a small interface card that plugs into the host computer’s motherboard. This interface offers transfer rates of up to 1 MB and can handle hard drives with a maximum capacity of 300 MB. Enhanced Integrated Drive Electronics (EIDE) The  Enhanced  Integrated  Drive  Electronics  (EIDE) was developed from the IDE standard. New features available  with  EIDE  include  Plug-n-Play  compatibility, increased   maximum   drive   capacity,   faster   data transfers, and the ability to use a CD-ROM or tape drive with an the interface. The IDE interface can address a hard drive with a maximum of 504MB. EIDE increases the maximum size of a hard drive by using an enhanced BIOS. The enhanced  BIOS  uses  a  different  geometry  when communicating with a program than it does when communicating with the hard drive. For example, the BIOS will tell a program that a hard drive with 2,000 cylinders and 16 heads is a drive with 1,000 cylinders with  32  heads.  The  BIOS  controls  the  address translation to keep track of where the data is physically located on the hard drive. The   EIDE   interface   uses   a   Programmed Input/output (PIO) mode to transfer data from the drive. There are five PIO modes that can be set to control data transfers. PIO Mode 0 is the slowest with a cycle time of 600 nanoseconds. Pio Mode 4 has a cycle time of 120  nanoseconds,  which  is  16.6  megabytes  per  second. Most high-end hard drives will support Mode 3 or Mode 4 operations. Using the enhanced BIOS, the hard disk responds   to   the   Identify   Drive   command   with information concerning the PIO and DMA modes the drive can support. The BIOS will automatically set the PIO mode to match the capability of the drive. If a drive is set to a higher mode than it is capable of supporting, data corruption will occur. I/O SERIAL DATA OPERATIONS Serial data operations exchange information via a single path, line, or wire. The channel/port itself is made up of several wires, but only one is used to transfer the  binary  data.  Bidirectional  channels  may  use  two wires for data, one for each direction or a single tristate bidirectional  line.  The  remaining  wires  are  used  for device addressing and to provide the  protocol  (channel control) for information exchange. The data is in the form of an asynchronous or synchronous bit stream. The bit stream is made up of a sequential series of data and/or control pulses in one of these two mutually 7-28 exclusive formats. Serial data operations can use a minimum of 4 conductors and up to 37 conductors to perform  serial  data  operations.  Serial  operations generally   exchange   information   between   data communications  equipment  (DCE)  and  data terminal equipment (DTE). The DCE configured device is considered the controller for the interface. The   DTE   is   either   the   computer   or   a   channel controller. There are variations in the channel pin connections  that  depend  on  the  device  mode  of operation (DCE or DTE). Asynchronous Data Exchanges Asynchronous  data  is  also  known  as  character framed data;  only one character at a time is sent. Each character is composed of either 7 or 8 bits (depending upon the coding scheme used), and is identified by a start and stop bit. At the minimum, each character is preceded by a start bit and followed by one stop bit. Asynchronous data transmission protocol allows for a maximum of the following in sequence: one start bit, eight data bits, a parity bit, and one stop bit for each character to be exchanged. The purpose of a start bit is to notify the modem that a character is being sent (or received). The bits that make up the character immediately follow the start bit. After all these bits have been transmitted, a stop bit is inserted to indicate the end of the character. Start and stop bits can immediately follow one another or there can be a period of idle time following the stop bit, depending upon the hardware device in use. During an idle  condition,  in  which  no  characters  are  sent,  a continuous MARK signal (equivalent to a logic 1) is transmitted   for   one   bit   time. Asynchronous transmission is normally used when transmission rates are between 600 to 2000 bps. The particular format used varies between computers and may be hardware or software controlled depending on the type of interface logic and devices used. Synchronous Data Exchanges When   more   speed   is   required   for   sending information,  synchronous  data  exchanges  fulfill  the requirement. Synchronous data is also known as message framed data.  The bit stream is divided into blocks  of  sequential  bits  grouped  into  individual messages, without the need for start and stop bits. Again, each character is composed of either 7 or 8 bits. There are two methods for controlling the exchange of messages. External control and timing signals may be