ISA Bus History
We Sell Computers with ISA Bus
WE SELL COMPUTERS WITH ISA Slot / ISA Bus. Change is inevitable. Why grumble about losing your ISA bus compatibility
when you update your computers? Preserve the usefulness of your data written
and stored in older formats with Intel servers in tower or rackmount enclosures.
Your business needs to stay flexible in the face of changing industry
standards. Let NIXSYS configure the hardware you need.
Using our PC with ISA Bus can make a temporary task safe and secure.
Use your data while you make the change to newer technology over time.
That way you won't have to rush or take the focus off of other business
matters. The features we offer give you enough muscle to keep working
without spending a fortune on transitional technology. It's all backed
up by 2-year warranty.
ISA Bus History
The ISA bus was developed by a team led by Mark Dean at IBM as part of
the IBM PC project in 1981. It originated as an 8-bit system and was extended
in 1983 for the XT system architecture. The newer 16-bit standard, the
IBM AT bus, was introduced in 1984. In 1988, the Gang of Nine IBM PC compatible
manufacturers put forth the 32-bit EISA standard and in the process retroactively
renamed the AT bus to "ISA" to avoid infringing IBM's trademark
on its PC/AT computer. IBM designed the 8-bit version as a buffered interface
to the external bus of the Intel 8088 (16/8 bit) CPU used in the original
IBM PC and PC/XT, and the 16-bit version as an upgrade for the external
bus of the Intel 80286 CPU used in the IBM AT. Therefore, the ISA bus
was synchronous with the CPU clock, until sophisticated buffering methods
were developed and implemented by chipsets to interface ISA to much faster
CPUs.
Designed to connect peripheral cards to the motherboard, ISA allows for
bus mastering although only the first 16 MB of main memory are available
for direct access. The 8-bit bus ran at 4.77 MHz (the clock speed of the
IBM PC and IBM PC/XT's 8088 CPU), while the 16-bit bus operated at 6 or
8 MHz (because the 80286 CPUs in IBM PC/AT computers ran at 6 MHz in early
models and 8 MHz in later models.) IBM RT/PC also used the 16-bit bus.
It was also available on some non-IBM compatible machines such as the
short-lived AT&T Hobbit and later PowerPC based BeBox.
In 1987, IBM moved to replace the AT bus with their proprietary Micro
Channel Architecture (MCA) in an effort to regain control of the PC architecture
and the PC market. (Note the relationship between the IBM term "I/O
Channel" for the AT-bus and the name "Micro Channel" for
IBM's intended replacement.) MCA had many features that would later appear
in PCI, the successor of ISA, but MCA was a closed standard, unlike ISA
(PC-bus and AT-bus) for which IBM had released full specifications and
even circuit schematics. The system was far more advanced than the AT
bus, and computer manufacturers responded with the Extended Industry Standard
Architecture (EISA) and later, the VESA Local Bus (VLB). In fact, VLB
used some electronic parts originally intended for MCA because component
manufacturers already were equipped to manufacture them. Both EISA and
VLB were backwards-compatible expansions of the AT (ISA) bus.
Users of ISA-based machines had to know special information about the
hardware they were adding to the system. While a handful of devices were
essentially "plug-n-play", this was rare. Users frequently had
to configure several parameters when adding a new device, such as the
IRQ line, I/O address, or DMA channel. MCA had done away with this complication,
and PCI actually incorporated many of the ideas first explored with MCA
(though it was more directly descended from EISA).
This trouble with configuration eventually led to the creation of ISA
PnP, a plug-n-play system that used a combination of modifications to
hardware, the system BIOS, and operating system software to automatically
manage resource allocations. This required a system with an advanced programmable
interrupt controller (APIC) replacing the Intel 8259 which the PC was
born with. In reality, ISA PnP can be troublesome, and did not become
well-supported until the architecture was in its final days since the
APIC chip was a serependitious addition to ISA by the PCI standard, which
required an APIC.
PCI slots were the first physically incompatible expansion ports to directly
squeeze ISA off the motherboard. At first, motherboards were largely ISA,
including a few PCI slots. By the mid-1990s, the two slot types were roughly
balanced, and ISA slots soon were in the minority of consumer systems.
Microsoft's PC 97 specification recommended that ISA slots be removed
entirely, though the system architecture still required ISA to be present
in some vestigial way internally to handle the floppy drive, serial ports,
etc., which was why the software compatible LPC bus was created. ISA slots
remained for a few more years, and towards the turn of the century it
was common to see systems with an Accelerated Graphics Port (AGP) sitting
near the central processing unit, an array of PCI slots, and one or two
ISA slots near the end. In late 2008, even floppy disk drives and serial
ports were disappearing, and the extinction of vestigial ISA (by then
the LPC bus) from chipsets was on the horizon.
It is also notable that PCI slots are "rotated" compared to
their ISA counterparts—PCI cards were essentially inserted "upside-down,"
allowing ISA and PCI connectors to squeeze together on the motherboard.
Only one of the two connectors can be used in each slot at a time, but
this allowed for greater flexibility.
The AT Attachment (ATA) hard disk interface is directly descended from
ISA (the AT bus). ATA has its origins in hardcards that integrated a hard
disk controller (HDC) — usually with an ST-506/ST-412 interface —
and a hard disk drive on the same ISA adapter. This was at best awkward
from a mechanical structural standpoint, as ISA slots were not designed
to support such heavy devices as hard disks (and the 3.5" form-factor
hard disks of the time were about twice as tall and heavy as modern drives),
so the next generation of Integrated Drive Electronics drives moved both
the drive and controller to a drive bay and used a ribbon cable and a
very simple interface board to connect it to an ISA slot. ATA, at its
essence, is basically a standardization of this arrangement, combined
with a uniform command structure for software to interface with the controller
on a drive. ATA has since been separated from the ISA bus, and connected
directly to the local bus (usually by integration into the chipset), to
be clocked much much faster than ISA could support and with much higher
throughput. (Notably when ISA was introduced as the AT bus, there was
no distinction between a local and extension bus, and there were no chipsets.)
Still, ATA retains details which reveal its relationship to ISA. The 16-bit
transfer size is the most obvious example; the signal timing, particularly
in the PIO modes, is also highly correlated, and the interrupt and DMA
mechanisms are clearly from ISA. (The article about ATA has more detail
about this history.)
At NIXSYS, we believe in building our relationship with our customers
even after our products are delivered. Receiving your PC with ISA Bus
on time and getting them up and running is just as important as selecting
the right components. We've put together a support team that provides
attentive and informed service. Don't hesitate to contact
us with any of your questions.
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