Hop on the High-Speed Bus

The first general purpose computer known as ENIAC (Electronic Numerical Integrator And Computer – circa 1946) was heralded as the “Giant Brain”. It was literally larger than a dozen passenger buses and weighed as much. It was made of tens of thousands of vacuum tubes and relays, hundreds of thousands of resistors and capacitors and millions of hand-soldered joints. It operated at lightning speed, a whopping 0.1 MHz.  Skip forward 67 years, when the average computer is more than ten thousand times faster and one hundred thousand times smaller.

Every computer since then has employed a system known as a “bus” for transferring signals and data both internally and to peripherals. For the past 60 years these computer buses were mainly parallel wires or circuit board traces that could contain hundreds of signals. In the heyday of mainframes and early minicomputers these buses were proprietary, highly guarded designs, and specific to particular models or families of computers. It was only in the 1970s-1990s that the proprietary nature of buses was turned on its head spurred on by the advent of the microprocessor (Intel, Texas Instruments, Motorola, Zilog and others) and availability of a wide range of general purpose integrated circuits (led by Fairchild).

A number of minicomputer vendors (including Digital Equipment Corporation – now part of HP) started documenting their computer bus architectures (Unibus, Q-Bus, LSI-11 bus). A whole circuit board or multiples could be dedicated to the CPU function, other boards for memory, still others for disk controllers and so on. These boards were often 19”x19” in size and connected by an expansion bus with gold plated fingers that slotted into a multi-connector backplane. Many third parties quickly developed massive add-in cards to supplement the vendors’ selection of peripherals.

As the general purpose microprocessor (8080, Z80, 8086, 68000) effectively replaced proprietary minicomputer CPUs during the 1980s, vendors building those systems immediately released their expansion bus specifications (S100, Multibus I & II, VMEbus 1-10 MHz) and the open-architecture add-on card industry began. However it was only after the release of the IBM PC in 1980, which was IBM’s first open architecture computer with an ISA bus (5MHz), did the add-on card industry (memory, video, network, disk, comms) grow to billions of dollars in that decade.

During the 1990s the popularity and power of the PC architecture (80286/386/486/Pentium, etc.) and follow-on improvements to the ISA bus (EISA and MCA) paved the way for more sophisticated buses which allowed 32-bit operation, higher speeds, multi-processor support, CPU independence and so on. So in 1993 Intel released the PCI bus (Peripheral Component Interconnect) which supported 32/64-bit transfers at 33 and 66 MHz and dominated for a decade.

Post 2000 we’ve seen a dramatic shift in the performance, miniaturization and transformation of computing devices. Most modern CPUs used in these systems have absorbed discrete functions into a 2 or 3 chip-set or a single System on Chip (SoC) so those old buses make little sense. However the need to add high-speed peripherals, storage, displays, and communication devices still exists. The PCI bus was found wanting and its physical attributes made it impractical. In 2004 the PCI bus evolved into PCI Express which was an ultra high-speed serial bus that implemented the nearly 100 pin PCI bus on a handful of wires on a board or on a cable to an external device. Not only that, it introduced the concept of lanes so that you could aggregate up to 16 channels into one for 256 Gbps transfers. Most modern systems support at least a “one-lane” PCIe interface. Add-on cards are palm-sized or smaller. Some are also available in a mini-card format such as wireless modules.

At Opengear our engineers have significant design and business experience that covers the major “open architecture” buses spanning the last 30 years. Many of our products and future products employ these popular buses. That’s the high-speed side covered. An article summarizing some key medium and low-speed serial buses, which are also industry stalwarts, will follow, so “don’t miss the bus”.

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