The  SCSI  interface  uses  one  50-pin  ribbon  cable  to connect  the  hard  disk  drive(s)  to  the  controller  card mounted  on  the  host  computer.  Some  computer manufacturers  include  the  SCSI  electronics  in  their motherboards  and  do  away  with  a  separate  controller card. RS-232 (EIA RS-232 and MIL-STD 188) An RS-232 interface uses a serial format. It can be used  for  asynchronous  and  synchronous  serial  transfers. It can be used with mainframes, minicomputers, and microcomputers for communication with external equipments, particularly with microcomputer systems. RS-232  channels/ports  are  capable  of  transmitting  from 50 to 19,200 baud of 7- or 8-bit asynchronous characters and 7- or 8-bit synchronous characters to 9600 baud. RS-232 limits cable transfers to 50 feet with a maximum transmission  speed  of  20,000  bits  per  second.  In microcomputers  and  their  external  equipments,  the configuration of the channel/port is normally hardware controlled  through  the  use  of  DIP  switches.  The number of bits per character (7 or 8), baud rate (110, 300, 600, 1200, 4800, 9600, or 19200), parity setting (odd,  even,  or  no  parity),  and  protocol  selection (ready/busy   or   X-ON/X-OFF)   are   examples   of controlled configuration parameters. Some computer systems allow for software control of these parameters but most peripherals that accept the RS-232 have a DIP switch configurat.ion to make them compatible with a variety of computer interfaces. RS-232 serial channel/port uses a 25-pin cable connector (DB-25) and transmits signal levels of +5 to +25 volts (HIGH or SPACE) and -5 volts to -25 volts (LOW or MARK). An RS-232 receives and recognizes transition difference of 6 volts (+3 volts and -3 volts) (fig. 7-19). A positive difference and more than  +3 Figure   7-19.—RS-232. volts indicates a HIGH and a negative difference and more   than   -3   volts   indicates   a   LOW.   Signal designations are discussed in serial I/O operations. An interface   that   uses   RS-232   interface   signals   is VACALES  (Variable Character Length Synchronous). It is synchronous to 32,000 baud transferring 1 to 16 bits. RS-422 (EIA RS-422) The RS-422 interface uses a serial format. RS-422 uses  RS-232  functional  specifications.  RS-422  uses two separate wires to allow transmission at a higher rate. This technique, called balanced circuitry, doubles the number of wires in the cable, but permits very high data rates  and  minimizes  the  problem  of  varying  ground potential. The high data rates include up to 10 megabits per second in distances of meters and 100 kilobits per second  at  1.2  kilometers. RS-422    grounding requirements are much less critical than RS-232. With the elimination of the grounding problem, the receiver transition period is narrower: .4 volt (+.2 volt and -.2 volt). Token  Ring  (IEEE  802.5) Token ring is used for work group solutions and work station intensive networks. It transfers serial I/O data. It has the ability to operate at a 4- or 16-megabits per second rate of data communication. It allows PCs and mainframes to operate as peers in the same network. In a token-passing ring network, a stream of data called a token circulates  through  the  network  stations  when they are idle. A station with a message to transmit waits until it receives a free token. It then changes the free token to a  busy token, and transmits a block of data called a frame immediately following the busy token. The frame contains all or part of the message the station has to send. The system does not operate by having one station accept a token, read it, and then pass it on. Instead, the stream of bits that make up a token or message might pass through as many as three stations. Once a station becomes a busy station, there is no free token on the line. That means other stations must wait until the receiving  station  copies  the  data  and  the  frame continues around the ring until it completes a round-trip back to the transmitting station. This guarantees that only one station at a time transmits data. A typical token   ring   (fig.   7-20)   provides   for   unlimited expandability  by  use  of  multistation  access  units (MAUs) and hubs (concentrators). 7-24