Jump to content

PDP-8

From RetroTechCollection
PDP-8
Table-top PDP-8 at the Science Museum, London (Daderot, CC0)
Specifications
ManufacturerDigital Equipment Corporation
Type12-bit minicomputer
Released22 March 1965[1][2]
Discontinued1968[2]
Intro priceUS$18,000 (FAQ) or US$18,500 (Wikipedia)[2][1]
Units sold1,450[2][3]
CPUDiscrete-transistor processor on R- and S-series Flip Chip modules; 1.5 µs memory cycle[4][3]
Memory4,096 12-bit words of core, expandable to 32,768 in 4,096-word fields[4]
Storage—
Display—
Sound—
Power115 V 60 Hz single-phase, 7.5 A; 780 W dissipation[4]
DimensionsTable-top model 34-1/16 × 21-1/2 × 21-3/4 in[4]
Weight225 lb (DEC specification)[4]
OS / Firmware—
PredecessorPDP-5[2]
SuccessorPDP-8/I[2]

The PDP-8 is the first model of Digital Equipment Corporation's 12-bit PDP-8 family, unveiled in New York on 22 March 1965.[2][1] Collectors call it the "Straight-8" because its name has no model suffix.[2] It is built from discrete-transistor diode-transistor logic on DEC's R-series Flip Chip modules, wire-wrapped into two hinged backplanes.[3][2] DEC made 1,450 of them before the cheaper PDP-8/I replaced it in 1968.[2][3]

A surviving Straight-8 has around 230 logic cards with roughly 10,000 diodes and 1,400 transistors, a Type 708 power supply full of large electrolytic capacitors, silver-contact margin switches, three fans under each backplane and dozens of incandescent console lamps.[5][6] The work is covered in PDP-8 Power Supply Restoration, PDP-8 General Maintenance and PDP-8 Troubleshooting.

Specifications

[edit | edit source]
PDP-8 specifications (DEC F-87, pp. 1-13 to 1-14)
Item Specification
Word length 12 bits
Memory cycle time 1.5 µs
Core memory 4,096 words, expandable to 32,768 in fields of 4,096
Logic levels Ground and −3 V
Internal supplies +10 V and −15 V (logic); 30 V floating (memory)
Mains 115 V, 60 Hz, single phase, 7.5 A (220 V or 50 Hz to special order)
Power dissipation 780 W (2,370 BTU/h)
Table-top model 34-1/16 in high, 21-1/2 in wide, 21-3/4 in deep, 225 lb
Operating temperature 0 to 55 °C

[4] David Gesswein gives his Straight-8 as about 180 lb, about 60 lb of it the power supply in the base, drawing about 400 W while it runs DEC's Disk Monitor System.[3]

Hardware

[edit | edit source]

Construction

[edit | edit source]

The table-top PDP-8 has a base holding the operator console and the power supply, with two hinged module mounting panels above it. The panels hinge at the rear and lock to the central strut with the same key as the console's lock switches; removing the plastic covers gives access to the modules, and swinging the panels outward exposes the wire-wrap and the marginal-check panels.[6] Three fans of 105 cubic feet per minute sit in the base of each module assembly and draw air through a dust filter under the power supply.[6] In the cabinet version the logic and console slide forward on rails while the power supply stays in the cabinet, and DEC warns that a PDP-8 in an ordinary radio rack can topple when drawn out on its slides.[6]

The logic is negative: a transistor pulls a signal to ground and a resistor pulls it to about −3 V, so several outputs can be wired together as a logical OR.[5] Logic modules are 2.5 by 5 inches with 18-contact edge connectors, and the backplanes are machine wire-wrapped with 24-gauge wire.[2]

Memory

[edit | edit source]

The basic 4K core memory is driven from the Type 708 supply's regulated read/write and inhibit outputs, which track array temperature through a thermistor in the stack. The optimum drive currents for each array were measured at the factory and printed on a label on the memory array.[7][8] Memory beyond 4K needs the Type 183 memory extension control and Type 184 4K modules, each in rack space of its own.[2]

Options

[edit | edit source]

DEC's maintenance manual covers the Type 182 extended arithmetic element, Type 183 memory extension control, Type 184 memory module, Type 188 memory parity, KR01 power-fail automatic restart, Type 189 analogue-to-digital converter and Type 681 data line interface.[9] The standard system came with a 110-baud current-loop Teletype interface and an ASR-33 Teletype.[2]

Power supply

[edit | edit source]

The Type 708 supply sits in the base and provides unregulated +10 V and −15 V for the logic from a ferroresonant transformer, a 40 V floating supply feeding the regulated inhibit and read/write supplies for the core, and a 0 to 20 V marginal-check supply with its own meter and polarity switch.[7] See PDP-8 Power Supply Restoration.

Known problems

[edit | edit source]

Restorers of surviving Straight-8s report the same short list. Electrolytic capacitors in the 708 need reforming, and some do not recover.[10][11] The silver-plated marginal-check switches corrode until rows of modules lose power.[12][11] Console lamps fail from corroded leads.[13] On the logic cards the failures are mostly diodes, with some transistors and solder joints: Gesswein repaired 15 cards, replacing 26 diodes and 3 transistors, and the University of Iowa restoration later began scanning every diode on the machine after finding many bad ones.[5][11] See PDP-8 Troubleshooting.

Documentation

[edit | edit source]
[edit | edit source]
  • Rack-mounted PDP-8 serial 85, University of Iowa (Douglas W. Jones, CC0)
    Rack-mounted PDP-8 serial 85, University of Iowa (Douglas W. Jones, CC0)
  • Serial 85 drawn out on its slides with the rack side removed (Douglas W. Jones, CC0)
    Serial 85 drawn out on its slides with the rack side removed (Douglas W. Jones, CC0)
  • DEC drawing RS-C-708, the Type 708 power supply
    DEC drawing RS-C-708, the Type 708 power supply
[edit | edit source]

References

[edit | edit source]
  1. ↑ 1.0 1.1 1.2 "PDP-8", Wikipedia, en.wikipedia.org/wiki/PDP-8. Introduction on 22 March 1965 at US$18,500; Straight-8 name.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 Douglas W. Jones, "What is a PDP-8?", PDP-8 Summary of Models and Options (comp.sys.dec FAQ), faqs.org. Introduction 1965 (unveiled 22 March in New York), withdrawal 1968, production run 1,450, price US$18,000, technology, standard configuration, options and the PDP-8/I as its replacement.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 David Gesswein, "PDP-8 (Straight 8) Computer", pdp8online.com. 1,450 made between 1965 and 1968; R, S and G series boards; weight about 180 lb with about 60 lb of power supply; about 400 W running the Disk Monitor System.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 DEC, F-87, pp. 1-13 to 1-14, "Specifications".
  5. ↑ 5.0 5.1 5.2 David Gesswein, "PDP-8 (Straight 8) Computer Functional Restoration", pdp8online.com. Card and component counts, repairs needed, R-series logic levels.
  6. ↑ 6.0 6.1 6.2 6.3 DEC, F-87, p. 1-11, "Physical Description".
  7. ↑ 7.0 7.1 DEC, F-87, pp. 4-12 to 4-13, "Current Source": Type 708 power supply, G808 and G809.
  8. ↑ DEC, F-87, p. 9-12, "Memory Current Check".
  9. ↑ Digital Equipment Corporation, PDP-8 Maintenance Manual, F-87, February 1966, contents (File:PDP-8 Maintenance Manual (F-87, 1966).pdf).
  10. ↑ David Gesswein, "PDP-8 (Straight 8) Computer Power Supply Restoration", pdp8online.com.
  11. ↑ 11.0 11.1 11.2 Douglas W. Jones, "The U of Iowa's DEC PDP-8 Problem List", homepage.cs.uiowa.edu. Bugs 10, 44 and 74: capacitor reforming, marginal-check switches, diode scan.
  12. ↑ David Gesswein, "PDP-8 (Straight 8) Computer Backplane Restoration", pdp8online.com.
  13. ↑ David Gesswein, "PDP-8 (Straight 8) Computer Front Panel Restoration", pdp8online.com.
  14. ↑ Directory listing, bitsavers.org/pdf/dec/pdp8/pdp8/, retrieved 2 October 2026.
Retrieved from "https://wiki.retrotechcollection.com/index.php?title=PDP-8&oldid=21489"