compatibility permits the computer and the peripheral
units to communicate intelligently. The compatibility
involves logic levels, timing or speed, and control.
When digital data is transmitted between two
units, the binary voltage or current levels must be
compatible. Logic-level conversion is often required
to properly interface different types of logic circuits.
For example, logic-level shifting is often required to
properly interface bipolar and MOS circuits. The
speed of the data transmission must also be
compatible. Some type of temporary storage between
the two units may be required as a buffer to match the
high-speed CPU to a low-speed peripheral unit.
Control is another function of the interface.
There are status lines that tell when the computer or
peripheral unit is ready or busy, and strobe lines that
actually initiate the data transfers. This process is
often referred to as handshaking.
The type of information exchanged between the
I/O unit and the peripheral devices includes data,
addressing, and control signals. Since multiple I/O
units can usually be connected to a computer, some
coding scheme is required to select the desired unit.
This is usually done with a binary word used as an
address. The address is transmitted to all the
peripheral devices. The unit recognizing the address
is connected to the I/O section. Data can then be
transmitted to or from the device over the
interconnecting data lines. The actual data transfers
are controlled by control signals between the two
Programmed data transfers that take place as the
result of executing an I/O instruction usually cause
the data to be transferred between the peripheral unit
and the accumulator register in the CPU. Other CPU
registers may also be used, depending upon the
computer architecture and the instruction. In some
computers, peripheral units are addressed as storage
locations, and all memory reference instructions can
be used in performing I/O operations. No special I/O
instructions are used in these computers.
PARALLEL VERSUS SERIAL DATA
There are two methods of transmitting digital
These methods are parallel and serial
transmissions. In parallel data transmission, all bits
of the binary data are transmitted simultaneously. For
example, to transmit an 8-bit binary number in
parallel from one unit to another, eight transmission
lines are required. Each bit requires its own separate
data path. All bits of a word are transmitted at the
same time. This method of transmission can move a
significant amount of data in a given period of time.
Its disadvantage is the large number of
interconnecting cables between the two units. For
large binary words, cabling becomes complex and
expensive. This is particularly true if the distance
between the two units is great. Long multiwire cables
are not only expensive, but also require special
interfacing to minimize noise and distortion
Serial data transmission is the process of
transmitting binary words a bit at a time. Since the
bits time-share the transmission medium, only one
interconnecting lead is required.
While serial data transmission is much simpler
and less expensive because of the use of a single
interconnecting line, it is a very slow method of data
transmission. Serial data transmission is useful in
systems where high speed is not a requirement. Serial
data transmission techniques are widely used in
transmitting data between a computer and its
peripheral units. While the computer operates at very
high speeds, most peripheral units are slow because of
their electromechanical nature. Slower serial data
transmission is more compatible with such devices.
Since the speed of serial transmission is more than
adequate in such units, the advantages of low cost and
simplicity of the signal interconnecting
PARALLEL DATA TRANSMISSION
line can be
In a parallel data transmission system, each bit of
the binary word to be transmitted must have its own
data path. There are a variety of ways to implement
this data path.
The two basic classifications of
transmission line circuits are single-ended and
balanced. Single-ended transmission systems use a
single-wire data path for each bit. When combined
with a ground or return reference, the electrical circuit
between the sending circuit and the receiving circuit
is complete. In a balanced transmission line system,
two conductor cables are used to send the data. The
data on the dual-transmission line is complementary.
The dual-transmission lines also use a ground return
reference. While a single-ended transmission line is
simpler and less expensive, it is subject to more noise
problems than the balanced or dual-transmission line
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