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Friden Model 1115 Desk Calculator

Friden 1115 external view Friden Model 1115, S/N 79982A
Functions: ASMD
Technology: MOS SSI/MSI (Hitachi, 46 chips)
Display: 12 digit, Nixie tubes
Dimensions: 260W x 330D x 110H, weight 4.66kg
Manufactured: Hitachi, Japan, 1970



Nameplate The Friden Calculating Machine Company built an extensive range of mechanical calculators from the early 1930s to the 1960s. In 1964 they introduced one of the first all-transistor electronic calculators, the EC-130. The mechanical calculator business collapsed world-wide in the mid-1960s, and Friden was taken over by the Singer (sewing machine) Company. In-house development was terminated, and subsequent "Friden" and "Singer" electronic calculators were sourced from Hitachi in Japan.

The first Hitachi Fridens were the "1100" series. This 1115 from 1970 is a basic four-function desk calculator with a 12-digit Nixie-tube display. The calculating circuitry is based on Hitachi's 3100 series of MOS/SSI chips, with four complex (for the time) MSI chips to provide register storage and arithmetic functions.

The circuitry of the 1115 represents a significant transition point in the development of calculator technology. The first-generation of IC-based calculators required 150 or more DTL-SSI chips. The introduction of MOS technology and medium-scale integration allowed the 1115 to be built with only 46 chips. Ever-higher integration led to machines with 5, 4, and eventually just one active device. The entire transition was completed in only about three years.


Internal View Internal view

Removal of the cover reveals a metal chassis sitting in a light plastic base. The chassis is held by a row of plastic retainers across the front and a single screw near the centre of each side. The keyboard sits on the chassis on four rubber-tipped supports and is held in place by the plastic cover alone. A multi-wire cable runs from the keyboard to a connector at the rear.

Chassis and boards Chassis and boards

With the keyboard removed we see a large circiut board covering the base ot the machine, with a shorter logic board and a display board above. The "black boxes" at either side of the display tubes contain the overflow and negative indicators. The three boards have metal frames around the front and sides to prevent flexing.

The two upper boards are held in place by a full-width metal retainer plate (removed for clarity). The front edge of the lower board (and the chassis) is held by the four large plastic retainers moulded into the base. The chassis must be removed from the base in order to remove the lower board.

Near the front left is a mains connector for the power switch on the keyboard, and an earth lead for the keyboard frame.

Rear View Rear view

This view shows the power supply and wiring at the rear of the machine. Below the power supply board is a hand-wired backplane for connection to the three circuit boards. The keyboard cable attaches to the 45-pin connector at the left.

The wiring is very congested behind the mains connector, to the extent that a special connector had to be mounted externally on the plastic base. This causes difficulty and potential damage when removing the chassis for maintenance, or for access to the lower board. Access is further restricted by the two conspicuous metal brackets. As the top cover is fastened from underneath in the usual manner, the brackets serve no obvious purpose in this machine.

The power supply produces a main regulated supply of negative 24V DC. Auxilliary supplies of -14V, -10V, and -5V are derived from a simple resistive divider across the main supply. The gas-discharge display tubes are driven from a bipolar supply of +/-100V DC. Total power consumption is rated at 12W.

Logic board 1 Logic boards and chips

The lower circuit board measures 260 x 230mm and carries 23 ICs. It connects to the backplane through two double-sided 22-way edge connectors.

The circuitry of the 1115 uses 46 IC ships, 58 transistors, and a quantity of discrete-component diode logic. All of the IC chips are from Hitachi's 3100 family of MOS logic, and most are SSI gates and flip-flops. Most of the gates require external load resistors. The date codes on the chips range from November 1969 to May 1970.

Device Pins Count Description
HD3101 14 1 Dual 8-bit quasi-static shift register
HD3103 16 9 Five MOS FETs (inverters).
HD3104 16 3 Dual 4-input AND gate plus 2 inverters.
HD3106 16 16 Dual 2-input and dual 3-input AND gates.
HD3107 16 9 Quad quasi-static D-type flip-flop.
HD3109 24 3 60+4+4 bit dynamic shift register with control gates.
HD3112 24 1 Delayed BCD full adder.
HD3115 16 4 Triple quasi-static RSS flip-flop.

The main work of the calculator is done by the four MOS-MSI chips in 24-pin packages. The HD3112 is a bit-serial BCD full adder/subtractor which handles all of the arithmetic functions. The three HD3109 chips are dynamic shift registers which provide recirculating storage for the three working registers of the machine. 56 bits are used to store 14-digit BCD numbers, but only 12 digits are displayed. 8 bits used for sign and decimal position data.

Logic board 2 Logic board 2

The second logic board measures 150 x 230mm, with similar metal frame and edge connectors. The four ICs in 24-pin packages are the 3112 full adder at the lower left, and the three 3109 register chips. Two of the nine transistors at the upper right produce an asymmetric two-phase master clock at around 60Khz, while the other 7 produce several buffered and gated clock outputs.

Display board Display board

The display board also measures 150 x 230mm with a cut-away section to clear the power transformer at the rear of the machine.. The twelve Hitachi CD-79 display tubes are 12.5mm diameter and 30mm high. The numerals are 9mm high with a decimal point at the lower right.

The numerals, decimal points, and indicators are multiplexed and driven by a total of 38 transistors and their associated R-C networks. The single transistors are the anode drivers, while the transistors in pairs are the cathode drivers for the numerals and the decimal points in each tube.

Gas-discharge tubes require an anode supply of about 200V DC to operate, but transistors rated above 100V were not readily available in 1970. The anode drivers in the 1115 use biploar +/-100V supplies and a capacitor "charge pump" arrangement to produce 200V pulses at the appropriate times.

Keyboard assembly (underneath) Keyboard assembly

The keyboard is a substantial unit which weighs over 1.25kg. It is built on a solid die-cast base plate which sits on four supports on the top of the chassis. In spite of its weight, it is only held in place by the lightweight plastic cover of the machine. The keyboard connects through a multi-wire cable to the plug-and-socket at the rear of the machine, and then via the backplane to the timing and coding circuits on the logic boards.

The keyboard uses glass reed switches, operated by moving magnets attached to the sides of the keystems. The switches are assembled into three heavy steel-framed modules, two in a 3x4 matrix and one 1x3. The Constant and Round-off keys (K and R/O) have latching mechanisms. A small slider switch selects 0, 2, or 5 decimal places in the result. The push-button mains switch has a separate plug-and-socket connector.



Keyboard and Display

Operation of the 1115 is quite conventional by modern standards. The four arithmetic keys work as expected. Latching the R/O key down enables 5/4 roundoff in the last digit of the result. Latching the K key retains the second factor of the previous calculation as a constant. The R (for Reverse) key reverses the contents of internal registers 2 and 3 (the input/result register and the previous factor storage register).

The display uses a floating decimal point on entry, but a fixed decimal position for the result. The output can be set to 0, 2, or 5 places by the slider switch at the left. The Overflow indicator lights and locks the machine if the display capacity is exceeded.

The calculator has a master clock frequency of only about 60kHz. It takes about 50mS for an addition or subtraction, and up to 400mS for a multiplication or division. There is no blanking of intermediate digits, which cycle rapidly through the display as the calculation is performed. The machine will happily attempt to divide by zero, and will continue trying until terminated with the Clear key.

Original text and images Copyright © John Wolff 2020.
Use at own risk; beware of errors; suggestions for improvement welcome.
Page created: 12 May 2020. Last Updated: 20 May 2020.

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