RS-232

Necessary knowledge: [HW] Controller module, [AVR] USART, [LIB] Serial Interface, [LIB] Alphanumeric LCD

Cable RS-232. Left plug is „male” and the right one is „female” plug.“

RS-232 is a standard of physical data interface, which is used for delivering binary data. The standard is used mainly in serial ports of computers, which are also called “COM” ports in everyday language. Nowadays is the RS-232 largely replaced by USB interface, but due to its simplicity it is still used very successfully in hobby applications, especially if there are USB – RS-232 converters. The RS-232 standard determines the plugs, electrical parameters and meanings of the signals but not the protocol.

The RS-232 interface is used mainly with UART hardware data transmission module which protocol is standardized, but it does not determine the plugs or other things. So the RS-232 enhances the UART. Since the UART is usually one module of periphery of the microcontroller, which digital input-output does not correspond to the electrical parameters of the RS-232, they are connected together with a special leveling-converter. One best known leveling-converters between RS-232 and TLL/CMOS is MAX232.

UART means universal asynchronous receiver/transmitter. USART is almost the same, with the difference that the data is sent with clock signal. The UART may be called also a serial interface. The serial interface is a data transferring mechanism, where each bit is transmitted one by one. For example, to transmit 1 bait, 8 bits are transmitted with certain interval. This means that on the serial interface line, which is one pin of the microcontroller, the value of voltage is changed after certain time, once low and then high. Usually there are two devices connected to the serial interface. One is transmitting the information (by changing the value of the pin) and the other is receiving it (by registering the value of the pin). Transmitting pin is TX, and receiving pin is RX. The info is moving always to one direction on one line. For sending data to the other direction an other line is used. If data is moved on two lines at the same time, it is called full duplex bus.

the frame of UART, where S is start-bit , 0-7 are data-bits, P is parity-bit (if existing) and T is stop-bit (or 2).

Transmitting data is done by frames of the UART interface, in which is 5-9 data bits (depending on the configuration). Most common is 8 bits (1 bait). In addition to the data bits also extra bits are transmitted with the frame, which are used to recognize the moments of arrival and ending of the data on the receiver’s side. The first is called start-bit and it is always 0. The second is called stop-bit (or bits), which is always 1. Before the stop-bit also parity bit may come. It is use to control regularity. The parity-bit shows whether in the amount of the data-bits is odd or even number of ones. Which reading it has depends on the configuration of the UART interface. The parity-bit is usually not used anymore and it can be banned in configuration. Like the parity-bit can be configured, also can the amount of data-bits and stop-bits.

In addition to the frame structure, there is one more important parameter – it is baud rate, with which the number of transmitted symbols in one second is determined. Baud shows the number of symbols. When we are dealing with UART then 1 baud is 1 bit and that is why we talked about bits when we were talking about frame. Basically it does not matter which baud rate is used for data transmitting, but there is a certain amount of commonly used baud rates, which should be used. For example: 9600 bps, 19200bps, 38400 bps, 57600 bps, 115200 bps;

Furthermore, it is worth to know that the RS-232 standard includes in addition to the data-signals (RX, TX) also data flow control pins DTR, DCD, DSR, RI, RTS and CTS, which are used for controlling the communication between the devices. For example they can be used to notify whether it is ready to receive data or not. Since the RS-232 interface’s original goal is to connect the computers to a modem, some signals are (were) useful rather for showing the state of the telephone lines.

The Controller module board is equipped with one RS-232 type male plug. Through that can controller be connected to computer or to an other controller. For connecting to a computer a usual not inverted cable must be used, which one end is male and other one is female. For connection to an other controller a cable must be used where RX and TX and current control signals are perpendicularly inverted and both plugs are female. The inverted cable is also called zero modem cable. The following is an example program of using UART serial interface. When the program is started, it transmits a welcome through a RS-232 interface and displays messages, which are received. LCD and USART libraries are used.

//
// Connecting the Controller module of the HomeLab to a computer through RS-232.
// The example is using digital input-output module with LCD.
// The text inserted in the terminal of the computer is displayed on the LCD.
//
#include <homelab/usart.h>
#include <homelab/module/lcd_alpha.h>
 
//
// Determining USART interface.
//
usart port = USART(0);
 
//
// Main program
//
int main(void)
{
	char c;
	unsigned char row = 1;
 
	// The set-up of the USART interface.
	usart_init_async(port,
		USART_DATABITS_8,
		USART_STOPBITS_ONE,
		USART_PARITY_NONE,
		USART_BAUDRATE_ASYNC(9600));
 
	// The set-up of the  LCD.
	lcd_alpha_init(LCD_ALPHA_DISP_ON_BLINK);
 
	// Displaying welcome message on the screen.
	lcd_alpha_write_string("Waiting for the message");
 
	// Putting the cursor in the beginning of the second row.
	lcd_alpha_goto_xy(0, row);
 
	// Saying hello to the computer.
	usart_send_string(port, "Hello, write something!\r\n");
 
	// Endless loop
	while (true)
	{
		// Reading the sign from the serial interface.
		if (usart_try_read_char(port, &c))
		{		
			// Are we dealing with the sign of changing the row?
			if (c == '\r')
			{
				// Changing the row.
				row = 1 - row;
 
				// Emptying the row from the previous message.
				lcd_alpha_clear_line(row);
			}
			else
			{
				// Issuing the sign directly to the screen.
				lcd_alpha_write_char(c);
			}
		}
	}
}

The window of the HyperTerminal

With Windows XP OS comes a program called HyperTerminal. It is opened from the Start menu by selecting Accessories → Communications → HyperTerminal. Select 9600 bps, 1 start-bit and 1 stop-bit without parity- and stream-control for configuration. When the HyperTerminal is opened during the time when the microcontroller is starting, there will be a welcoming message on the display. The letters inserted through the window are displayed in the alphanumerical LCD. By pressing Enter button the row is changed on the LCD.