Page 1056 - The Mechatronics Handbook

completion of data transfer does not have to be acknowledged. In asynchronous transmission mode, the
two systems are using clocks, that are not synchronized and may run at frequencies slightly out of step.
Thus, for asynchronous systems, data validation requires a separate scheme called handshaking.
Data Flow-Control

Another problem in asynchronous communication systems is the speed of data processing. If one system
is significantly slower in processing the data, a flow-control must be implemented to avoid data loss. Data
flow-control may require additional handshaking. Similar problems may arise in multitasking systems in
which, due to other tasks, the system is unable to handle incoming data during the period of high workload.

Handshaking
In order to ensure efficient transmission of data without errors, the sending system will use a separate
signal to indicate that valid data has been presented to the interface. Because the instant at which the
receiving device can process the data is not known, the sending device must wait for an acknowledgment
signal before presenting new data to the interface. The handshaking can be implemented in either hard-
ware or software.

Communication Protocol
Operation of a communication system is governed by a set of rules which must ensure reliable data transfer
without errors and data loss. Such a set of rules is called a communication protocol.

Error Handling
Data transmitted over a communication line are subjected to noise and can thus be corrupted. Since it is
essential to maintain the integrity of data, a number of different schemes for error detection have been
developed. The simplest remedy after error detection is retransmission of the corrupted data. More sophis-
ticated communication protocols can involve complex error correction schemes implemented at protocol
level.

Simplex, Half-Duplex, Full-Duplex
In its simplest form, communication can be established with a single pair of wires. The data transmission
mode, in which data can pass in one direction only, is called simplex or unidirectional channel. In most
applications it is required that the communication takes place in both directions. If the cost of the data
transmission line is high, it can be arranged that signals can pass in either direction over a single trans-
mission line using additional circuitry on both ends of the transmission line but only in one direction
at a time. This type of data communication mode is called half-duplex. Additional handshaking is re-
quired to implement the time sharing of the transmission line.
If signals can pass in either direction over a single transmission line simultaneously, the data commu-
nication mode is called full-duplex. An example of a full-duplex is a telephone line where the two channels
are created as separate frequency bands. Cost permitting, two separate transmission lines can be estab-
lished in which case the full-duplex communication is conducted over two simplex channels. This requires
duplication of all the functions of a simple data communication system as shown in Fig. 37.1.

Unbalanced vs. Balanced Transmission
Implementation of the electrical transmission line can take two basic forms, unbalanced (single-ended) or
balanced (differential). For unbalanced operation, a single conductor is used to carry the signal voltage,
which is referenced to a signal ground. The signal ground is usually common return for all signals in the
interface. Figure 37.2 shows an example of an unbalanced data transmission system with two channels
and three wires. Symbol D represents driver and symbol R receiver. Unbalanced data transmission is

©2002 CRC Press LLC