We Owe A Debt To Early Computer Bus Development
       By: Maxwell Rubin
Posted: 2007-04-03 09:06:55
Why was there such a rush in the development of the vintage computer bus?All in all there were a number of ongoing improvements. These included. 32 bit widths. Bus mastering. Less susceptible to noise in that they were "quieter' in signal transmission. More convenience of setup of add on boards via software.The ISA bus came in only 8 bit and 16 bit formats. Whereas the later 386 and 486 chips , in both the DX and SX formats while they did have a 32 bit path were hobbled by the ISA bus. As a result they could never realize their true 32 bit speed potential. Later buses such as the MCA and EISA busses were able to overcome these inherent limitations.Thee "data highways" referred to as ": busses" are the data transmission lines around the PCs. The bus serves as the path for information transmission around the PC. True this routing is controlled by the CPU. However as that point in vintage computing history this was not an issue. The PCs were stand alone single CPU units.However as time went on and PCs got faster and more complicated with less expensive additional CPUs handling other tasks within the PCs events became dicer.Onboard peripherals themselves began to have built in CPUs. CPUS in different manners began to be found in such peripherals as hard drives, sound and video cards.The overall computer system may become much more efficient if these in essence peripheral CPUs can communicate directly with each other. without having to use the main CPU as an intermediary. Hence MCA and EISA were developed with these roles in mind. The concept became known as "bus mastering".Bus mastering involves the concept that the peripheral CPUS could request permission to take over the bus for a short period of time. The main CPU would grant permission for them to take over the bus, and it would temporarily drop "out of the loop", enabling swift communications between for example the hard drive and floppy disk drive.As computer busses developed and had the inherent abilities to transfer more and more data in a given time period noise became an issue. The ISA bus was fairly noise prone because it relied on triggered interrupts. Whenever the voltage level on the data line of the bus exceeded a given threshold value then "Edge Triggering" would result.The alternative to this situation where "Edge Triggering" could result is "level triggering" where it is required that the transmitting hold and archive the higher voltage level in order for data to be recognized by the devices on the bus. Edge Triggering however can lead to "transients" - that is brief power surges that can confuse the devices on the bus into thinking that data is on the bus when it is not. Luckily level triggering lowers the noise level and both MCA and EISA employ it.As a result of all of these inherent benefits MCA and EISA came to support the idea, which we know take for granted, of instant software configurations. There were no switches or jumpers on add in MCA or EISA boards. Although we take plug and play instant configuration of mother boards and peripherals such as sound, video or network cards for granted it was not always that way. We owe a lot to these early computer innovations of improving the computer bus.
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