What is VMEbus? (Explained)

What is VMEbus?

VMEbus refers to the particular standard of computer bus developed by Motorola with the primary intention to use in the Motorola 68000 line of processors.

Also used widely for several other applications later, this bus was standardized as ANSI/IEEE 1014-1987 by the IEC.

Technically, this standard is based on the physical sizes of the Eurocard and its mechanicals and connectors such as DIN 41612. However, it uses its own signals, which are not defined by Eurocard.

KEY TAKEAWAYS

• The VMEbus is a computer data path or an OEM bus that is built on the overall design trends of the PCB and electronic industry.
• It can be integrated into many different hardware systems used for weapons control systems, robotic systems, traffic control systems, or other significant hardware setups.
• Built in different sizes and available in different varieties, this bus can fit into any specific rack space.
• The bus can operate in various states and it uses its own signaling and command syntax to indicate them.
• Its design, features and functionality make it a worthy competitor of other models such as CompactPCI.

Understanding VMEbus

VMEbus was introduced in 1981 by Motorola and can send data through it at multiple bits ranging from 8 bit to 64 bits at a time.

Also known as Versa Module Europa or Versa Module Eurocard bus, there are several extensions included with this interface.

Over the years, VME has evolved and each of them follows a specific topology and bus cycle to offer variable speed such as:

• VMEbus32 Parallel Bus Rev. A, released in 1981, follows the BLT bus cycle and operates at a maximum speed of up to 40 Mbps
• VMEbus IEEE-1014, released in 1987, follows the BLT bus cycle and operates at a maximum speed of up to 40 Mbps
• VME64, released in 1994, follows the MBLT bus cycle and operates at a maximum speed of up to 80 Mbps
• VME64x, released in 1997, follows the 2eVME bus cycle and operates at a maximum speed of up to 160 Mbps and
• VME320, released in 1997, follows the 2eSST bus cycle and operates at a maximum speed of up to 320 Mbps.

These inclusions offer sideband channels for communication to VME itself in parallel.

A few examples of them are:

• IP Module
• SCSA or Signal Computing System Architecture
• RACEway Interlink
• Gigabit Ethernet on VME64x Backplanes
• StarFabric
• RapidIO
• PCI Express and
• InfiniBand.

In addition to that, the VMEbus was also used to build other standards that are closely related, such as VPX and VXIbus.

With such developments, this bus had a very strong influence on several computer buses that came up later on, such as STEbus.

The Eurocard may hold one bus request line out of four to become the bus master, irrespective of the arbitration mode.

It also decoded all of the interrupt levels on a 7-pin interrupt bus based on one of the vectored and prioritized interrupts.

All transfers on the VMEbus are usually DMA in nature and every card in it is either a master or a slave.

The transfer type is relatively simpler in comparison to other bus standards such as ISA in terms of master/slave selection.

However, each card needs a more complex controller to be more powerful.

VMEbus Specification

One of the main specifications of the VMEbus is its flat memory model of 32 bits which does not consist of any anti-features or memory segmentation.

VMEbus uses two separate Eurocard connectors to offer a 32 bit data bus and an address bus. These cards are named P1 and P2.

It operates at 5 volts, +12 volts and -12 volts DC voltages and has three key signal lines such as:

• ACFAIL
• PG or Power Good and
• SYSRESET.

As for the sizes of the VMEbus cards, there are three length and height specifications as follows:

• 3U Size A – 160 mm by 100 mm
• 6U Size B – 160 mm by 233 mm or Size C 340 mm by 233 mm and
• 9U Size D – 340 mm by 360 mm

Typically, the different widths or board thickness of VME cards accepted are 1.6 +/- 0.2 mm or 0.063 +/- 0.008 inches. Any thicker or thinner card may not fit properly in the card guide.

A 0.90 board, on the other hand, may fit in the card guide if the card is milled down to 0.63 at the top and bottom.

The VMEbus follows the VITA or VMEbus International Trade Association and IEEE or Institute of Electrical and Electronic Engineers specifications. Each of these specs has different features and functionalities.

The Versa module bus specification has been refined several times to come up with different revisions such as:

• B
• C
• C.1
• IEC 821
• IEEE 1014-1987 and
• ANSI/VITA 1-1994.

Out of these the VITA, ANSI, IEEE and IEC standards are significant because these particular standards make it publicly defined.

The users and vendors need not worry about it becoming obsolete due to the whims and fancies of one single manufacturer because there are no proprietary rights assigned to it.

VMEbus Protocol

The VMEbus protocol typically follows asynchronous 4-edge handshake and specific cycle that depends on the operating speed of the master and slave. It also follows several high and active low signals.

Some of the most important signals with their respective descriptions are:

• BBSY – Bus Busy
• A[31..1] – Address lines
• D[31..0] – Data lines
• AM[5..0] – Address modifier
• DS0 and DS1 – Data strobes
• LWORD – Defines transfer size and carries data during multiple block transfers
• AS – Address Strobe
• WRITE – Defines direction of data transfer
• DTACK – Data acknowledge
• BERR – Bus error
• IRQ1… IRQ7 – Interrupt request lines and
• IACK – Interrupt acknowledgement.

As for the addressing protocol, the master drives the Address, AM, and LWORD first and waits 35 ns to finally drive AS for validating any information. The slave then decodes the information in about 40 ns.

The master continues transferring data until it receives an acknowledgement DTACK or an error message BERR.

Since it follows an asynchronous, handshake protocol there is no set transfer rate. Typically, the parameters for timing set the upper limit.

In single cycle transfer for typical performance, it is 1 µs per transfer and as follows:

• D8 = 1 Mbps
• D16 = 2 Mbps
• D32 = 4 Mbps.

And for block transfers, it is:

• D32 = 20…25 Mbps, though the theoretical limit is 40 Mbps
• D64 = 40…50 Mbps, though the theoretical limit is 80 Mbps.

The performance during write posting decouples VMEbus, and PCI cycle increases to about 10 Mbps for D32.

VMEbus Pinout

The pins of the VMEbus run out on a backplane and are analogous or equivalent to those of the 68000. There are three rows of 32 pins each in the P1.

These pins implement the first 24 address bits and 16 data bits along with all control signals. The P2 connector, on the other hand, has an additional row in it for the remaining 16 data bits and 8 address bits.

Also known as J1/J2 connectors, the pinout of the bus according to the specification is as follows:

• According to the IEEE 1014-1987 specification, the VMEbus pinout of P1 and P2 is 96 pins in three rows of 32 pins each
• According to the ANSI/VITA 1-1994 specification, the VMEbus pinout of P1 and P2 is 160 pins in five rows of 32 pins each and
• According to the ANSI/VITA 1-1994 specification, the VMEbus pinout of P0 is 95 pins in five rows of 19 pins each where the pinout is defined only for Myrinet, ATM I/O, PMC and GbE.

However, according to the SEM E VITA 18-1997 specification, the VMEbus pin out is for military MIL-STD-1389 Format-E boards and backplanes.

VMEbus Architecture

The architecture of this bus is based on VERSAbus, also developed by Motorola in the late 1970s. From the point of view of the VME64 and VME64x standards, the VMEbus architecture can be described on the basis of its functional modules.

These modules are actually a conceptual tool and in some cases also describe the real hardware.

All of the functional modules that are typically found in the VMEbus architecture and also have different classes of modules are:

• Master module to initiate data transfer
• Slave module to respond to the master
• Interrupter module to send an interrupt usually to the slave
• Interrupt handler to receive and handle interrupts
• Arbiter to arbitrate bus access and monitor bus status
• Location monitor for monitoring data transfer and asserting on-board signals
• Bus timer to measure data transfer cycles
• IACK daisy chain driver
• Requester for requesting ownership of the data transfer bus
• System clock driver to offer a 6 MHz stable utility clock and
• Power monitor to generate ACFAIL and SYSRESET signals.

There are also different sub-buses such as:

• Data transfer bus for transferring data and address information
• Data transfer arbitration bus for obtaining ownership of the bus
• Priority interrupt bus for sending interrupts between modules
• Utility bus to perform several utility functions such as utility clock and system reset signal and
• A two-wire serial bus.

There are several bus cycles found as well, impressed on the sub-buses where the standard is READ/WRITE cycle that can transfer 8, 16, 24 or 32 bits of data in each transaction. Some of the other specialized data transfer bus cycles are:

• Read-modify-write Cycle
• Block Transfer Cycle
• Multiplexed Block Transfer Cycles
• Two-edge Cycles
• Address-only cycle

There is also a data transfer arbitration bus cycle for granting ownership of the bus and a priority interrupt bus cycle or IACK cycle to send interrupts across the backplane.

What is VMEbus Used for?

The effectiveness and speed of VMEbus allow it to be used in different applications including industrial controls, office and factory automation, military, telecommunications, and instrumentation systems.

VMEbus is used as a standardized computer bus in several embedded applications and is designed to support several other specialized applications as well.

It is all due to its sheer speed and effectiveness that this bus is used widely in some of the most mission critical applications, industries and systems all over the world.

These are:

• Research
• Industrial controls
• Medical
• Transport
• Semiconductor process control
• Defense sectors as in U.S. Military in satcom military systems
• Nuclear industries and nuclear medical systems
• Industrial automation industries
• Semiconductor robots
• Telecommunications
• Federal Aviation Administration operations
• Injection molding machines
• Sawmill control
• Metal working
• Cardboard cutters
• Steel manufacturing
• Ground and flight radar control systems
• Battlefield command and control systems
• Tank and gun controls
• Avionics
• Fly-by-wire control systems
• Spacecraft experiment control
• In-flight video servers
• Missile countdown sequencers
• Railway controls
• Smart highway systems
• Advanced Intelligent Node or AIN switch gear
• Satellite uplinks and downlinks
• Telephone switches
• Cellular phone base stations
• Aircraft flight as well as metal fatigue, earthquake, and different military simulation systems
• CT SCAN and MRI imaging
• Acoustical systems
• High energy physics including particle accelerators and detectors
• Network routers
• Servers
• High-speed printers and
• Copy machines.

In simple words, Versa buses can be used for general as well as specialized purposes and in several proprietary systems for communicating with each other.

The VME support for inter-slot communications of today seems to be astounding. There seems to be no limitation to it, whether it is used for star interconnects or mesh fabrics and even for full switching in a 21-slot chassis.

Is VMEbus Still Used?

Yes, this bus is still used in several applications and industries today. This is because using this bus makes it easier for the system engineers to move data between different boards in the system.

There is no doubt VMEbus compatible systems still command a major share of the market today, whether it is in the field of transportation, aerospace, defense, or industrial automation segments.

The features and functionality of the bus influence critical embedded computing significantly, which is one particular reason for its success and such an extended life.

Systems seem to work impeccably while protecting life, property, environment and equipment.

The utility of the bus makes the systems much more able in several dimensions such as:

• Dependable
• Configurable
• Supportable
• Reliable and
• Serviceable.

It is the deterministic capability in real time that makes this bus an ideal solution to use in more complex and larger computing systems, especially those that need a very high degree of data processing and real time control.

Moreover, its effectiveness in both I/O and processing capability allows it to be emulated in even high-end systems such as VPX or VITA 46.

The backplane-based topology helps it to perform well when used in I/O intensive applications that motherboards with pizza box-style configurations and architectures cannot.

It also paved the way for an open standard to be used in rugged systems that can function in extreme conditions.

This bus is going to go further forward and will be conceded to the backplane-based solutions of the next generation.

It is justly deterministic and real-time optimized. It is flexible and scalable – in fact, second to none.

It is true that perhaps no one is working to develop it further, but VMEbus is expected to gain more ground and live on for several more years due to the experience it has provided to the users in the past forty years and counting.

Conclusion

Right from its launch, VMEbus is used in several sectors and many manufacturers produced VME compatible products such as VME boards, mechanical hardware, bus interface chips and software.

With its utility, even at forty, it is here to stay and its range of use will continue to grow.