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Texas Instruments announced on September 17, 1971 with the TMS1802NC the first available standard calculator building block on a chip, it was later renamed into TMS0102. The chip integrates 3,520 Bits Read-Only program Memory (ROM, 320 Words x 11 Bits), a 182-bit Serial-Access Memory (SAM, 3 Registers * 13 Digits, 2 * 13 Bit-Flags) and a decimal arithmetic logic unit as well as control, timing, and output decoders but no drivers for the display. These function blocks of the chip add up to an overall complexity of roughly 5,000 transistors.

While the TMS0100 product family was an impressive achievement combining the electronics of two Integrated Circuits (ICs) housed in large 40-pin packages into a single IC with a much smaller 28-pin package, could Texas Instruments’ advertisement "calculator-on-a-chip" be misleading. The Bill of Materials (BOM) of a typical calculator based on the TMS0100 design like the TI-2500 Datamath included not only the calculator chip, housing, keyboard assembly, display assembly, rechargeable batteries and a power [ON-OFF] switch, we did count on the printed circuit board (PCB) of the already cost-optimized TI-2500B introduced in January 1974 a myriad of components:

• TMS0119 Calculator Chip • 2 SN75494 Digit Driver Chips • 2 SN27915 Segment Driver Chips • 5 Transistors • 12 Diodes • 1 Inductor • 20 Resistors • 7 Capacitors

Consequently the suggested retail price of the TI-2500B was marked with an astonishing $69.95, about $349 in 2000’s money and more than $500 in 2020’s money!

Texas Instruments was able to significantly drop the manufacturing costs of the Datamath with the introduction of the TI-2500 II in November 1974 based on the TMS0800 Chip with the integration of both the segment drivers and clock oscillator into a small 28-pin package.

Credits for the first "real" single-chip calculator circuit go to the TMS0950 Chip introduced early in 1975 based on the TMS1000, the World’s first Microcomputer. While switching from the chip-size efficient Register Processor architecture of the TMS0100 design to a more flexible Digit Processor architecture of the TMS1000, more than doubling the program memory from 3,520 Bits to 8,192 Bits, increasing the data memory from 182 Bits to 256 Bits resulting in an overall complexity of about 8,000 transistors, could the BOM of a typical electronic handheld calculator be reduced dramatically due to a series of measures:

• Integration of both segment and digit drivers for a 9-digit LED display • Lowering the two supply voltages from -10.0V and -15.8V to a single supply voltage of -9V provided by a disposable alkaline battery • Integration of clock circuitry and power-on reset circuitry

Due to a smart pin-out of the 28-pin package matching both the pin-out of the display assembly and keyboard assembly and the use of only 6 discrete components (4 resistors, 1 capacitor and [ON-OFF] switch) avoiding any cross-points in the layout of the circuit, the whole electronics of a calculator could be placed easily on a simple, single-sided PCB. One side effect of integrating additional functionality to the specifictions of the TMS1000 while keeping the pin-count limited to 28 was a reduction of the number of digit outputs from 11 to 9.

The TI-1200 was introduced in March 1975 with a MSRP of $24.95, ditching the rather expensive Klixon keyboard and paving the way to the sub-$10 calculator leading to casualties like Bowmar Industries in 1976 during the Calculator War.

Texas Instruments even further optimized the design of the TMS0950 with the introduction of the pin-compatible TMS0970 in March 1976 and avoiding any passive components other than the [ON-OFF] switch on the PCB.

Due to its short product cycle of just one year only three calculators are based on the original TMS0950 design, while the TMS0970 – later renamed to TMC0970 and TMC0900 - could be considered one of the most successful calculator chip designs next to the TMC0980.

Type	Calculators	Keyboard	Digits	Fixed DP	Special Functions	(6,7,9) Font	Entry Overflow	Calculating Overflow	Memory Indicator
TMS0952	TI-1200, TI-1250	[+][-][=]	sign + 8	F	[Memory][%]
TMS0954	TI-1260	[+][-][=]	sign + 8	F	[Memory][%] [Conversions]
TMS0956	Sharp EL-204	[+][-][=]	sign + 8	F	[Memory][%] [√x]

 

	Description	Comments
Architecture	Single-chip Calculator	Basic
Category	Digit Processor	4-bit digits
Related	TMS1000 Portfolio TMS0970	Improved design
ROM Size	8,192 Bits	1024 Words * 8 Bits
RAM Size	256 Bits	64 Digits
Outputs	9 Digits, 8 Segments	Integrated Digit and Segment Drivers
Inputs	4 Keyboard 1 Oscillator Mode 1 Reset	Segments to Keyboard Scan-Matrix

Item	Min	Typ	Max	Unit	Comments
VSS		0		V
VDD		-9.0		V
CK		180		kHz	Rext= 68 kOhm, Cext= 68 pF

Pin	IO	Function	Pin	IO	Function
1	O	Digit driver 7	28	O	Digit driver 6
2	O	Digit driver 8 (MSD)	27	O	Digit driver 5
3	O	Digit driver 9 (sign)	26	O	Digit driver 4
4	V	Negative Voltage VDD	25	O	Digit driver 3
5	I	Keymatrix input 1	24	O	Digit driver 2
6	I	Keymatrix input 2	23	O	Digit driver 1 (LSD)
7	I	Keymatrix input 4	22	R	6k8 to VDD
8	I	Keymatrix input 8	21	V	Reference voltage
9	I	Key or power on reset	20	V	Common Voltage
10	O	Segment driver DP	19	I	OSC2 (Ext. CLK = VSS)
11	O	Segment driver G	18	I	OSC1 (Cext, Rext) or Ext
12	O	Segment driver F	17	O	Segment driver A
13	O	Segment driver E	16	O	Segment driver B
14	O	Segment driver D	15	O	Segment driver C

The Segment drivers A-G and DP (Decimal Point) are connected to the display in the pictured way.

The keyboards of all calculators based on the TMS0950 family consist of an x/y-matrix connected to six or seven segment-driver outputs and the keymatrix inputs K1 to K4. The Power-on reset is usually not connected to a key.

Example for the TI-1250 with TMS0952:

	K1	K2	K4	K8
O7 (Seg. DP)	MC	MR	M−	M+
O6 (Seg. G)
O5 (Seg. F)	C	+/−	%	÷
O4 (Seg. E)	7	8	9	×
O3 (Seg. D)	4	5	6	−
O2 (Seg. C)	1	2	3	+
O1 (Seg. B)	CE	0	.	=
O0 (Seg. A)

Example for the Sharp EL-204 with TMS0956:

	K1	K2	K4	K8
O7 (Seg. DP)	CM	RM	M−	M+
O6 (Seg. G)
O5 (Seg. F)	C	%	√x	÷
O4 (Seg. E)	7	8	9	×
O3 (Seg. D)	4	5	6	−
O2 (Seg. C)	1	2	3	+
O1 (Seg. B)	CE	0	.	=
O0 (Seg. A)			+/−

• Display

Calculators based on the TMS0950 make use of 9-digit LED (Light-Emitting-Diode) Displays with common cathode architecture. The TI-1200/TI-1250 uses the LED-stick DIS233.

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

© Joerg Woerner, February 02, 2001. No reprints without written permission.