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.

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.

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.

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.

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.

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.

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.

The keyboard operates according to the "reverse Polish" logic system:

• key the first number (stored in the display register)
• press "Enter 1st Number" (copies number to internal register)
• key the second number (if any)
• press the operation key ASMD.

The "C" key is Constant rather than Clear - it is a latching key which locks the contents of the internal register for use in a series of calculations. The percent key functions as expected.

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

The Store key copies the display into a separate memory storage register and lights an "S" indicator to the left of the display. The "ENT S" key copies the stored value back into the display.

Machine M2971 shows a number of inconsistencies in the keyboard operations, but it is not known whether these are design features or malfunctions of this particular example. I would be keen to compare notes with owners of similar or "known good" machines.

• The Bell Punch page gives an overview of the company's mechanical calculators.
• The Technical Section of this site has a detailed description of the workings of a Bell Punch "PLUS" mechanical calculator.
• Nigel Tout from England has a detailed history of the rise and fall of the Bell Punch Company in his Vintage Calculators Web Museum. He has illustrations of many of their other models, both mechanical and electronic, and tells the fascinating story of how a bus-ticket company came to build the world's first electronic calculator.
• The label attached to the underside of the case contains many references to patent documentation that may be available from repositories around the world.