Unibus

Last updated
Unibus
Unibus
Unibus.jpg
Unibus backplane (left) and two expansion cards
Year created1969;54 years ago (1969)
Created by Digital Equipment Corporation
Width in bits18 address, 16 data
Style Parallel
A DEC Core Memory Unibus card (16K x 18) DEC Core Memory 16K x 18 H217C.png
A DEC Core Memory Unibus card (16K × 18)

The Unibus was the earliest of several computer bus and backplane designs used with PDP-11 and early VAX systems manufactured by the Digital Equipment Corporation (DEC) of Maynard, Massachusetts. The Unibus was developed around 1969 by Gordon Bell and student Harold McFarland while at Carnegie Mellon University. [1]

Contents

The name refers to the unified nature of the bus; Unibus was used both as a system bus allowing the central processing unit to communicate with main memory, as well as a peripheral bus, allowing peripherals to send and receive data. Unifying these formerly separate busses allowed external devices to easily perform direct memory access (DMA) and made the construction of device drivers easier as control and data exchange was all handled through memory-mapped I/O. [2]

Unibus was physically large, which led to the introduction of Q-bus, which multiplexed some signals to reduce pin count. Higher performance PDP systems used Fastbus, essentially two Unibusses in one. The system was later supplanted by Massbus, a dedicated I/O bus introduced on the VAX and late-model PDP-11s.

Technical specifications

A Unibus connector and extension cable Unibus cable.jpg
A Unibus connector and extension cable

The Unibus consists of 72 signals, usually connected via two 36-way edge connectors on each printed circuit board. When not counting the power and ground lines, it is usually referred to as a 56-line bus. It can exist within a backplane or on a cable. Up to 20 nodes (devices) can be connected to a single Unibus segment; additional segments can be connected via a bus repeater.

The bus is completely asynchronous, allowing a mixture of fast and slow devices. It allows the overlapping of arbitration (selection of the next bus master) while the current bus master is still performing data transfers. The 18 address lines allow the addressing of a maximum of 256 KB. Typically, the top 8 KB is reserved for the registers of the memory-mapped I/O devices used in the PDP-11 architecture.

The design deliberately minimizes the amount of redundant logic required in the system. For example, a system always contains more slave devices than master devices so most of the complex logic required to implement asynchronous data transfers is forced into the relatively few master devices. For interrupts, only the interrupt-fielding processor needs to contain the complex timing logic. The result is that most I/O controllers can be implemented with simple logic, and most of the critical logic is implemented as a custom MSI IC.

Pinout

NumberNameTypeDescription
18A00-A171Address Lines
16D00-D151Data Lines
4BR4-BR71Bus (Interrupt) Requests at priorities 4 (lowest) through 7 (highest)
4BG4-BG72Bus (Interrupt) Grants at priorities 4 (lowest) through 7 (highest)
1NPR1Non Processor (DMA) Request
1NPG2Non Processor (DMA) Grant
1MSYNC1Master Sync
1SSYNC1Slave Sync
1BBSY1Bus Busy
1SACK1Selection Acknowledge
1INIT1Bus Init
1INTR1Interrupt Request
1PA1Parity control
1PB1Parity control
2C0-C11Control Lines
1ACLO3AC Low
1DCLO3DC Low
2+5v-Power Lines (not counted as part of the 56)
14Gnd-Ground Lines (not counted as part of the 56)
Unibus grant request continuity card Unibus grant continuity card.jpg
Unibus grant request continuity card

Type 1 lines are a normal multi-sender wired-OR bus with pull-up resistors at each end of the bus, typically on a terminator card. [3]

Type 2 lines are selectively propagated by each card to the next slot – if the card wants to keep the request grant it will assert the SACK line and not propagate the request to the next slot. If a slot is empty, it is necessary to install a "grant continuity card" in the slot to propagate the four type 2 signals to the next card. [3]

Type 3 signals are generated by the power supply and have only a single sender. They warn the devices on the bus when the power is about to fail, so those devices can execute an orderly shutdown, and disable operations to prevent spurious writes. [3]

The two control lines (C0 and C1) allowed the selection of four different data transfer cycles:

Related Research Articles

<span class="mw-page-title-main">Bus (computing)</span> System that transfers data between components within a computer

In computer architecture, a bus is a communication system that transfers data between components inside a computer, or between computers. This expression covers all related hardware components and software, including communication protocols.

<span class="mw-page-title-main">Data General Nova</span> 16-bit minicomputer series

The Data General Nova is a series of 16-bit minicomputers released by the American company Data General. The Nova family was very popular in the 1970s and ultimately sold tens of thousands of units.

<span class="mw-page-title-main">Peripheral Component Interconnect</span> Local computer bus for attaching hardware devices

Peripheral Component Interconnect (PCI) is a local computer bus for attaching hardware devices in a computer and is part of the PCI Local Bus standard. The PCI bus supports the functions found on a processor bus but in a standardized format that is independent of any given processor's native bus. Devices connected to the PCI bus appear to a bus master to be connected directly to its own bus and are assigned addresses in the processor's address space. It is a parallel bus, synchronous to a single bus clock. Attached devices can take either the form of an integrated circuit fitted onto the motherboard or an expansion card that fits into a slot. The PCI Local Bus was first implemented in IBM PC compatibles, where it displaced the combination of several slow Industry Standard Architecture (ISA) slots and one fast VESA Local Bus (VLB) slot as the bus configuration. It has subsequently been adopted for other computer types. Typical PCI cards used in PCs include: network cards, sound cards, modems, extra ports such as Universal Serial Bus (USB) or serial, TV tuner cards and hard disk drive host adapters. PCI video cards replaced ISA and VLB cards until rising bandwidth needs outgrew the abilities of PCI. The preferred interface for video cards then became Accelerated Graphics Port (AGP), a superset of PCI, before giving way to PCI Express.

<span class="mw-page-title-main">PDP-8</span> Minicomputer product line

The PDP-8 is a family of 12-bit minicomputers that was produced by Digital Equipment Corporation (DEC). It was the first commercially successful minicomputer, with over 50,000 units being sold over the model's lifetime. Its basic design follows the pioneering LINC but has a smaller instruction set, which is an expanded version of the PDP-5 instruction set. Similar machines from DEC are the PDP-12 which is a modernized version of the PDP-8 and LINC concepts, and the PDP-14 industrial controller system.

<span class="mw-page-title-main">PDP-11</span> Series of 16-bit minicomputers

The PDP–11 is a series of 16-bit minicomputers sold by Digital Equipment Corporation (DEC) from 1970 into the late 1990s, one of a set of products in the Programmed Data Processor (PDP) series. In total, around 600,000 PDP-11s of all models were sold, making it one of DEC's most successful product lines. The PDP-11 is considered by some experts to be the most popular minicomputer.

Direct memory access (DMA) is a feature of computer systems that allows certain hardware subsystems to access main system memory independently of the central processing unit (CPU).

<span class="mw-page-title-main">Universal asynchronous receiver-transmitter</span> Computer hardware device

A Universal Asynchronous Receiver-Transmitter is a computer hardware device for asynchronous serial communication in which the data format and transmission speeds are configurable. It sends data bits one by one, from the least significant to the most significant, framed by start and stop bits so that precise timing is handled by the communication channel. The electric signaling levels are handled by a driver circuit external to the UART. Common signal levels are RS-232, RS-485, and raw TTL for short debugging links. Early teletypewriters used current loops.

<span class="mw-page-title-main">Expansion card</span> Circuit board for connecting to a computer system to add functionality

In computing, an expansion card is a printed circuit board that can be inserted into an electrical connector, or expansion slot on a computer's motherboard to add functionality to a computer system. Sometimes the design of the computer's case and motherboard involves placing most of these slots onto a separate, removable card. Typically such cards are referred to as a riser card in part because they project upward from the board and allow expansion cards to be placed above and parallel to the motherboard.

Memory-mapped I/O (MMIO) and port-mapped I/O (PMIO) are two complementary methods of performing input/output (I/O) between the central processing unit (CPU) and peripheral devices in a computer. An alternative approach is using dedicated I/O processors, commonly known as channels on mainframe computers, which execute their own instructions.

PCI configuration space is the underlying way that the Conventional PCI, PCI-X and PCI Express perform auto configuration of the cards inserted into their bus.

<span class="mw-page-title-main">System bus</span> Single computer bus that connects the major components of a computer system

A system bus is a single computer bus that connects the major components of a computer system, combining the functions of a data bus to carry information, an address bus to determine where it should be sent or read from, and a control bus to determine its operation. The technique was developed to reduce costs and improve modularity, and although popular in the 1970s and 1980s, more modern computers use a variety of separate buses adapted to more specific needs.

The Q-bus, also known as the LSI-11 Bus, is one of several bus technologies used with PDP and MicroVAX computer systems previously manufactured by the Digital Equipment Corporation of Maynard, Massachusetts.

<span class="mw-page-title-main">VAXBI bus</span>

The VAXBI bus is a computer bus designed and sold by the Digital Equipment Corporation (DEC) of Maynard, Massachusetts.

<span class="mw-page-title-main">DEC Professional (computer)</span>

The Professional 325 (PRO-325), Professional 350 (PRO-350), and Professional 380 (PRO-380) are PDP-11 compatible microcomputers. The Pro-325/350 were introduced in 1982 and the Pro-380 in 1985 by Digital Equipment Corporation (DEC) as high-end competitors to the IBM PC.

<span class="mw-page-title-main">Low Pin Count</span> Low-bandwidth computer motherboard bus

The Low Pin Count (LPC) bus is a computer bus used on IBM-compatible personal computers to connect low-bandwidth devices to the CPU, such as the BIOS ROM, "legacy" I/O devices, and Trusted Platform Module (TPM). "Legacy" I/O devices usually include serial and parallel ports, PS/2 keyboard, PS/2 mouse, and floppy disk controller.

<span class="mw-page-title-main">STEbus</span> Non-proprietary, processor-independent, computer bus

The STEbus is a non-proprietary, processor-independent, computer bus with 8 data lines and 20 address lines. It was popular for industrial control systems in the late 1980s and early 1990s before the ubiquitous IBM PC dominated this market. STE stands for STandard Eurocard.

The Massbus is a high-performance computer input/output bus designed in the 1970s by Digital Equipment Corporation (DEC). The architecture development was sponsored by Gordon Bell and John Levy was the principal architect.

<span class="mw-page-title-main">VAX 8000</span> Family of superminicomputers by Digital Equipment Corporation

The VAX 8000 is a discontinued family of superminicomputers developed and manufactured by Digital Equipment Corporation (DEC) using processors implementing the VAX instruction set architecture (ISA).

The PDP-11 architecture is a 16-bit CISC instruction set architecture (ISA) developed by Digital Equipment Corporation (DEC). It is implemented by central processing units (CPUs) and microprocessors used in PDP-11 minicomputers. It was in wide use during the 1970s, but was eventually overshadowed by the more powerful VAX architecture in the 1980s.

<span class="mw-page-title-main">Europe Card Bus</span>

The Europe Card Bus is a computer bus developed in 1977 by the company Kontron, mainly for the 8-bit Zilog Z80, Intel 8080 and Intel 8085 microprocessor families.

References

  1. Gardner Hendrie, Interviewer (June 23, 2005). "Bell (Gordon) Oral History". Reference number: X3202.2006. Computer History Museum. Retrieved May 20, 2011.{{cite web}}: |author= has generic name (help)
  2. "PDP-11 Buses". University of Sydney.
  3. 1 2 3 Digital Equipment Corporation (1979). "Unibus Specification" (PDF).