Data Acquisition

interrupt lines (1RQ13, 8, 2, 1 and 0) are used by the system board and are not available inthe bus./IOCHKAn expansion board can assert /IOCHK (I/O channel check) to indicate that a serious errorhas occurred. Assertion of /IOCHK causes an NMI (non-maskable interrupt) to the CPU ifPort 61h bit 3 is 0 and if NMIs are enabled with Port A0h. Parity errors and uncorrectablehardware errors are examples of where expansion boards might assert /IOCHK.4.8.2 Microchannel busIn 1986 IBM included the newly released Intel 32-bit 80386 (‘386’) microprocessor into anew family of PC’s called the PS/2 systems. At the same time, and rather unexpectedly, IBMintroduced with this family of machines a new and proprietary expansion bus called themicrochannel architecture (MCA) bus.This bus provided many enhancements, which included:• Bus mastering.• Burst mode data transfers (where data is transferred a predefined block size at atime).• Bus arbitration, which permitted up to eight processors and eight other devices,such as DMA controllers, to share the single data bus without interfering witheach other.4.8.3 EISA busHowever, the MCA bus was not compatible at all with ISA bus expansion boards, the mostobvious difference being the change in connector size, layout and pin spacing (the MCAconnector is much smaller, pin configurations were rearranged and pin spacing decreased to0.050"). Moreover, there were two versions of the MCA bus slot (a 16-bit version and 32-bitversion), each with different pinouts.IBM’s refusal to co-operate with the industry to create just one standard (development ofthe EISA standard bus was already in progress), has meant that this bus was largely shunnedfrom the beginning. MCA systems have achieved limited success in the data acquisitionenvironment, due mainly to the incompatible expansion bus and the availability of highperformance, low cost, easily expandable ISA machines.The introduction of the Intel 32-bit 80386 (‘386’) microprocessor marked the first departureby the industry from following developments in the IBM PC system architectures. WhileIBM produced the PS/2 family of machines incorporating the MCA bus, the rest of theindustry produced 386-based ISA machines operating at 16, 20, 25, 33 and 40 MHz CPUspeeds. Initially these were AT clones with 386 CPUs, the split bus architecture allowing theexpansion bus to continue to operate at around 8 MHz, thus maintaining ISA compatibility,and access to the increasing range of ISA compatible expansion cards.In 1988, and after almost two years of meetings, nine manufacturers collaborated toproduce the extended industry standard architecture (EISA) bus specification, in directopposition to IBM's MCA bus. This published standard encompassed all of the ISA features,almost all of the MCA enhancements and added some new features while maintaining thebackward compatibility with existing expansion boards. The EISA bus is a full 32-bit dataand address bus.