DEC PDP-11/20

S/N S-1294

This PDP-11/20 was donated by Thomas Holmes on 12/19/23.

The PDP-11/20 was the first 16-bit system built by DEC and was introduced in January 1970. It was the first of the PDP-11 family, and the first to use the Unibus I/O bus. There are eight general purpose registers in the processor that can be used as accumulators, pointers, or index registers. Register R6 is the Stack Pointer, and R7 is the Program Counter. Because it has a Stack Pointer it supports reentrant code. The instruction set and addressing modes are much more complex than the earlier PDP-8 and PDP-9 minicomputers. The base system had no memory management, so the 16-bit address limited the memory to 32k words (64k bytes). The chassis could be a BA11-CC Table Top, or a BA11-CS Rack Mount , both with an H720 Power Supply. A BA11-EC or BA11-ES Table Top of Rack Mounted Extension Mounting Box could be connected to the processor chassis to provide room for more Unibus devices. A year after the introduction the KT11-B Paging option was introduced that increased the address size to 18-bits and the maximum memory to 256k bytes.

The system originally shipped with just paper tape based software development tools. Later a disk based DOS operating system and tools were available. A PDP-11/20 equipped with the KE11-A Extended Arithmetic Element and an RF11/RS11 256K-word fixed head disk could run UNIX V1.

The PDP-11/20 is built upside-down like the PDP-8/L system. Later PDP-11 systems had the backplane at the bottom of the chassis. At the far right are three 4-slot backplanes wired for the KA11 processor. The next 4-slot DD11A backplane is empty and can be used for small peripheral controller boards. The two 4-slot MM11 backplanes at the left each hold 4k of MM11-F core memory.

The following boards are installed in the chassis.

Slot P/N Description

ABCDEF xxxx KY11-A Front Panel
- A xxxx Unused
- B xxxx KM11A Maintenance Panel
- CDEF M727 State Control
- A G772 Power Connector
- B M823 Codes, Data
- CDEF M728 Timing & States
- A Reserved
- B M825 Power Fail & Control
- CDEF M726 Instruction Register Decode
Casting Space for Backplanes
- A Reserved
- B Reserved
- CDEF M724 Bus & Console Control
- A Unused
- B Reserved
- CDEF M725 Bus Interface & Instruction Register
- A G772 Power Connector
- B Reserved
- CDEF M820 Data Path Control
- A Reserved
- B Reserved
- CD M224 Data Paths #1
- EF M224 Data Paths #2
Casting Space for Backplanes
- AB M930 Unibus Terminator
- CD M822 Flag Control
- EF M821 Register Control
- A Unused
- B M787 KW11A Option
- CD M824 Priority
- EF M225 Register
- A G772 Power Connector
- B Reserved
- CDEF M7800-YA Teletype Transmitter & Receiver
- AB M920 Unibus Out Connector
- C Optional Device Control
- D G727 Grant Continuity if no Peripheral Installed
- E M105 Optional Device Selector
- F M782 Optional Interrupt Control
Casting Space for Backplanes
- AB M920 Unibus In Connector
- C Optional Device Control
- D G727 Grant Continuity if no Peripheral Installed
- E M105 Optional Device Selector
- F M782 Optional Interrupt Control
- A Unused
- B Reserved
- C Optional Device Control
- D G727 Grant Continuity if no Peripheral Installed
- E M105 Optional Device Selector
- F M782 Optional Interrupt Control
- A G772 Power Connector
- B Reserved
- C Optional Device Control
- D G727 Grant Continuity if no Peripheral Installed
- E M105 Optional Device Selector
- F M782 Optional Interrupt Control
- AB M920 Unibus Out Connector
- C Optional Device Control
- D G727 Grant Continuity if no Peripheral Installed
- E M105 Optional Device Selector
- F M782 Optional Interrupt Control
Casting Space for Backplanes
- AB M920 Unibus In Connector
- CD M7290 Control Logic
- E Used with optional parity memory
- F G103 Levels & Gates
- A Unused
- B M1091 Device Select
- C G226 Decoder & Switch
- D G102 Sense inhibit
- E G102 Sense inhibit
- F G226 Decoder & Switch
- A G772 Power Connector
- B G225 Current Generator
- C G226 Decoder & Switch
- D G102 Sense inhibit
- E G102 Sense inhibit
- F G226 Decoder & Switch
- AB M920 Unibus Out Connector
- CDEF H207 4k x 16-bit Core Memory
Casting Space for Backplanes
- AB M920 Unibus In Connector
- CD M7290 Control Logic
- E Used with optional parity memory
- F G103 Levels & Gates
- A Unused
- B M1091 Device Select
- C G226 Decoder & Switch
- D G102 Sense inhibit
- E G102 Sense inhibit
- F G226 Decoder & Switch
- A G772 Power Connector
- B G225 Current Generator
- C G226 Decoder & Switch
- D G102 Sense inhibit
- E G102 Sense inhibit
- F G226 Decoder & Switch
- AB M930 Unibus Terminator
- CDEF H207 4k x 16-bit Core Memory

This is the KA11, PDP-11/20 Processor. The M821 was in the wrong slot and was moved to slot 11.

These are the two MM11-F 4k x 16-bit core memories.

Page updated

Report abuse