Craig Model 4518

Kovac Corporation of Osaka, Japan, started already in 1972 with the Model K80 manufacturing of battery-operated electronic calculators. Most of Kovac's later products were sold not only under the Kovac brand, but made their way as OEM products into the distribution channels of Craig Electronics, Homeland, Prinztronic, Rambler, Unitrans, Ventron and others. Most of Kovac's calculators are based on Texas Instruments' early single-chip calculator circuits, the earliest designs used the TMS0105 chip. The Beetle series with their unique design was introduced in November 1973 and came in two sizes, small with five rows of keys and medium with six rows of keys. The smaller four- or five-function calculators were designed around chips from the TMS0800 Product Family while the more enhanced products made use of the TMS0600 Product Family.

This Craig Model 4518 uses the electronics of the Kovac LE-808MR calculator in a completely redesign housing with a different coloring scheme, different texture and even slightly modified shape.

Here at the Datamath Calculator Museum we acquired this Craig Model 4518 calculator for its TMS0611 single-chip calculator circuit on our quest to complete the Characterization of Single-Chip Calculator Circuits of the TMS0600 Product Family.

Dismantling the featured Craig Model 4518 calculator manufactured in October 1974 in Japan reveals - a beautiful mess with many colorful wires. Three different printed circuit boards (PCBs) are stuffed into the rather small package of the calculator, with a large Display-PCB connected with 17 wires to the Main-PCB and the Keyboard-PCB using a connector for 11 of its 14 signals to the Main-PCB and 3 discrete wires for the remaining signals. Some additional wires connect the battery terminals and charging port to the electronics. Definitely not designed for easy servicing but having a Return-of-Invest calculation in mind. In OEM Projects like this electronic calculator, there are four numbers from importance:

With the first two bullet points negotiated upfront, the OEM needs to balance manufacturing costs vs. development costs - with labor costs in 1974 still very low in Japan, it is obvious that Kovac decided to keep development costs of the electronics rather low and use the money rather for a fancy housing design to accommodate for an unusual large LED display.

Calculating Unit: The TMS0611 is a member of the TMS0600 Product Family and tracing back to the TMS1802NC, the first available standard calculator building block on a chip, later renamed into TMS0102. The TMS0600 increased the size of the Instruction ROM (Read-Only Memory), kept the Data Memory and added integrated segment drivers for the LED display.

Display: This Craig Model 4518 calculator manufactured in October 1974 makes use of nine Fairchild FND357 Seven-Segment displays soldered onto a small Display-PCB which is connected with 17 wired to the Main-PCB of the calculator. The red LED display with a digit height of 9.2 mm / 0.36" is certainly the main differentiator of the Craig Model 4518 calculator from other offerings in the 1974/1975 time frame. While its bold numerals clearly stand out, are they not as fancy as the sales box illustrates. Fun fact: The TMS0611 chip would support the "Fancy-Four" with its additional Segment H, but the FND357 displays use only Segments A to G.

Display Driver: The PCB of the disassembled Craig Model 4518 makes use of an unusual combination of one SN75491 Segment Driver and one SN75492 Digit Driver - inspecting the layout of the PCB reveals that the SN75491 Segment Driver was repurposed to act as a Digit Driver for three digits of the LED display while the SN75492 Digit Driver serves the other six digits. The calculator design utilizes the integrated Segment Drivers of the TMS0611 chip.

Clock: The Craig Model 4518 uses an astable multivibrator composed of two 2SC711 NPN bipolar junction transistors (BJTs), two 100 pF ceramic capacitors and four resistors (47 kOhm and 1.5 kOhm) for the clock generation of the TMS0611 single-chip calculator circuit. We calculated with this values a clock frequency of around 150 kHz, but measured an actual clock frequency of about of 130 kHz.

Power Supply: The Craig Model 4518 is powered by a disposable 9 Volt battery and can be operated with an external, DC adapter, too. The Main-PCB of the calculator hosts a discrete power converter to generate the V_DD and V_GG supply voltages for the TMS0611 chip. We observed with the featured calculator manufactured in October 1974 voltages of V_DD = -6.3 V and a pulsed V_GG supply with voltages between -5.1 V and -12.7 V while operated with V_BAT = 9.0 V. We measured the operating current of the featured Craig Model 4518 calculator for two different cases:

The power consumption for the featured Craig Model 4518 results in about 360 mW displaying a '0.' and 1,440 mW with all segments but the minus sign illuminated. The peak current of 160 mA with all segments lit is unusual high and a result of the strong integrated Segment Drivers of the TMS0611 and operating the display directly from the 9 Volt battery. The Brother Model 827R (Version 2) calculator sporting a similar TMS0603 chip and operating the display from its 4.5 Volt battery clocks in at 280 mW and 440 mW, respectively. Typical 9 V alkaline batteries have a capacity of around 500 mAh, meaning the calculator would drain the battery within about 3 hours displaying '88888888'.

Keyboard: The Craig Model 4518 calculator makes use of a complex keyboard assembled with individual long-stroke push-button switches pushing conductive elements against small contacts etched into the Keyboard-PCB. The switches are arranged in an 11*3 matrix and require consequently 14 connections to the TMS0611 single-chip calculator circuit. Eleven of the 14 signals are brought to 12 connector pads on the bottom of the Keyboard-PCB while the remaining 3 signals are hand-soldered onto small pads on the top of the PCB. The 12 connector pads are most likely a relict from the TMS0800 chips using a 9*3 matrix for the keyboard.

Preparing our DCM-50A Platform to allow the Characterization of Single-Chip Calculator Circuits of the TMS0600 Product Family, we studied the featured Craig Model 4518 calculator manufactured in October 1974. In a first step did we observe with a Mixed Signal Oscilloscope (MSO) the signals at the TMS0611 chip to verify its pin-out before disassembling the calculator completely to analyze its printed circuit board (PCB) wiring. When we desoldered the TMS0611 from the Main-PCB, we made an unexpected discovery:

Confirming the pin-out of the calculator chip and reverse-engineering the keyboard matrix, we were able to fully operate the TMS0611 with our DCM-50A Platform and not only measure precisely the timing of its Display and Keyboard interface but digging deeper into the algorithm embedded in its firmware. Analyzing the Feature Sets and Calculator Logic Implementations of the six known members of the (not so successful) TMS0600 Product Family, we can summarize: Too Little, Too Late. When Texas Instruments introduced the TMS0600 early in 1974, with custom-specific designs not available before Summer 1974, the market of pocket calculators diverted into two directions:

With the rather limited program memory (384 Words * 11 Bits) and small data memory (3 * 13 Digits Registers), the functionality of all six known TMS0600 designs wasn't competitive and the restriction to LED displays with external digit drivers and missing an internal clock oscillator didn't help with the manufacturing costs of an electronic calculator.

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

© Joerg Woerner, December 23, 2024. No reprints without written permission.