Bowmar BD5026 Digit Driver Chip

• Features

When Texas Instruments announced on September 17, 1971 with the TMS1802NC - later renamed to TMS0102 and the first member of the TMS0100 Product Family - the first available standard calculator building block on a chip, it did not include any display drivers to allow customer to pick between available technologies, like LED (Light Emitting Diode) Displays, Panaplex Displays, VFDs (Vacuum-Fluorescent Displays) and LCDs (Liquid-Crystal-Displays) and, with some reprogramming of the Output Decoder PLA, Nixie type tubes.

Most handheld calculators built around the TMS0100 make use of 7-segment LED displays. To simplify the interface and to reduce the component count, Texas Instruments designed two special integrated circuits that allow direct interface between the TMS0100 and the TIL360, a multi-digit display. The circuits are designated SN75491 (segment driver) and SN75492 (digit driver). They meet the driving requirements of multiplexed, common-cathode displays and include the necessary pull-down resistors for the TMS0100 outputs.

The SN75491 segment driver includes four stages and SN75492 digit driver includes six stages. Two SN75491 and two SN75492 were necessary for a handheld LED calculator designed around the TMS0100 devices, not a big deal when a boxy Bowmar 901B sold in Fall 1971 for $240. Driven by the early success of electronic handheld calculators and the resulting fierce competition between dozens - and during the Calculator War even hundreds - of companies, component prices and manufacturing costs grew more and more important. A first step towards a component count reduction for handheld LED calculators was in 1973 the integration of segment drivers into the single-chip calculator (TMS0700, 8- and 10-digits plus minus sign), before the clock oscillator was integrated, too (TMS0800, 8-digits plus minus sign). A major breakthrough for LED calculators was the TMS0830 introduced in 1974, reducing the power supply requirements from two voltages to a single 9 Volt rail.

Designing a compact, low-cost LED calculator around the TMS0800 Product Family was very inviting - but engineers struggled to use two of the 6-channel SN75492-style devices for a 9-digit LED display and used either one digit driver chip and three discrete, transistor based drivers or omitted the digit driver chip completely and used nine discrete drivers as seen in the Canon LE-84. To win their customers back, Texas Instruments consequently introduced with the SN75498 a 9-channel digit driver using a 0.3” wide 20-pin DIP (Dual In-line Package with a 0.1” / 2.54 mm lead pitch) instead the 14-pin DIP of the SN75492 to accommodate the additional 3-input and 3-output pins. But the SN75498 was optimized for calculators using 2, to 4 NiCd or Alkaline batteries and wasn't compatible with a 9 Volt battery powered handheld calculator design with the new TMS0830.

Enter Bowmar! Bowmar addressed with the BD5026 9-channel digit drivers not only the 6.6 Volt power supply limitation of the SN75498, but even reduced its real estate on the printed circuit board (PCB) from a 20-pin DIP to a 16-pin DIP. Wait, nine inputs, nine outputs and two power supply pins - that doesn't fit into a 16-pin package! Maybe not in Dallas, TX, but in Acton, MA. Engineers at Bowmar knew very well that most single-chip calculator circuits scan the 9 digits of the display in a sequential way, the TMS0800 for example is starting with the leftmost digit (D9, minus sign), continues with the next digit to the right (D8, most significant digit), then D7 to D2 and finally the rightmost digit (D1, least significant digit) before inserting with D10 a dead cycle (no digit activated) and repeating the loop.

The innovative BD5026 makes use of this sequential digit scanning and is connected only to every second digit output of the TMS0800, namely D9, D7, D5, D3, and D1. These five signals are connected directly to the corresponding input pins of the BD5026 and reflected on the respective output pins Y9, Y7, Y5, Y3, and Y1 - technically so far a 5-channel digit driver. The remaining output pins Y8, Y6, Y4, and Y2 are "interpolated" with integrated Flip Flops, Y8 for example is activated after Y9 was deactivated and deactivated at the moment when Y7 is activated.

While the BD5026 was from the engineering point of view a very interesting design, was its market success negligible. Too little, too late - Texas Instruments introduced with the TMS0950 early in 1975 the first "true" single chip calculator circuit with integrated segment and digit drivers for 9-digit LED displays and it took together with its successor TMS0970 the already diminishing market of 5-function handheld calculators with by storm. Think about the Far East Generic Design I, Design II and Design III calculators flooding in 1977 and 1978 the market with sales prices well below USD10 and leaving no margin for American calculator manufacturers. And Bowmar was alreday financially struggling, not really helpful for their Marketing Department.

• Family Members and Applications

TMS0830 based LED calculators

• Technical Specifications

• Applications Information

GENERAL INFORMATION

The applications information is based on the reverse-engineering of an MBO Expert calculator utilizing a TMS0833 single-chip calculator circuit in concert with a BD5026 9-digit display driver and a WB7-44 9-Digit 7-Segment LED display module from an unknown manufacturer. We retrieved both the TMS0833 and the BD5026 from the donor calculator and made various measurements described below with a simplified breadboard setup.

The characterization of the digit scanning outputs was conducted with manual measurements but we used for the Output Voltage vs Input Voltage transfer function an automated setup originally created for 75492-style devices. Here at the Datamath Calculator Museum we use this transfer function as an easy obtainable "Signature" of the underlying circuit design and manufacturing processes of digit drivers.

DRIVER CHARACTERISTICS

The TMS0830 is manufactured in a PMOS process and its digit scanning outputs are high-side PMOS transistors, an activated digit corresponds to a logical 1 (around 9 V). The multiplexed, common-cathode LED display on the other hand require the digit pins connected to GND (around 0 V) to lit the LEDs. A typical digit driver for the TMS0830 can be realized with a NPN bipolar junction transistor (BJT) in common-emitter topology or the SN75492-style devices integrating 6 bipolar NPN Darlington transistors with DC current gain of at least 100.

We characterized here at the Datamath Calculator Museum one channel of a BD5026 9-digit display driver retrieved from an MBO Expert calculator and found an unusual high DC current gain, less than 100 uA input current are sufficient for more than 50 mA output current. The trip voltage is slightly higher than 2.0 V, at an input voltage of around 2.5 V, the output is capable to sink more than 50 mA.

Increasing the input voltage up to 9 V shows a the input current increasing almost linear to to around 0.8 mA, suggesting a "pull-down" resistor around 10k Ohm. The output characteristics of the BD5026 Device-under-Test is showing a linear relationship between output current and output voltage drop, around 0.3 V at a load of 50 mA.

The "Signature" Output Voltage vs Input Voltage transfer characteristics is looking typical for a NPN BJT Darlington transistor design but shows a remarkable low saturation voltage at the programmed test parameters of 9 V supply voltage and 100 Ohm load resistor.

SCANNING

The BD5026 9-digit display driver incorporates five traditional digit drivers for the D9, D7, D5, D3, and D1 inputs and reflects them on Y9, Y7, Y5, Y3, and Y1 outputs. The remaining output pins Y8, Y6, Y4, and Y2 are "interpolated" with integrated Flip Flops, assuming that the Inputs are sequentially scanned in D9 → D1 direction. An active (log.1) level on the D9 input is directly output (inverted as log.0) on the Y9 pin. At the end of the D9 digit time, signaled with an inactive (log.0) level on the D9 input, the corresponding Y9 output is turned off (log.1) and a Flip Flop sets the Y8 output active. This Flip Flop is reset with an active level on the D8 input, hence "interpolated" the BD5026 logic the missing D8 signal.

Additional testing of the BD5026 with some variations of the input signals demonstrated that the four Flip Flops for the Y8, Y6, Y4, and Y2 output pins are set edge-triggered with the previous D input and reset level-triggered with both the previous and following D inputs.

The TMS0830 single chip calculator circuit was programmed to accomplish so-called inter-digit blanking, basically inserting a small delay between the deactivation of one digit and the activation of the next digit to avoid ghosting of the digits. The "interpolation" of the Y8, Y6, Y4, and Y2 signals of the BD5026 is removing as seen in the timing diagrams above these gaps and consequently every second digit of the LED display should appear slightly brighter than the others. Due to the additional segment blanking of the TMS0830 does this design choice from the Bowmar engineering team not matter.

We measured the on- and off-delays of the direct Y5 output and the Flip Flop generated Y6 output and noticed that the BD5026 is turning its outputs much faster from off to on than vice versa.

• Technology

• Packaging

The BD5026 uses a standard 0.3” wide 16-pin DIP (Dual In-line Package with a 0.1” / 2.54 mm lead pitch).

• Pin Configuration