Litton Royal Digital V

Litton Industries, headquartered in Beverly Hills, California, acquired in 1958 Monroe Calculating Machine Company, an American manufacturer of mechanical calculators. In 1966 Litton acquired Imperial Typewriter Company Ltd, a very successful manufacturer of typewriters and merged it with its own Royal Typewriter division. In 1969 the conglomerate was further growing with the acquisition of Triumph-Adler, the merger of UK Triumph Cycle Company and Adler, a German manufacturer of bicycles, typewriters, sewing machines and calculators. Early in the 1970s, Litton sold, depending on the region, electronic calculators under five different brands: Adler, Imperial, Monroe, Royal and Triumph.

Litton Industries introduced with the Royal Digital III in November 1971 their first handheld electronic calculator based on General Instrument's 250 single-chip calculator circuits, better known as PICO1. The Digital III was soon complimented with the Digital IV, replacing the unusual 4-digit Vacuum Fluorescent Display (VFD) with a more common 8-digit LED display while maintaining its stylus and gold-plated keyboard contacts to operate the calculator. This Litton Royal Digital V calculator combines a traditional keyboard with an 8-digit VFD while the Litton Monroe 20 calculator combines a traditional keyboard with the 8-digit LED display from the Digital IV.

Dismantling the featured Litton Royal Digital V calculator manufactured in May 1972 in Japan reveals a compact design based on a double-sided printed circuit board (PCB) for the main electronics, a single-sided PCB for the power supply and powered by six internal rechargeable NiCd batteries or an external power adapter.

The Main-PCB is centered around a General Instrument 251F single-chip calculator circuit and the other remaining components on the PCB are mainly used to interface the display and keyboard. The Power Supply and Clock Module mounted next to the rechargeable batteries generates the different supply voltages for the GI 251F and the Vacuum Fluorescent Display (VFD).

To gain some knowledge about the differences between the GI 251F located in this Litton Royal Digital V and the GI 250 used with this Royal Digital III, we decided here at the Datamath Calculator Museum to give it a "Teardown Treatment", sharing our findings accordingly.

Calculating Unit: The GI 251F used with the Litton Royal Digital V is the second implementation of the PICO1 single-chip calculator - on the silicon die labeled as F 1971 76250 PICO11 76251:

Display: The featured Litton Royal Digital V calculator manufactured in May 1972 makes use of an 8-Digit low-voltage VFD manufactured by Futaba and known as Type 8-CT-01, soldered with its 18 pins directly to the Main-PCB.

Display Driver: Early "low-voltage" Vacuum Fluorescent Displays are usually operated between 30 Volts and 45 Volts, significantly higher than the operating voltage of single-chip calculator circuits. Consequently do most calculators use some discrete or integrated display drivers on the PCB as "level-shifters" between the segment and digit outputs of the calculator chip and the anodes and grids of the VFD. The General Instrument C251 chip uses only 4 digit outputs, labeled D15, D26, D37, and D48, requiring an external "demultiplexer" to operate an 8-digit display. The dismantled Digital V calculator uses an interesting and complex approach to interface with the VFD:

Measuring the supply voltage of the GI 251F chip results in a value of 24 Volts while the VFD tubes operate with a rather high voltage of 44 Volts.

Clock: The Power Supply and Clock Module of the Litton Royal Digital V is providing the clock signal for the GI 251F Chip with a frequency of about 80 to 100 kHz.

Power Supply: The Litton Royal Digital III calculator is powered with six internal, AA-sized rechargeable batteries or an external 7.2 Volt power adapter and uses a complex DC/DC converter to generate a total of four voltages:

We measured the operating current of the featured Litton Royal Digital V calculator for two different cases:

Calculating the power consumption at 7.5 Volts for the Litton Royal Digital V results in about 890 mW displaying a '00000000' and about 900 mW with all segments illuminated.

Keyboard: The keyboard assembly of the Litton Royal Digital V uses 18 spring-supported plastic keys pushing fingers on a stamped sheet-metal piece against large, gold-plated contacts on the Main-PCB. The sheet-metal piece itself is connected through two additional contacts with the Main-PCB.

Here at the Datamath Calculator Museum we use the DCM-50A Platform to Characterize and Reverse-engineer Single-chip Calculator Circuits. Many designs of electronic calculators do not use all features of their calculator brains and it would be difficult to unleash the full potential of the calculator chips in these cases. Additionally are electronic calculators "closed systems" with limited flexibility to measure signals, change voltages or clock frequencies, provide additional input keys or even change the display technology or specifications additional digits. Core idea of the DCM-50A is providing a generic platform to access all features of a single-chip calculator circuit and with the DCM-50A (PLAYGROUND) we increased the scope from Texas Instruments products to offerings from their competitors in the 1970s, namely AMI, Cal-Tex, Commodore/MOS Technology, Electronic Arrays, General Instrument, Hitachi, Litronix, Matsushita, Mitsubishi, Mostek, National Semiconductor, NEC, Omron, RFT, Rockwell, Sharp, Toshiba, and Western Digital.

On our quest to document Pico Electronics' PICO1 Chip and its many descendants like the General Instrument C-500, C-550, C-560, C-570 and CZL-550, we developed here at the Datamath Calculator Museum three additional tools for our DCM-50A (PLAYGROUND):

Comparing the Calculator Logic Implementation of the GI 251F with the Calculator Logic Implementation of the GI 250 chip retrieved from the "Teardown" Litton Royal Digital III reveals no differences other than the missing toggle key.

Don't miss the "Picolator" on the German Richi's Lab site, an Emulator for General Instrument's C-550 single-chip calculator circuit - still using the PICO1 program code from the original GI 250 chip.

If you have additions to the above article please email: joerg@datamath.org.

© Joerg Woerner, July 9, 2025. No reprints without written permission.