In The Beginning

The Industry Standard Architecture or ISA bus began as part of IBM's revolutionary PC/XT released in 1981. However, it was officially recognized as "ISA" in 1987 when the IEEE (Institute of Electrical and Electronics Engineers) formally documented standards governing its 16-bit implementation.

This first XT bus was intended to allow the addition of system options which could not be fit onto the motherboard. This XT bus was completely under the microprocessor's direct control, and its addressing width was limited to the 8-bit level of the processor. To make the bus useful, control lines were added to signal interrupts for input/output ports. Bus speed was also limited to match the processor. The PC/XT's 8088 was a one-byte wide 4.77 MHz processor. Thus the XT bus, which required two clock cycles for data transfer, was limited to an excruciatingly slow (by today's standards) 2.38 Mbps, that could be curtailed even further if the system was busy with other tasks.

Quatech's first data communication adapters were designed for the PC/ XT, and some of these are being used in older systems running extremely simple, low-speed applications. However, the ISA bus has come a long way since 1981, and its modern incarnation is much better suited to the PCs we use today and the applications we run on them.

Modern 16-bit ISA

This "modern" ISA bus emerged in 1984 when it became clear to IBM developers that the advances made on the processor front had rendered the XT bus archaic. The first of these new processors--a 286--was designed to run at 8MHz with a full 16-bit data bus. IBM's PC/AT used this processor and contained an 8MHz bus to match it. Still limited by the two-clock-cycle data transfer, the new AT bus can reach speeds of only 8Mbps. The AT bus provides 16 data lines and 24 address lines, thereby taking full advantage of the 16-bit addressing limit of the microprocessor. This improvement over the XT expands a PC's capability to accept add-in boards by including additional interrupts, and DMA (direct memory access) channels. The ISA system however, does not have a central registry from which to allocate system resources. Consequently, each device behaves as though it has sole access to system resources such as DMA, I/O ports, IRQs, and memory. Obviously, this can cause problems when using multiple add-in boards in a single system.

Another problem is caused by the limited number of available ports and interrupts on the system. Quatech has solved this problem for the ISA bus by developing a series of drivers which allow multiple devices to share the same interrupt. These drivers are available for DOS, OS/2, Windows 3.1, 95/98/ NT and Windows 2000/XP. They permit any of Quatech's multi-port ISA serial adapters to share interrupts amongst themselves and with any other Quatech ISA serial device. To avoid confusion, this driver contains a register which will indicate the source of any interrupt.

Backward and Forward Compatibility

IBM's PC/AT was designed with an expansion bus which not only provided for taking advantage of the new technology, but which also remained compatible with the older style 8-bit XT add-in boards. Anticipating that advances in processors would again out pace advances in bus technology, the PC/AT was designed with two separate oscillators. This way the microprocessor and expansion bus could be run on different clocks with different speeds. Therefore, a computer running a newer processor with 33MHz clock speed could also run its expansion bus at an 8MHz clock rate. This forward-looking IBM design is largely responsible for the dominant role the ISA bus has played in the market for the past 15 years. Indeed, until very recently, ISA expansion slots were the only kind available for data communication expansion boards installed in desktop PCs. But, it comes as no surprise that people running high speed PCs began to balk at having their expansion boards limited to 8 Mbps. IBM attempted to remedy this situation with the release of MicroChannel in 1987 (MCA Overview), however it never caught on.

Not until the introduction of the 33MHz PCI bus in 1992 did expansion boards come close to matching processor speeds. (See more on PCI.) It is enthusiasm for the potential 1Gbyte/sec+ speeds of PCI which is driving the rapid replacement of ISA slots with PCI slots in today's PCs. It is very likely that ISA slots will soon disappear from new PCs altogether, and users will be forced to choose PCI expansion boards not because they are faster, but because they will be all the PC is able to accommodate.

ISA For Data Communication Applications

Theoretically PCI blows ISA out of the water, however in practice there is no speed difference between running many serial communication peripherals using a PCI rather than an ISA bus. (Though the PCI advantage is obvious for high-speed devices such as video cards.) This is because of limitations inherent in the serial communication protocols (see discussion of serial communication). Thus Quatech's DSC-100 two port RS-232 serial PCI board has the same maximum baud rate as does our DS-100S two port RS-232 enhanced serial ISA adapter--921.6 kbps, well below the maximum 8 Mbyte/sec ISA limitation. ISA cards are more cumbersome to install than other cards because I/O addresses, interrupts and clock speed must be set using jumpers and switches on the card itself. The other bus options which use software to set these parameters are called Plug & Play. While there is nothing inferior about using jumpers and switches, it can be more intimidating for novice users.

The bottom line is that there is no reason to convert your current ISA serial communication systems to PCI, as ISA will provide equivalent functionality, generally at a lower price. However, if you are starting a new installation using a PC with few or no (as is increasingly the case today) ISA slots, or you prefer using Plug & Play cards, then you should consider using PCI adapters.

ISA Specs
Bus Clock Signal		8 MHz
Bus Width		16-bit
Theoretical Max. Transfer Rate		8 Mbytes/sec (64 Mbits/sec)
Advantages		low cost, compatibility, widely used
Disadvantages		low speed, Jumpers & DIP switches. becoming obsolete

Click here to see how ISA compares with other busses.