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VMEbus

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VME64 crate with, from left, an ADC module, a scaler module and a processor module

VMEbus (Versa Module Eurocard[1] bus) is a computer bus standard physically based on Eurocard sizes.

History

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In 1979, during development of the Motorola 68000 CPU, one of their engineers, Jack Kister, decided to set about creating a standardized bus system for 68000-based systems.[2] The Motorola team brainstormed for days to select the name VERSAbus. VERSAbus cards were large, 370 by 230 mm (14+1⁄2 by 9+1⁄4 in), and used edge connectors.[3] Only a few products adopted it, including the IBM System 9000 instrument controller and the Automatix robot and machine vision systems.

VERSAbus memory card

Kister was later joined by John Black, who refined the specifications and created the VERSAmodule product concept. A young engineer working for Black, Julie Keahey designed the first VERSAmodule card, the VERSAbus Adaptor Module, used to run existing cards on the new VERSAbus. Sven Rau and Max Loesel of Motorola-Europe added a mechanical specification to the system, basing it on the Eurocard standard that was then late in the standardization process. The result was first known as VERSAbus-E but was later renamed to VMEbus, for VERSAmodule Eurocard bus (although some refer to it as Versa Module Europa).[3]

At this point, a number of other companies involved in the 68000's ecosystem agreed to use the standard, including Signetics, Philips, Thomson, and Mostek. Soon it was officially standardized by the IEC as the IEC 821 VMEbus and by ANSI and IEEE as ANSI/IEEE 1014-1987.

The original standard was a 16-bit bus, designed to fit within the existing Eurocard DIN connectors. However, there have been several updates to the system to allow wider bus widths. The current VME64 includes a full 64-bit bus in 6U-sized cards and 32-bit in 3U cards. The VME64 protocol has a typical performance of 40 MB/s.[3] Other associated standards have added hot-swapping (plug-and-play) in VME64x, smaller 'IP' cards that plug into a single VMEbus card, and various interconnect standards for linking VME systems together.

In the late 1990s, synchronous protocols proved to be favourable. The research project was called VME320. The VITA Standards Organization called for a new standard for unmodified VME32/64 backplanes.[3] The new 2eSST protocol was approved in ANSI/VITA 1.5 in 1999.

Over the years, many extensions have been added to the VME interface, providing 'sideband' channels of communication in parallel to VME itself. Some examples are IP Module, RACEway Interlink, SCSA, Gigabit Ethernet on VME64x Backplanes, PCI Express, RapidIO, StarFabric and InfiniBand.

VMEbus was also used to develop closely related standards, VXIbus and VPX. The VMEbus had a strong influence on many later computer buses such as STEbus.

VME early years

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The architectural concepts of the VMEbus are based on VERSAbus,[3] developed in the late 1970s by Motorola. This was later renamed "VME", short for Versa Module European, by Lyman (Lym) Hevle, then a VP with the Motorola Microsystems Operation. (He was later the founder of the VME Marketing Group, itself subsequently renamed to VME International Trade Association, or VITA).

John Black of Motorola, Craig MacKenna of Mostek and Cecil Kaplinsky of Signetics developed the first draft of the VMEbus specification. In October 1981, at the System '81 trade show in Munich, West Germany, Motorola, Mostek, Signetics/Philips, and Thomson CSF announced their joint support of the VMEbus. They also placed Revision A of the specification in the public domain.

In 1985, Aitech developed, under contract for US Army TACOM, the first conduction-cooled 6U VMEbus board. Although electrically providing a compliant VMEbus protocol interface, mechanically, this board was not interchangeable for use in air-cooled lab VMEbus development chassis.

In late 1987, a technical committee was formed under VITA under the direction of IEEE to create the first military, conduction-cooled 6U × 160 mm, fully electrically and mechanically compatible, VMEbus board co-chaired by Dale Young (DY4 Systems) and Doug Patterson (Plessey Microsystems, then Radstone Technology). ANSI/IEEE-1101.2-1992 was later ratified and released in 1992 and remains in place as the conduction-cooled, international standard for all 6U VMEbus products.

In 1989, John Peters of Performance Technologies Inc. developed the initial concept of VME64: multiplexing address and data lines (A64/D64) on the VMEbus. The concept was demonstrated the same year and placed in the VITA Technical Committee in 1990 as a performance enhancement to the VMEbus specification.

In 1993, new activities began on the base-VME architecture, involving the implementation of high-speed serial and parallel sub-buses for use as I/O interconnections and data mover subsystems. These architectures can be used as message switches, routers and small multiprocessor parallel architectures.

VITA's application for recognition as an accredited standards developer organization of ANSI was granted in June 1993. Numerous other documents ( including mezzanine, P2 and serial bus standards) have been placed with VITA as the Public Domain Administrator of these technologies.

Evolution of VME
Topology Year Bus cycle Maximum speed (MB/s)
VMEbus32 Parallel Bus Rev. A 1981 BLT 40
VMEbus IEEE-1014 1987 BLT 40
VME64 1994 MBLT 80
VME64x 1997 2eVME 160
VME320 1997 2eSST 320

Description

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In many ways the VMEbus is equivalent or analogous to the pins of the 68000 run out onto a backplane.

However, one of the key features of the 68000 is a flat 32-bit memory model, free of memory segmentation and other "anti-features". The result is that, while VME is very 68000-like, the 68000 is generic enough to make this not an issue in most cases.

Like the 68000, VME uses separate 32-bit data and address buses. The 68000 address bus is actually 24-bit and the data bus 16-bit (although it is 32/32 internally) but the designers were already looking towards a full 32-bit implementation.

In order to allow both bus widths, VME uses two different Eurocard connectors, P1 and P2. P1 contains three rows of 32 pins each, implementing the first 24 address bits, 16 data bits and all of the control signals. P2 contains one more row, which includes the remaining 8 address bits and 16 data bits.

A block transfer protocol allows several bus transfers to occur with a single address cycle. In block transfer mode, the first transfer includes an address cycle and subsequent transfers require only data cycles. The slave is responsible for ensuring that these transfers use successive addresses.

Bus masters can release the bus in two ways. With Release When Done (RWD), the master releases the bus when it completes a transfer and must re-arbitrate for the bus before every subsequent transfer. With Release On Request (ROR), the master retains the bus by continuing to assert BBSY* between transfers. ROR allows the master to retain control over the bus until a Bus Clear (BCLR*) is asserted by another master that wishes to arbitrate for the bus. Thus a master that generates bursts of traffic can optimize its performance by arbitrating for the bus on only the first transfer of each burst. This decrease in transfer latency comes at the cost of somewhat higher transfer latency for other masters.

Address modifiers are used to divide the VME bus address space into several distinct sub-spaces. The address modifier is a 6 bit wide set of signals on the backplane. Address modifiers specify the number of significant address bits, the privilege mode (to allow processors to distinguish between bus accesses by user-level or system-level software), and whether or not the transfer is a block transfer. Below is an incomplete table of address modifiers:

Hex Code Function Explanation
3f Standard Supervisory block transfer Block transfer A24, privileged
3e Standard Supervisory Program access A24 instruction access, privileged
3d Standard Supervisor Data Access A24 data access, privileged
3b Standard Non-privileged block transfer A24 block transfer for normal programs
3a Standard Non-privileged Program access A24 instruction access, non-privileged
39 Standard non-privileged Data Access A24 data access, non-privileged
2d Short supervisory Access A16 privileged access.
29 Short non-privileged Access A16 non-privileged access.
0f Extended supervisory Block transfer A32 privileged block transfer.
0e Extended supervisory Program access A32 privileged instruction access.
0d Extended supervisory Data Access. A32 privileged data access.
0b Extended Non-privileged Block transfer A32 non-privileged block transfer.
0a Extended Non-privileged Program access A32 non-privileged instruction access.
09 Extended non-privileged data access. A32 non-privileged data access.
Note An as in A16, A24, A32 refers to the width of the address

On the VME bus, all transfers are DMA and every card is a master or slave. In most bus standards, there is a considerable amount of complexity added in order to support various transfer types and master/slave selection. For instance, with the ISA bus, both of these features had to be added alongside the existing "channels" model, whereby all communications was handled by the host CPU. This makes VME considerably simpler at a conceptual level while being more powerful, though it requires more complex controllers on each card.

Development tools

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When developing and/or troubleshooting the VME bus, examination of hardware signals can be very important. Logic analyzers and bus analyzers are tools that collect, analyze, decode, store signals so people can view the high-speed waveforms at their leisure.

VITA offers a comprehensive FAQ to assist with the front end design and development of VME systems.

Computers using a VMEbus

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Computers using VMEbus include:

Pinout

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Seen looking into backplane socket.[5][6]

P1

Pin a b c
1 D00 BBSY* D08
2 D01 BCLR* D09
3 D02 ACFAIL* D10
4 D03 BG0IN* D11
5 D04 BG0OUT* D12
6 D05 BG1IN* D13
7 D06 BG1OUT* D14
8 D07 BG2IN* D15
9 GND BG2OUT* GND
10 SYSCLK BG3IN* SYSFAIL*
11 GND BG3OUT* BERR*
12 DS1* BR0* SYSRESET*
13 DS0* BR1* LWORD*
14 WRITE* BR2* AM5
15 GND BR3* A23
16 DTACK* AM0 A22
17 GND AM1 A21
18 AS* AM2 A20
19 GND AM3 A19
20 IACK* GND A18
21 IACKIN* SERCLK A17
22 IACKOUT* SERDAT* A16
23 AM4 GND A15
24 A07 IRQ7* A14
25 A06 IRQ6* A13
26 A05 IRQ5* A12
27 A04 IRQ4* A11
28 A03 IRQ3* A10
29 A02 IRQ2* A09
30 A01 IRQ1* A08
31 −12V +5VSTDBY +12V
32 +5V +5V +5V

P2

Pin a b c
1 User Defined +5V User Defined
2 User Defined GND User Defined
3 User Defined RESERVED User Defined
4 User Defined A24 User Defined
5 User Defined A25 User Defined
6 User Defined A26 User Defined
7 User Defined A27 User Defined
8 User Defined A28 User Defined
9 User Defined A29 User Defined
10 User Defined A30 User Defined
11 User Defined A31 User Defined
12 User Defined GND User Defined
13 User Defined +5V User Defined
14 User Defined D16 User Defined
15 User Defined D17 User Defined
16 User Defined D18 User Defined
17 User Defined D19 User Defined
18 User Defined D20 User Defined
19 User Defined D21 User Defined
20 User Defined D22 User Defined
21 User Defined D23 User Defined
22 User Defined GND User Defined
23 User Defined D24 User Defined
24 User Defined D25 User Defined
25 User Defined D26 User Defined
26 User Defined D27 User Defined
27 User Defined D28 User Defined
28 User Defined D29 User Defined
29 User Defined D30 User Defined
30 User Defined D31 User Defined
31 User Defined GND User Defined
32 User Defined +5V User Defined

P2 rows a and c can be used by a secondary bus, for example the STEbus.

See also

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References

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  1. ^ "VMEbus FAQa". Retrieved 17 January 2023.
  2. ^ Black, John Arthur (1992). The System engineer's handbook: a guide to building VMEbus and VXIbus systems. Morgan Kaufmann. p. 563. ISBN 978-0-12-102820-6. A team of engineers at Motorola Microsystems led by Jack Kister, designed a 68000 development system called the EXORmacs. The backplane of the EXORmacs was called VERSAbus. While coordinating the efforts of his team, Jack wrote a 41-page bus description of VERSAbus which was published in November of 1979. The first EXORmacs was shipped in January 1980.
  3. ^ a b c d e "VME Technology FAQ". Vita.com. 3 January 1999. Retrieved 1 August 2013.
  4. ^ "HP VME Products - Alimar Technology Corp". Alimartech.com. Retrieved 1 August 2013.
  5. ^ From Table 7 - 1 J1/P1 Pin Assignments, ANSI/VITA 1-1994 (R2002)
  6. ^ From Table 7 - 2 J2/P2 Pin Assignments, ANSI/VITA 1-1994 (R2002)
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