970.

Keyboard PCB Keyboard PCB

The keyboard PCB measures 85 x 200mm, and contains the key switches and a diode encoding matrix. The connecting cable plugs in to the IC board. The power switch at the left has two separate quick-connect spade lugs which carry the incoming mains voltage.


Keyboard construction Key switch detail

The keyboard uses simple mechanical switches with a bronze spring and a fixed stud mounted directly onto the circuit board. The keytops and return springs are mounted on a 3-legged base which clips into slots in the board. The keytops are easily removed for cleaning. The input circuit includes an R-C filter to eliminate contact bounce.



The instruction manual for the 1011-LSI gives examples of keystroke sequences for performing various types of calculations, but provides no information at all about the overall architecture of the machine or the internal register operations. The notes following summarise what appears to happen in response to the function keys.

Keyboard and display

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 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.

The calculator normally operates to 10 significant figures, which are displayed left-aligned with trailing zeros. The decimal point floats within this range. For out-of-range results with more than 10 figures to the left of the decimal point or 10 zeros to the right, the machine still displays 10 significant figures but shifts the decimal point 10 places in the appropriate direction, giving an effective 20-digit operating range.

Resources for further information

Original text and images Copyright © John Wolff 2009-10.
Use at own risk; beware of errors; suggestions for improvement welcome.
Last Updated: 28 February 2010

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