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PDP-8/m

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PDP-8/M
DEC PDP-8M (Dave Ruske)
PDP-8/M with the KC8-ML programmer's console (Dave Ruske, CC BY 2.0)
Specifications
ManufacturerDigital Equipment Corporation
Type12-bit minicomputer (OEM version of the PDP-8/F)
Released1972[1]
Discontinued1978[1]
CPUPDP-8/E processor (M8300, M8310, M8330)[1][2]
MemoryMM8-E core, 4K per G104/H220/G227 set[2]
Storage—
Display—
Sound—
ExpansionOMNIBUS; expandable by 40 slots with an expander box[3]
PowerH740 switching supply: 90–135 or 180–270 V AC, 47–63 Hz, 325 W at full load[4]
OS / Firmware—
PredecessorPDP-8/E[5]
SuccessorPDP-8/A[5]

The PDP-8/M is the OEM version of Digital Equipment Corporation's PDP-8/F, introduced in 1972 for companies that built the computer into their own systems.[1] Behind the front panel it is a PDP-8/E: the same OMNIBUS modules, CPU and core memory, in a shorter box with a smaller H740 switching power supply in place of the 8/E's H724 linear supply.[5][6] The H740 was the first switching supply in the PDP-8 family.[5] DEC withdrew the 8/F and 8/M in 1978, when the PDP-8/A 800 offered similar expansion for less.[5]

The 8/M's standard front panel is a minimal operator's panel, so a machine needs either the KP8-E power-fail and auto-restart option or the MI8-E hardware bootstrap loader to start a program. The KC8-ML programmer's console with lamps and switches was an option; with it fitted, an 8/M differs from an 8/F only in the front panel artwork, colour scheme and labels.[1][3] David Gesswein notes that 8/Ms often lacked the full programmer's console; his has one, as does the machine pictured here.[7]

Specifications

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H740 power supply ratings (DEC specification 5409728-0-8)
Output Load range Notes
+5 V 0–20 A static at 50 °C (0–17 A at 60 °C) Switching regulator; crowbar at 5.7–6.8 V blows the fuse
−15 V 0–5, 0–7 or 0–8 A by regulator board revision Switching regulator; crowbar at 17.5–20.5 V
+15 V 0–1 A Linear; current limit 1.3–1.7 A
Mains input 90–135 V or 180–270 V, 47–63 Hz, 5 or 2.5 A 325 W at full load; 25 ms hold-up at low line and full load

[4] The processor, memory, cycle times and instruction set are those of the PDP-8/e.[5][2]

Hardware

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Front panels

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The KC8-ML has light-emitting diodes in place of the 8/E's incandescent lamps. The H740 has no +8 V lamp supply, so an 8/E KC8-EA console cannot be used on an 8/F or 8/M, although the LED console fits an 8/E.[5][2] With the introduction of the M8330 timing board, DEC required it to sit next to the console.[5]

Power supply

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The H740 transforms the mains to 28 V AC, centre-tapped, and derives every DC output from that. The +5 V and −15 V regulators are switching circuits running at about 10 kHz; the +15 V regulator is linear.[8][9] A thermostat on the regulator board's heat sink cuts the transformer and fan power when it overheats.[8][4] The original 8/F box had a defective power supply, corrected in a revised, slightly larger box, and the KP8-E power-fail option as first sold did not work with the 8/F supply.[5] David Gesswein also notes that the first H740s had problems and later ones were better.[9] See PDP-8/m Capacitor Replacement Guide and PDP-8/m Troubleshooting.

Documentation

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See PDP-8/e Documentation.

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  • PDP-8/M with a paper tape reader at VCF Berlin 2019 (stiefkind, CC0)
    PDP-8/M with a paper tape reader at VCF Berlin 2019 (stiefkind, CC0)
  • DEC assembly drawing of the H740 regulator board
    DEC assembly drawing of the H740 regulator board
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 Douglas W. Jones, "What is a PDP-8/M?", PDP-8 Summary of Models and Options (comp.sys.dec FAQ), faqs.org. Introduced 1972, withdrawn 1978, OEM packaging of the 8/F, minimal front panel requiring KP8-E or MI8-E, KC8-M option.
  2. ↑ 2.0 2.1 2.2 2.3 DEC, DEC-8E-HMM1A-D-D, pp. 5-5 to 5-6, Tables 5-5 and 5-6, PDP-8/F and PDP-8/M first-level spares (M8300, M8310, M8330, G104, G227, LEDs, regulator board 54-09728).
  3. ↑ 3.0 3.1 Digital Equipment Corporation, PDP-8/E, PDP-8/F, PDP-8/M Maintenance Manual, Volume 1: Processor, DEC-8E-HMM1A-D-D, September 1973, p. 1-2 (File:PDP-8E Maintenance Manual Volume 1 (DEC-8E-HMM1A-D-D).pdf). KC8-M operator's panel; expansion by 40 slots.
  4. ↑ 4.0 4.1 4.2 Digital Equipment Corporation, engineering specification SP-5409728-0-8, "Power Supply Specification – H740", sheets 8 to 13, in H740 Power Supply Engineering Drawings, revision H (File:H740 Power Supply Engineering Drawings (Rev H, 1974).pdf), PDF pp. 16 to 19.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 Douglas W. Jones, "What is a PDP-8/F?", faqs.org. First switching supply in the family, LED front panel and the +8 V restriction, M8330 placement, defective original supply and revised box, KP8-E incompatibility, replacement by the PDP-8/A 800.
  6. ↑ DEC, DEC-8E-HMM1A-D-D, p. 3-172, section 8: the 8/F and 8/M use the H740.
  7. ↑ David Gesswein, "PDP-8/E and PDP-8/M Computer Information", pdp8online.com. The 8/M was an OEM version of the 8/F and frequently lacked the full front panel.
  8. ↑ 8.0 8.1 DEC, DEC-8E-HMM1A-D-D, pp. 3-177 to 3-181, section 3.48, PDP-8/F and PDP-8/M power supply.
  9. ↑ 9.0 9.1 David Gesswein, "PDP-8 h724 Power Supply Information", pdp8online.com. H740 switching at about 10 kHz; early H740s had problems.
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