Brother Model 827R (Version 2)

Brother Industries, Ltd. was founded already in 1908 as Yasui Sewing Machine Co, in Nagoya, Japan and is today mainly known for their printers and sewing machines and vaguely remembered for their typewriters. Nevertheless entered Brother already in October 1966 with the fully-transistorized Calther 130 the business of electronic calculators before switching in October 1969 with the Calther 412 (12 Nixie tubes) and Calther 514 (14 Nixie tubes) to a mix of Integrated Circuits (ICs) and transistors. Shortly after Texas Instruments introduced in October 1971 the TMS1802NC "calculator-on-a-chip", Brother introduced with the PRO-CAL 408 one of the earliest battery powered, portable electronic calculators.

Here at the Datamath Calculator Museum we acquired our first Brother Model 827R calculator in 2024 on our quest to complete the Characterization of Single-Chip Calculator Circuits of the TMS0600 Product Family, anticipating the missing TMS0603 single-chip calculator circuit but locating a General Instruments C-595-1 Chip instead. Comparing online pictures of various Brother Model 827R calculators, we found a small but important difference between them! Some Model 827R calculators sport an orange [C] key and some have a red key, instead.

Dismantling the featured Brother Model 827R (Version 2) calculator manufactured in September 1974 in Hong Kong reveals indeed a design centered around a TMS0603 Chip but we noticed some other cost-reduction measures. The first noticeable difference to the original design introduced just 6 months earlier is a large piece of foam in the battery compartment, reducing the number of AA-sized rechargeable NiCd batteries from four to three.

Comparing the printed circuit boards (PCBs) of a Model 827R (Version 1) manufactured in June 1974 and a Model 827R (Version 2) manufactured in September 1974 shows a change in the PCB technology from a two-sided Epoxy design to a single sided Phenol design and the display drivers in hybrid technology were replaced with two SN75494 Digit Drivers.

One interesting feature of the original Model 827R did not change, the small discrete LED soldered next to the left-most digit of the LED display is still present. The General Instruments C-595-1 Chip provides a dedicated Memory Indicator Output Pin to signal a value other than Zero in the (accumulating) Memory, a feature not known from the TMS0600 Family of single-chip calculator circuits.

Power Supply: While the first Version of the Model 827R calculator was powered by four rechargeable NiCd batteries, uses this second version only three batteries. We measured the operating current of both calculators for two cases:

The power consumption for the Model 827R (Version 1) results in about 250 mW displaying a '0.' and 470 mW with all segments but the minus sign illuminated and for the Model 827R (Version 2) we calculate about 280 mW and 440 mW, respectively. Truly a cost-cutting measure, less battery capacity and roughly the same power consumption results in less operating time for the user of the calculator.

Preparing our DCM-50A Platform to allow the Characterization of Single-Chip Calculator Circuits of the TMS0600 Product Family, we studied the featured Brother 827R (Version 2) calculator manufactured in September 1974. In a first step did we observe with a Mixed Signal Oscilloscope (MSO) the signals at the TMS0603 chip to verify its pin-out before disassembling the calculator completely to analyze its printed circuit board (PCB) wiring.

Confirming the pin-out of the calculator chip and reverse-engineering the keyboard matrix, we were able to fully operate the TMS0603 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, leading to some unexpected discoveries:

Automatic Constant: When Texas Instruments introduced with the TMS1802 the first member of the TMS0100 Product Family, the direct predecessor of the TMS0600 Product Family, it did not even sport a [=] key. Its [+=] and [−=] keys clearly demonstrated that the chip was designed to replace adding machines used in offices and not slide rules. Consequently were these two keys used to accumulate numbers in a register and some calculator designs even labeled the Clear key with [CA] for Clear Accumulator. With Adding Machine Logic the [+=] and [−=] keys always complete operations, meaning the key sequence [2] [x] [3] [+=] [4] [x] [5] [+=] is resulting in 20 from the evaluation of 4 x 5, the previous calculation of 2 x 3 = 6 was cleared in the moment the [4] key was pressed. The [CHAIN-CONST] switch often used with TMS0100 single-chip calculator circuit designs is using the Constant Mode to simplify repeating multiplications and divisions but does no affect the outcome of the key sequence illustrated above.

Later algorithm designs omitted the costly [CHAIN-CONST] switch and started using the concept of an Automatic Constant, meaning the answer from a multiplication or division is entered as a Constant with the [=] key. For practical reasons most implementation use for Multiplications the 1^st operand as a Constant and for Divisions, Additions, and Subtractions the 2^nd operand as a Constant. Here at the Datamath Calculator Museum we annotate this implementation as M-D-A-S (1-2-2-2). This approach mimics old-school Multiplication and Division Table Charts used in Elementary Schools all over the world. The Constant Mode feature simplifies raising a number to a power and calculating the reciprocal of a number, too.

Example operations for a calculator with M-D-A-S (1-2-2-2) Automatic Constant:

Memory Indicator Output: While reverse-engineering the electrical connections of the Memory Indicator LED of the Brother Model 827R (Version 2) calculator to the TMS0603 Chip, we noticed that it is wired between the H-Segment of the TMS0603 and the common cathode of Digit 8 of the LED display.

The H-Segment Output of the TMS0600 device is a relict inherited from the TMS0100 Product Family to render the "Fancy Four" used with some early Vacuum Fluorescent Displays but not available with 7-Segment LED Displays like the one used with the Brother Model 827R calculator. While the General Instruments C-595-1 Chip features a dedicated Memory Indicator Output, is the H-Segment Output a multiplexed signal and wiring it to Digit 8 of the LED Display suggests that it might serve other purposes during the other Digit Times. With the DCM-50A Platform using eleven H-Segment LEDs and most of them turning on and off at seemingly random times, we started to analyze each of the eleven signals during different sample calculations. To our surprise could we decipher the "Flag-Information" encoded in ten of the 11 Digit Times:

Implemented Algorithm: Texas Instruments introduced the TMS0600 Product Family about two years after the introduction of the TMS0100 and the only differentiator was a slightly larger program memory,  increasing the maximum number of instructions from 320 to 384. The first two designs of the TMS0600 Product Family labeled TMS0601 for the TI-2550 and TMS0602 for the SR-11 were just algorithm enhancements of existing designs labeled TMS0119 and TMS0120, respectively:

Looking into the feature set of the Brother Model 827R calculator with just the Percent Function, a Four-Key Memory and Automatic Constant, we questioned the reasoning for the enhanced instruction memory found with the TMS0603. Using the DCM-50A Platform supporting a maximum of 44 keys and switches and the TMS0603 Chip salvaged from the featured calculator, we discovered five additional functions hiding behind some unused key locations:

It is obvious that the TMS0603 was targeted against the General Instruments C-590 Product Family but with only two known calculator implementations we wouldn't call the design successful. Neither was the TMS0604 but this is a different story. Don't miss the rare Dittel TMP608 that should have been named Dittel TMS604.

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, May 12, 2024. No reprints without written permission.