Industry Standard Architecture - ISA Bus Architecture

ISA Bus Architecture

The PC/XT-bus is an eight-bit ISA bus used by Intel 8086 and Intel 8088 systems in the IBM PC and IBM PC XT in the 1980s. Among its 62 pins were demultiplexed and electrically buffered versions of the eight data and 20 address lines of the 8088 processor, along with power lines, clocks, read/write strobes, interrupt lines, etc. Power lines included -5V and +/-12 V in order to directly support pMOS and enhancement mode nMOS circuits such as dynamic RAMs among other things. The XT bus architecture uses a single Intel 8259 PIC, giving eight vectorized and prioritized interrupt lines. It has four DMA channels originally provided by the Intel 8237, three of the DMA channels are brought out to the XT bus expansion slots; of these, two are normally already allocated to machine functions (diskette drive and hard disk controller):

The PC/AT-bus, a 16-bit (or 80286-) version of the PC/XT bus, was introduced with the IBM PC/AT. This bus was officially termed I/O Channel by IBM. It extends the XT-bus by adding a second shorter edge connector in-line with the eight-bit XT-bus connector, which is unchanged, retaining compatibility with most 8-bit cards. The second connector adds four additional address lines for a total of 24, and eight additional data lines for a total of 16. It also adds new interrupt lines connected to a second 8259 PIC (connected to one of the lines of the first) and four 16-bit DMA channels, as well as control lines to select 8 or 16 bit transfers.

The 16-bit AT bus slot originally used two standard edge connector sockets in early IBM PC/AT machines. However, with the popularity of the AT-architecture and the 16-bit ISA bus, manufacturers introduced specialized 98-pin connectors that integrated the two sockets into one unit. These can be found in almost every AT-class PC manufactured after the mid-1980s. The ISA slot connector is typically black (distinguishing it from the brown EISA connectors and white PCI connectors).