SERIAL PROTOCOLS
One main aspect of a bus is whether the data is transferred in a serial or parallel fashion. In serial mode, the bits of each character are transmitted one at a time, one after another. Contrast with parallel transmission, where the bits of a character or data are transferred simultaneously. The speed of a serial bus is generally expressed in bits per second (bps) and the speed of the parallel bus is expressed in bytes per second (Bps) or characters per second. The bits per second rate are also called as baud rate. When comparing with the parallel interface, the number of wires is reduced to one or two in serial communication. The serial approach is cost-effective, especially for long distances.
Both the serial and parallel communication has numerous modes of operation. They are:
Simplex -
The simplex mode uses a single channel or frequency to exchange information between two or more terminals. Communications is in one direction only.
Half Duplex -
The half-duplex mode has one-way flow of information between terminals. Technical arrangements often permit transmission in either direction, but not simultaneously.
Semi Duplex –
The semi duplex uses an arrangement of equipment where one terminal is simplex configured and the other uses two channels or frequencies in full duplex. A clarifying example is a ship in a simplex mode terminated full duplex with a shore station. The ship may send or receive but not do both at the same time.
Full Duplex –
The full-duplex mode is a method of operation in which telecommunications between stations takes place simultaneously in both directions using two separate frequencies. The term “full duplex” is synonymous with “duplex.”
Broadcast -
Broadcast is the type of operation in which one station transmits information on one or more channels directed to more than one station and/or unit. The broadcast system has no provision for receipt or reply; however, special arrangements may require the receiving station to reply or receipt for the message at a later time by other means.
The serial communication is internally divided into synchronous and asynchronous. In synchronous systems there is a common clock, which synchronizes the data transfer. One of the devices generates the clock, the other computer or peripheral device receives the clock. Inter Integrated Circuit (I2C) and Serial Peripheral Interface (SPI) are synchronous types of communications.
In asynchronous system, the devices involved in communication uses separate clocks. The clocks must be adjusted to an agreed frequency. Furthermore, the receiver needs an indication that the transmission will start. Logically, the sender inserts a low bit before the actual byte to advice the receiver that data bits are to come. Finally, a high stop bit completes the data frame. The long distance data transfers are comes under the category of asynchronous type of communication, eg: RS232, RS485, USB, Ethernet and CAN.
You can read about these in upcoming posts
- RS232
- RS485
- INTER-INTEGRATED CIRCUIT (I2C)
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