The Tektronix 21 and Tektronix 31 are programmable desktop calculators. They feature a complete set of transcendental functions. The 21 supports simple programs (sequences of keystrokes) whereas the 31 supports conditional branching and subroutines.
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 supporting DMA access at up to 125 kbyte/s. The bus uses open-collector TTL drivers and can be daisy-chained through multiple peripherals (which all have two matching bus connectors for this purpose). It must be terminated at the last unit.
The 31/10 Graphic Calculator System was a bundle of a model 31 with a 4010-1 graphics terminal, interface and software collection.
- Tek 21 / Tek 31 @ oldcalculatormuseum.com / Spec sheet / blog post
- Tektronix 31 Programmable Desktop Calculator running a counter program from tape @ YouTube
- Tek 31 picture / back side
The 21 and 31 are built around a custom LSI chip set, the 156-0243-00 Adder-Subtracter, 156-0238-00 Working Register Storage, 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 and 156-0241-00 A & C Operation Control ROM. These chips were manufactured by American Microsystems, Inc. as full custom parts.