Editing PDP-11/20

{{Infobox computer
| name         = PDP-11/20
| image        = [[File:DEC PDP-11-20 at the Computer History Museum.jpg|260px|frameless]]
| caption      = PDP-11/20 table-top model at the Computer History Museum (Don DeBold, CC BY 2.0)
| manufacturer = Digital Equipment Corporation
| type         = 16-bit minicomputer
| release date = 1970<ref name="wp" />
| price        = US$10,800 with 4K words of core and an ASR-33 (November 1970)<ref name="pl" />
| cpu          = KA11, TTL logic with hard-wired control<ref name="sm2" /><ref name="chw" />
| memory       = MM11-E core, 4K 16-bit words per unit, expandable to 28K words<ref name="sm2" />
| storage      = Paper tape on the basic system; DECtape, disk and magnetic tape as options<ref name="pl" />
| power        = H720 power supply; 120 V 6 A or 230 V 3 A single phase<ref name="h720ds" />
| dimensions   = 10½ × 19 × 23 in (rack-mounting BA11-CS box)<ref name="h720ds" />
| weight       = About 90 lb with processor, console and 4K of memory<ref name="h720ds" />
| successor    = PDP-11/40 and PDP-11/05<ref name="wp" />
}}
The '''PDP-11/20''' is the first model of Digital Equipment Corporation's 16-bit PDP-11 family. DEC announced the family in January 1970 and shipped the 11/20 that year.<ref name="wp" /> Its KA11 processor is built from small-scale TTL on quad-height modules and has hard-wired control; every later PDP-11 is microprogrammed.<ref name="chw" /> The 11/20 introduced the Unibus, the single bus that connects processor, memory and peripherals, and the first officially named version of Unix ran on an 11/20 in 1970.<ref name="sm2" /><ref name="wp" />

A basic system fills one 10½-inch BA11-C box: the KA11 processor, the KY11-A programmer's console on the front, 4K words of MM11-E core memory, a KL11 Teletype control and the H720 switching power supply at the rear.<ref name="sm2" /> These machines are now more than fifty years old. The power supply, the core memory and the console lamps are the parts that need most attention; see [[PDP-11/20 General Maintenance]], [[PDP-11/20 Power Supply Service]] and [[PDP-11/20 Troubleshooting Guide]].

== Models and prices ==
<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:70%; text-align:center;"
|+ '''PDP-11/20 models, DEC price list of 1 November 1970'''
! Model !! Mounting !! Mains !! Price (US$)
|-
| PDP-11/20-AA || Rack-mountable, slides included || 115 V 60 Hz || 10,800
|-
| PDP-11/20-AB || Rack-mountable, slides included || 230 V 50 Hz || 10,800
|-
| PDP-11/20-BA || Table-top, cover included || 115 V 60 Hz || 10,800
|-
| PDP-11/20-BB || Table-top, cover included || 230 V 50 Hz || 10,800
|-
| PDP-11/20-CA || Rack-mounted in a cabinet || 115 V 60 Hz || 11,450
|-
| PDP-11/20-CB || Rack-mounted in a cabinet || 230 V 50 Hz || 11,450
|}
<ref name="pl" />
Each price covers a KA11 processor, 4K words of core memory, the programmer's console, the basic mounting box and power supply, and an ASR-33 Teletype with its control.<ref name="pl" /> Wikipedia gives the 11/20's price as $11,800; DEC's own list gives the figures above.<ref name="wp" /><ref name="pl" /> DEC's H720 manual also lists twelve OEM configurations (codes D to K), physically the same as the standard systems but with the Teletype and front-panel logo optional and no documentation, training or maintenance included.<ref name="h720i" />

== Hardware ==
=== Processor and console ===
The KA11 has eight 16-bit general registers, R6 being the stack pointer and R7 the program counter. It addresses 32K words directly: 28,672 words of memory and 4,096 words of device registers at the top of the address space. It has four hardware bus-request priority levels with sub-levels and vectored interrupts; interrupt response takes 7.2 µs. The processor itself draws 8 A from +5 V.<ref name="sm2" /> Power-fail and automatic restart are standard: the processor traps when the mains falls below 105 V and needs 2 ms to save its state, using the AC LO and DC LO signals from the power supply.<ref name="sm2" />

The KY11-A programmer's console has an 18-bit ADDRESS REGISTER display, a 16-bit DATA display, eight state lamps (FETCH, SOURCE, DESTINATION, two ADDRESS, EXEC, BUS, RUN), an 18-bit switch register, the key-operated OFF/POWER/PANEL LOCK switch and the LOAD ADDR, EXAM, CONT, ENABLE/HALT, S-INST/S-CYCLE, START and DEP keys.<ref name="sm2" /> Its lamps run from an unregulated, unfiltered +8 V supply.<ref name="sm2" /><ref name="h720" />

Options for the 11/20 included the KE11-A extended arithmetic element, which does multiply and divide as a separate device on the Unibus, programmed through its registers, and the rare KT11-B paging option.<ref name="chw" /><ref name="bits" />

=== Memory ===
The MM11-E is a coincident-current, 3-wire planar core memory of 4,096 16-bit words with a 1.2 µs cycle and 500 ns access time. The basic box holds the first unit and has room for two more; more than that needs a BA11-E extension box. Memory expands to 28K words in 4K steps, and each 8K segment can be interleaved. The MM11-F (950 ns), MM11-FP with parity, MM11-H (1K) and MM11-J (2K) could be fitted instead.<ref name="sm2" /> Core memory keeps its contents with the power off. A DC OK signal inhibits the memory when the supply voltages are low, so that it does not write spuriously during power changes.<ref name="sm2" />

=== Unibus ===
The Unibus carries 16 data lines, 18 address lines and control lines between all devices, with transfers of up to 2,500,000 words per second. Inside the box it runs from system unit to system unit on M920 jumper modules; a 120-conductor BC11 cable joins boxes, with a total Unibus length of no more than 50 ft.<ref name="sm2" /><ref name="h720i" />

=== Power supply ===
The H720 gives +5 V at 16 A (models A and B) or 22 A (E and F) and −15 V at 10 A from switching regulators, plus unregulated supplies for the lamps and Teletype interface, a line-frequency clock signal and the AC LO and DC LO signals. Models A and C are 120 V, B and D 240 V; C and D are door-mounted versions, and E and F add a circuit breaker, remote power control and a thermal cut-out.<ref name="h720" /> After the basic processor, console, 4K memory and Teletype control, an 11/20 box has 3.4 A at +5 V and 1.7 A at −15 V left for options.<ref name="sm2" /> See [[PDP-11/20 Power Supply Service]].

== Specifications ==
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+ '''Basic PDP-11/20 system'''
! Item !! Specification
|-
| Word length || 16 bits, byte addressable
|-
| Core memory || 4K words basic, 28K words maximum; 1.2 µs cycle (MM11-E)
|-
| Unibus transfer rate || 2,500,000 words per second maximum
|-
| Mains || 120 V ±10%, 47–63 Hz, 6 A (H720-A); 218–240 V ±10%, 3 A (H720-B)
|-
| Heat || 400 W for the computer, 150 W for an ASR-33
|-
| Operating environment || 10–50 °C, 20–95% relative humidity, non-condensing; DEC recommends 20–30 °C
|-
| Size and weight || BA11-CS rack box 10½ × 19 × 23 in, about 90 lb; BA11-CC table-top 11 × 20 × 23⅝ in, about 100 lb
|}
<ref name="sm2" /><ref name="h720i" /><ref name="h720ds" />

== Known problems ==
* The H720's unregulated supplies rest on two 22,000 µF 50 V reservoir capacitors, and its regulator board carries a further seven electrolytics on the +5 V and −15 V outputs, all now about fifty years old.<ref name="drw" /> The +5 V crowbar fires above about 6 V and holds the output down, which looks like a dead supply.<ref name="h720" /> See [[PDP-11/20 Power Supply Service]].
* The −15 V regulator is switched off whenever +5 V is below about 4.5 V, to protect the core memory, so a fault on +5 V also removes −15 V.<ref name="h720" />
* AC LO and DC LO from every H720 in a system are wired together on the Unibus. A fault in an expansion box's supply halts or hangs the whole machine.<ref name="h720" /><ref name="ka5" />
See [[PDP-11/20 Troubleshooting Guide]].

== Documentation ==
* ''PDP-11/20 System Manual'', DEC-11-HR1B-D: [[:File:PDP-11-20 System Manual (DEC-11-HR1B-D).pdf]]
* ''KA11 Processor Manual'', DEC-11-HR2B-D: [[:File:KA11 Processor Manual (DEC-11-HR2B-D).pdf]]
* ''MM11-E Core Memory Manual'', DEC-11-HR3A-D: [[:File:MM11-E Core Memory Manual (DEC-11-HR3A-D).pdf]]
* ''H720 Power Supply and Mounting Box Manual'', DEC-11-HR5B-D: [[:File:H720 Power Supply and Mounting Box Manual (DEC-11-HR5B-D).pdf]]
* ''H720 Power Supply and Mounting Box Engineering Drawings'': [[:File:H720 Power Supply and Mounting Box Engineering Drawings (1971).pdf]]
* PDP-11/20 price list, 1 November 1970: [[:File:PDP-11-20 Price List (1 November 1970).pdf]]
See [[PDP-11/20 Documentation]] for what each covers.

== Gallery ==
<gallery mode="packed" heights="200">
File:DEC PDP-11-20 console switches.jpg|KY11-A console keys and state lamps (Don DeBold, CC BY 2.0)
File:DEC H720 power supply schematic (H720-0-1).png|DEC drawing D-CS-H720-0-1, the H720 power supply
</gallery>

== Related pages ==
* [[PDP-11/20 General Maintenance]]
* [[PDP-11/20 Troubleshooting Guide]]
* [[PDP-11/20 Power Supply Service]]
* [[PDP-11/20 Documentation]]
* [[PDP-11/34]]
* [[PDP-10]]

== References ==
<references>
<ref name="wp">"PDP-11", ''Wikipedia'', [https://en.wikipedia.org/wiki/PDP-11 en.wikipedia.org/wiki/PDP-11]. Announcement in January 1970, the 11/20 as the first model, its $11,800 price, Unix on the 11/20, successors.</ref>
<ref name="pl">Digital Equipment Corporation, PDP-11/20 price list, 1 November 1970, p. 2 ([[:File:PDP-11-20 Price List (1 November 1970).pdf]]).</ref>
<ref name="sm2">Digital Equipment Corporation, ''PDP-11/20 System Manual'', DEC-11-HR1B-D, sixth printing May 1973, chapter 2, pp. 2-1 to 2-6, Tables 2-2 to 2-8 ([[:File:PDP-11-20 System Manual (DEC-11-HR1B-D).pdf]]).</ref>
<ref name="h720">Digital Equipment Corporation, ''H720 Power Supply and Mounting Box Manual'', DEC-11-HR5B-D, fifth printing November 1972, chapter 2, pp. 2-1 to 2-8 ([[:File:H720 Power Supply and Mounting Box Manual (DEC-11-HR5B-D).pdf]]).</ref>
<ref name="h720i">DEC, DEC-11-HR5B-D, chapter 4, pp. 4-1 to 4-3. Configurations, environment, power requirements and cabling.</ref>
<ref name="h720ds">DEC, DEC-11-HR5B-D, data sheets for models AA/AB and BA/BB (pp. 4-4 to 4-5).</ref>
<ref name="ka5">Digital Equipment Corporation, ''KA11 Processor Manual'', DEC-11-HR2B-D, chapter 5, section 5.7, "Multiple Box Systems" ([[:File:KA11 Processor Manual (DEC-11-HR2B-D).pdf]]).</ref>
<ref name="drw">Digital Equipment Corporation, ''H720 Power Supply and Mounting Box Engineering Drawings'', 1971, D-CS-H720-0-1 parts list and regulator board schematic D-CS-5408475-0-1 (PDF pp. 10–11) ([[:File:H720 Power Supply and Mounting Box Engineering Drawings (1971).pdf]]).</ref>
<ref name="chw">"PDP-11/20", ''Computer History Wiki'', [https://gunkies.org/wiki/PDP-11/20 gunkies.org/wiki/PDP-11/20]. KA11 built without microcode, KE11-A and KT11-B options, RSTS-11.</ref>
<ref name="bits">Bitsavers directory [https://bitsavers.org/pdf/dec/pdp11/1120/ bitsavers.org/pdf/dec/pdp11/1120/] (KE11-A extended arithmetic element and KT11-B paging option manuals), retrieved 2 October 2026.</ref>
</references>

[[Category:Digital Equipment Corporation]]