31
The Tektronix 21 and Tektronix 31 are programmable desktop calculators introduced in August 1973. They feature a complete set of transcendental functions. The 21 supports simple programs (sequences of keystrokes) whereas the 31 supports conditional branching and subroutines.
Key Specifications
Functions
Add, subtract, multiply, divide, reciprocal, logarithm (natural and base 10), exponentiation, square, square root, integerize, Pi recall, triangle formula, factorial
Number of digits
The 21 and 31 displays 10-digit signed numbers but calculates to 12 digits internally (i.e. there are two guard digits). In scientific notation, a signed 2-digit exponent is shown. The number range is 1 × 10-99 ≤ |x| ≤ 9.999999999 × 1099. The manual says,
A googol is defined as 1 × 10100 or 1 followed by 100 zeros. The range of the Tektronix calculator is limited to slightly less than one googol. Is this really a limitation? How big is a googol? [...]
A googol is large indeed. In our world there might not be a googol of anything.
Display
The 31's display makes use of seven-segment planar gas-discharge display modules. Each display module contains two or three digits and a triangular-shaped decimal point to the right of the digits. The mantissa display is made up of one three-digit SP-333 module and four two-digit SP-332 modules, the exponent display is one SP-333. Leading zeroes are not suppressed, i.e. entering the number "5" displays 0000000005.
The display segments glow a bright orange-red when energized. A red filter in situated in front of the display makes the digits appear a deep red to the user.
The calculator's status is indicated by a number of miniature incandescent lamps that are pre-heated to glow faintly when off, for quicker response and longer life.
Printer
The printer in the 21 and 31 is a thermal matrix printer with a fixed 80-element head. It can print 2.4 lines per second on standard 2¼ (57 mm) thermal paper (originally supplied on 250' rolls with a ⅞" core, although smaller rolls will also work). Each line can hold up to sixteen 5×7 matrix digits; on the 31, it has alphanumeric capability as well (uppercase only).
Memory
The base configuration of 1KByte RAM (eight Intel 2102 chips) stores 512 program steps and 64 numeric registers. On the base memory board, there are 24 more sockets for 2102s, allowing expansion in 1K steps up to 4K. An optional auxiliary memory board contains 64 more sockets for up to 8K additional memory. A strap on this board allows the user to change memory partitioning (program step memory vs storage register space). Later models of the Tek 31 had a three-position rocker switch under the tape cartridge slot for this purpose (restart required when changing).
There is also a slot for ROM cartridges on top of the machine, toward the right rear. Tek offered a service to create (P)ROM cartridges from user-supplied programs.
| Steps | Registers | |||||
|---|---|---|---|---|---|---|
| 128 | 192 | 256 | 448 | 640 | 1000 | |
| 1024 | Option 2 | |||||
| 1536 | Option 3 | - | ||||
| 2048 | Option 4 | Option 5 | Option 8 | |||
| 3584 | Option 6 | |||||
| 5120 | Option 7 | Option 9 | ||||
| 8192 | Option 10 | |||||
Storage
Model 31 cartridges (P/N 020-0082-00) contain 24 feet of 1/4" magnetic tape arranged in an endless loop. The tape stores 6 blocks (numbered 0 to 5) identified by groups of holes in the tape. Each block can hold 1000 program steps or 100 R registers. Density is ~285 bits/inch, tape speed ~7 inches per second.
Model 21 uses a special cartridge that allows a 7.5" magnetic card (P/N 119-0473-00) to be fed into the front of the cartridge, read or written by the transport, then returned to the user through another slot in the front of the cartridge. The card stores 2048 bits.
The 067-0792-00 cartridge is used for aligning the drive.
Interfaces
Several interfaces were available:
- Model 152 General Purpose BCD Interface
- Model 153 Tektronix Instrument Interface
- Model 154 RS-232C Interface
These connect to the calculator through a 50-pin card-edge bus interface with 48 mA open-collector TTL drivers (such as a 7438). The bus can be daisy-chained through multiple peripherals (which all have two matching bus connectors for this purpose), and must be terminated at the last unit. Transfer rates can reach up to 125 kbyte/s.
The bus uses two BCD digits for addressing.
There are separate digit-serial BCD and byte-serial External data buses. Via the BCD bus, the calculator can send numbers to or receive numbers from an external device, in either fixed or scientific notation, or an external device can write directly to the display. The External bus transfers alphanumeric characters (bit 7 = 1), program steps (bit 7 = 0), or binary register content. It supports DMA access to read or write program or register locations. It can also be used to attach an external printer, or execute a subroutine under external control.
System configurations
- The 31/10 Graphic Calculator System is a bundle of a model 31 with a 4010-1 graphics terminal, interface and software collection (see brochure A-2797-1).
- The 31/53 Instrumentation Package (1974) is a bundle of a model 31 with a 153 interface in a TM503 mainframe, and a choice of DM501 or DC503 plug-ins (see Tekscope V6N3).
- The DPO/31, later WP1100, is a bundle of a model 31 with a 7704A/P7001 digitizer and 021-0127-00 interface (see flyer and 1973 advertising).
Links
- Tektronix Calculators @ vintagetek.org
- August 31, 1973 TekWeek: Introduction of the Tek 21 and 31
- Tek 21 / Tek 31 @ oldcalculatormuseum.com / Spec sheet / blog post
- Tektronix 31 Desktop Calculator @ vintagecalc.com
- Tektronix 31 Programmable Desktop Calculator running a counter program from tape @ YouTube
- Tek 31 picture / back side
Documents referencing 31
| Document | Class | Title | Authors | Year | Links |
|---|---|---|---|---|---|
| Tekscope 1973 V5 N4 Jul 1973.pdf | Article | PROGRAMMING – As Easy As Writing A Formula | 1973 | 21 • 31 | |
| Tekscope 1973 V5 N4 Jul 1973.pdf | Article | Reliable By Design | 1973 | 21 • 31 | |
| Tekscope 1973 V5 N4 Jul 1973.pdf | Article | The Tektronix Calculator - It All Adds Up | Larry Mayhew | 1973 | 21 • 31 |
| Tekscope 1973 V5 N4 Jul 1973.pdf | Article | The Calculator and You | 1973 | 21 • 31 | |
| Tekscope 1973 V5 N4 Jul 1973.pdf | Article | A Close-Up Look | 1973 | 21 • 31 | |
| A-2797-1.pdf | Brochure | Tektronix 31/10, the first interactive graphic calculator system | 1974 | 31 • 4010 | |
| A-2968.pdf | Brochure | TEKTRONIX 154 Calculator RS-232-C Interface brochure | 1974 | 31 • 154 | |
| Tekscope 1974 V6 N3.pdf | Article | The 31/53 Calculator-based Measurement Instrumentation Package | John Mulvey | 1974 | 31 • 153 • DM501 • DC503 |
Patents that may apply to 31
| Page | Title | Inventors | Filing date | Grant date | Links |
|---|---|---|---|---|---|
| Patent US 3760370A | Calculator with a number processing system | Michael J. Cochran | 1971-04-26 | 1973-09-18 | 909 • 911 • 31 • 21 (calculator) |
| Patent US 3831199A | Common transport for magnetic tape cartridge or magnetic card cartridge | Howard Vollum • Sid Broughton | 1973-03-19 | 1974-08-20 | 21 (calculator) • 31 • 020-0082-00 |
Prices
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The 31 is last listed in the 1978 catalog as an "OEM Scientific Calculator", and mentioned indirectly in 1979 where the WP1100 last appeared (without a price).
Internals
The 21 and 31 are built around a custom LSI chip set, the 156-0243-00 Adder-Subtracter, 156-0238-00 Working Register Storage (2×), 156-0239-00/156-0240-00/156-0242-00 Function Decoding, 156-0235-00 Microcode Routine Address ROM, 156-0231-00 through 156-0234-00 Microcode ROM/Next Address Latch, 156-0237-00 Timing and Constant Storage ROM, 156-0236-00 K Register Storage (2×) and 156-0241-00 A & C Operation Control ROM. All of these are packaged in 40-pin DIP. These chips were manufactured by American Microsystems, Inc. as full custom parts.
Pictures
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Tektronix 21 and 31 calculators
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Tektronix 31 with custom function key overlay
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Tektronix 31 with magnetic card reader plug-in cartridge
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Tektronix 31 originally used in a medical lab application, with operator "cheat sheet" number markings for loading tape contents
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Display and printout
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"Just married" advertising from 1973
Internal
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Top cover removed – printer, power supply, tape drive; ROM cartridge slot on top right
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Top of calculator with printer, power supply, tape drive and keyboard/display removed
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Tek 31 rear of case with fan, backplane board, and ROM module slot
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Tek 31 calculator board, component side
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Tek 31 calculator board, solder side
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Tek 31 programmer board with main memory board mounted on top
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Tek 31 programmer board, component side
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Tek 31 programmer board, solder side
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Tek 31 memory board, component side − 1KB soldered, 2KB populated in sockets
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Tek 31 memory board, solder side
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Tek 31 display board
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Tek 31 display board, incandescent lamp detail (note faint glow on inactive lamp)
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3-digit display module, Beckman SP-333
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2-digit display module, Beckman SP-332
Replacement display board
When the 31's gas-discharge display modules are failing, they are not only difficult to obtain, but also very hard to replace due to the conformal coating of the board. Volunteers at the VintageTek museum have therefore developed a drop-in replacement display board with a microcontroller driving modern LEDs. This board can take the place of the original without requiring further modifications of the calculator.
The Museum sells the board through their eBay store to support the Museum's operations.
Components
Some Parts Used in the 31
| Part | Part Number(s) | Class | Description | Used in |
|---|---|---|---|---|
| 150-0048-01 | 150-0048-01 | Discrete component | 5 V, 60 mA, T-1 miniature incandescent lamp | 21 (calculator) • 31 • 576 • SW503 |
| DM8880 | 156-0284-00 | Monolithic integrated circuit | display driver | 21 (calculator) • 31 |
| Intel 2102 | 156-0291-00 • 156-0291-01 • 156-0893-00 • 156-0893-01 | Monolithic integrated circuit | 1K×1 static RAM | 21 (calculator) • 31 • 833 • DF1 • DF2 • P7001 • Nicolet 526 |
| SP-332 | 150-1015-00 | Gas Discharge Tube (display tube) | seven-segment planar gas-discharge display | 21 (calculator) • 31 |
| SP-333 | 150-1016-00 | Gas Discharge Tube (display tube) | seven-segment planar gas-discharge display | 21 (calculator) • 31 |