MODES OF TRANSMISSION
When you send a signal from one device to another, must first pass through an interface to the transmission medium. There are two basic ways to do this: in parallel or serially.
- PARALLEL TRANSMISSION
Is sending data one byte at a minimum of 8 on parallel lines through a parallel interface. The parallel interface is best known for Centronics printers. IEC-6257/IEEE-488 The bus is often found in laboratory measurement systems, is another example.
- BROADCAST SERIES
Is sending data on a bit-serial interface. Requires fewer wires than parallel transmission, but the transmission time increases the size horn string of bits to be transmitted. Examples are the RS-232 and RS-485.
- INTERFACE
It has the task of placing the digital signal generated by the device in the network transmission medium. The string of bits is mostly transmitted as alternating current or modulated amplitude modulated in frequency and phase. After reception, the signal is demodulated by electronics interface and recovers the original information. The electronic modules that perform the modulation and demodulation, are part of every interface and are built under a particular standard. Through the interface then, which is the physical link data transmission is that it sends the information, the characteristics and structure are given under a particular protocol, subject to be discussed later.
- TIMING
An interface can transmit in one of two modes, asynchronously, which basically means that transmission will occur at any time or synchronously, which means that transmission depends on a common system clock. The asynchronous transmission is particularly true for short messages and is often found in field bus systems (fieldbus). Each byte to be transmitted and packaged between a start bit and a stop. The start bit tells the receiver that what follows is a byte of data, the stop bit tells the transmission is completed.
Figure 2 asynchronous mode
Naturally the contents of the message should be more than one byte, so that agreement is required in the sequence of data sets and types of information. Asynchronous transmission requires a relatively small technical effort and can be used in virtually all situations.
In synchronous transmission, the clock of the system both the transmitter and receiver must be in phase. This requires sending the call before the start of preamble transmission. This usually involves a burst of the carrier signal to synchronize the receiver, bit patterns to synchronize the timing of the bits and then control patterns to synchronize messages.
During transmission, the synchronization character should be repeated at regular intervals. Synchronous transmission time, has more technical problems than asynchronous. Its advantage lies in yet possible to transmit large blocks of data efficiently is, with a high proportion of useful data.
Figure 3 Synchronous Mode
COMMUNICATION MODES
Another feature of digital communication between two devices is the communication mode that is the way in which they will talk to each other. There are three possible ways:
Communication simplex, for which the information flows in one direction. Confirmation of receipt of the message is not possible in this way. Examples of this are radio and television.
half-duplex communication, through which information flows in both directions, first transmits a device. At the end, the other responds. An example of this is the fax, where the communication must be first established before transmitting the message. This is the preferred mode of communication for the field.
full-duplex communication, where it can transmit and receive simultaneously. One example is the telephone conversation between two people. For communication between two machines, however, requires separate lines of transmission and reception, otherwise the information could not be decoded.
Figure 4 duplex communication modes
BAUD
The last feature of digital communication is the transmission speed. This indicates how many bits per second can be transmitted from one device to another. All network devices must operate at the same transmission rate. The maximum speed is limited by the type of nterfaz and the transmission medium used. This is also a function of cable length. This is because the probability of electromagnetic interference increases with increasing distance, but decreases with a transmission speed.
Depending on the standard baud rates from 1200 bit / s to 37.5 kbit / s are obtained relatively easily. For speeds of 1 Mbit / s or more special cables are required copper or optical fiber.
