Ask A Scientist© Computer Science Archive

Index Key:   CSI001
Author:      SJW
Subject:     PC Questions. PC, EISA, LOCAL BUS, QIC-80
Text:        I have some questions about PC's.  What is EISA, LOCAL BUS, and 
Micro Channel?  For QIC-80 Tape Driver, should I remove one of FD if I want to 
add a QIC-80 Driver.  If I have an ESDI HD, could I add a SCSI HD or Syquest 
HD?  For SCSI control, what is Direction Kit, master Kit, SVP, and DNK?

Response #: 1 of 1
Author:     Chris Baker
Text:       This will be a multi-part answer.  ISA, EISA, MCA, and Local Bus:  
All of these are 'BUSes'.  On the inside of a computer, data must flow between 
I/O devices (hard disk, video, etc.), memory (special I/O device or mass 
storage), ROM (hard coded instructions), and the CPU.  Data is passed between 
these devices via a 'BUS.'  When a PC is purchased, many times the processor 
will be referred to as 16 bit or 32 bit.  For example, a 386dx is a 32 bit 
processor.  This means it internally operates at 32 bits and can also access 
memory at 32 bits.  This memory addressing   is done over a 32 bit 'BUS.'  The 
memory bus is dedicated for just accessing memory.  ISA (Industry Standard 
Architecture), EISA (Extended Industry Standard Architecture), and MCA 
(MicroChannel Arch.) refer to a different type of bus called the I/O bus.  
Usually, all peripherals on a PC are attached to the I/O bus or the expansion 
bus.  Items like the disk controllers, video, and communication ports 
traditionally plug into the expansion bus.  The key to ISA, EISA, and MCA is 
data size and compatibility.  ISA is the most common.  The original PC's 
(XT's) were built by IBM using an 8 bit bus.  As their popularity grew, 
companies began to make clones.  Then Intel released the 16 bit 286 chip, and 
IBM and its competitors  developed the 16 bit bus.  These IBM compatible 
companies created a standard called the ISA standard which basically means a 
16 bit bus.  Intel then came out with the 32 bit 386dx chip, allowing for the 
expansion of the bus.  (NOTE: Some machines have 386dx chips, but use the ISA 
16 bit bus.)  Up to this point, the companies selling compatibles were really 
compatible.  A device that could plug into an IBM, could plug into a 
compatible.  These compatible companies developed EISA, a 32 bit bus standard.  
The problem is that IBM went its own way and developed the MCA, its 
proprietary 32 bit bus.  No big deal, right?  Wrong!  EISA accepts ISA boards.  
In other words, if you have a video board in a  286 (ISA) machine, it will 
work in an EISA machine.  But, MCA is not compatible with any of its previous 
buses (ISA or EISA).  Because of this and the fact that only IBM and its 
closest companies were making components for MCA, prices were not driven down 
by competition. Meanwhile, EISA could use the huge selection of ISA devices 
already available.  Importantly, the EISA standard was written by a group of 
companies instead of one.  This has made EISA pricing a little  more 
competitive.  Recently, IBM has released a line of computers that have used 
the ISA and EISA, I believe, that are competitively priced to compatibles.
LOCAL BUS: Video has, in past systems, used the expansion bus to transport its 
data and the CPU to do its processing.  LOCAL BUS is a bus that is designed to 
help relieve the bottleneck of data flow over the expansion bus by creating 
its own video bus.  I believe this bus has the ability to access memory 
directly, without burdening the CPU.  This step is importanttoward creating a 
more user friendly Graphical User Interface (GUI) - OS/2 and Windows - which 
take a lotof processing for video.  As for the Quarter Inch Cartridge (QIC) - 
80 driver: That depends on what type of controller it requires.  You can get 
all types: IDE, SCSI, Parallel, ...  This is really specific for the type of 
QIC you get.  SCSI and ESDI Hard disk: You can add an SCSI drive if you have 
an ESDI, but you need to have a new controller for the SCSI.  The SCSI drive 
will not work with a ESDI controller.  Syquest: Usually Syquest HD are SCSI 
devices.  If you add an SCSI controller, you can have up to 7 SCSI, depending 
on the controller (some only allow 2 devices).  An ST506, IDE, and ESDI hard


disks all only allow a max of 2 of the same devices per controller.  A SCSI 
controller will allow a max of 7 devices for one controller.  But, be careful, 
not all SCSI devices work on all controllers and not all controllers will 
allow 7 devices.  As for the other SCSI questions, I need more information.  
Where or what are these related to?  Setup of a SCSI device?  I am not 
familiar with these terms.