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
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 configuration 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
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
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.
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
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).