The concept of serial communication is simple, in serial communication data is sent one bit at a time. Serial communication requires only one line for the data; second line for ground & possible a third line for clock. Although this is slower than parallel communication, which allows the transmission of an entire byte at once, it is simpler & requires less physical wires for communication so can be used over longer distances. IEEE 488 specifications for parallel communication state that the cabling between equipment can be no more than 20 meters total, with no more than 2 meters between any two devices; serial, however, can extend as much as 1200 meters. Synchronous and asynchronous transmissions are two different methods of transmission. In Synchronous transmission the transmitter & receiver are synchronized through external clock, while in asynchronous transmission the transmitter & receiver are synchronized by special signals along the transmission medium. SYNCHRONOUS TRANSMISSION Figure 1 shows the block diagram of Synchronous transmission. As you can see a common clock line is required between the transmitter & receiver.
Figure 1 ASYNCHRONOUS TRANSMISSION In asynchronous transmission since the transmitter & receiver are synchronized by special signals along the transmission medium there is no need of the clock signal. Asynchronous transmissions is advantageous over long distance, moreover it is easy to set up & well suited for application where messages are generated at irregular intervals. There is no clock pulse so the receiver must know at what rate the data is transmitted (Baud Rate). It should also know the number of bits per word (DATA BITS) the transmitter is using as well as it should know when the data begins (START BIT) & where the data stops (STOP BIT).All the above parameter make up the PROTOCOL. If the receiver & transmitters protocol is the same then receiver will receive the correct data. BAUD RATE It indicates the number of bits transferred per second. So when we say 9600 BAUD RATE it means 9600 BITS per second. (Please note its is Bits per Second not Bytes per Second)
DATA BITS It indicates the number of bits in a Transmission WORD. In asynchronous communication other that the data which is normally 8-bit at least two more bits i.e. START bit & STOP bits are transmitted. So the actual number of bits transmitted is ten. START and STOP BITS The receiver should know when the data transmission has started; this is indicated by the START BIT. In most protocols the start bit is LOGIC 0. So when no data is being transmitted the data line is HIGH. And the receiver waits for the 1 to 0 transition & once this transition occurs the receiver knows that the data is being transmitted. The receiver knows the baud rate as well as the number of bits to be received i.e. eight so it reads the eight bits after the start bit & then checks if the next bit is a STOP BIT which is basically a LOGIC 1. If the last bit is not LOGIC 1 then it assumes error in communication & then dumps the data. RS 232 Standard RS232 is a most basic & the oldest form of UART. It is basically used for POINT TO POINT Communication. Since transmit & receive lines are different it can operate in full duplex mode. The RS 232 standard defines different voltage level corresponding to Logic 1 & Logic 0. Logic 1 is defined as negative voltage & Logic 0 is defined as positive voltage. Valid voltages are in range of plus or minus 3 to 15 Volts for Logic 0 & Logic 1 correspondingly. Since the voltage level is higher than the used in the digital circuit’s special driver circuit are required to convert the logic levels.
HANDSHAKING Communication can occur even without the handshaking. Handshaking increases the number of lines between the two devices but if any one of the devices is busy with some other task it can to stop the transfer & resume only when it finishes its task MAX 232 The Max 232 is a dual RS-232 receiver / transmitter that meets all EIA RS232C specifications while using only a +5V power supply. It has 2 onboard charge pump voltage converters which generate +10V and –10V power supplies from a single 5V power supply. It has four level translators, two of which are RS232 transmitters that convert TTL\ CMOS input levels into + 9V RS232 outputs. The other two level translators are RS232 receivers that convert RS232 inputs to 5V TTL\CMOS output level. These receivers have a nominal threshold of 1.3V, a typical hysterics of 0.5V and can operate up to + 30V input. 1. Suitable for all RS232 communications. 2. +12V power supplies required. 3. Voltage quadruple for input voltage up to 5.5V (used in power supply Section of computers, peripherals, and modems). Three main sections of MAX232 are 1. A dual transmitter 2. A dual receiver 3. + or - 5V to + or - 10V dual charge pump voltage converter.
PIN DIAGRAM OF MAX 232
TRANSMITTER SECTION: Each of the two transmitters is a CMOS inverter powered by + or 10V internally generated supply. The input is TTL and CMOS compatible with a logic threshold of about 26% of Vcc. The input if an unused transmitter section can be left unconnected: an internal 400KΩ pull up resistor connected between the transistor input and Vcc will pull the input high forming the unused transistor output low. The open circuit output voltage swing is guaranteed to meet the RS232 specification + or - 5v output swing under the worst of both transmitter driving the 3KΩ . Minimum load impedance, the Vcc input at 4.5V and maximum allowable ambient temperature typical voltage with 5KΩ and Vcc = + or - 9 v. The slow rate at output is limited to less than 30V/µ s and the powered done output impedance will be a minimum of 300Ω with + or - 2V applied to the output with Vcc = 0V.The outputs are short circuit protected and can be short circuited to ground indefinitely.
5.1.3 RECEIVER SECTION The two receivers fully conform to RS232 specifications. They’re input impedance is between 3KΩ either with or without 5V power applied and their switching threshold is within the +3V of RS232 specification. To ensure compatibility with either RS232 IIP or TTL/CMOS input. The MAX232 receivers have VIL of 0.8V and VIH of 2.4V the receivers have 0.5V of hysterisis to improve noise rejection. The TTL\CMOS compatible output of receiver will be low whenever the RS232 input is greater than 2.4V. The receiver output will be high when input is floating or driven between +0.8V and –30V.