FMA Unit 4.6 RS 232

RS-232

·        In communication, RS232, (Recommended Standard 232) is standard introduced in 1960 for serial communication transmission of data.

• An RS232 serial port was once a standard feature of a personal computer, used for connections to modems, printers, mica, data storage,   uninterruptible power supply, and other peripheral devices.
•  In RS232 standard a bit 0 is represented by +3 to +25 V, while a bit 1 is represented by -3 to -25 V.
• Hence, for connecting any RS232 to micro-controller voltage converters like MAX232 are required.
• MAX232 is a line driver (Voltage converter) that converts TTL logic level to RS232 voltage level and vice versa.
• This communication is useful for point to point communication at slow speed.   Ex. COM1 port in PC can be used for mouse, COM2 port in PC can be used for modem.

RS232 male connector

Pin Description DB-9 Connector:

PIN No.	Pin Name
1	DCD (Data Carrier Detect)
2	RxD (Received Data)
3	TxD (Transmitted Data)
4	DTR (Data Terminal Ready)
5	GND (Signal ground)
6	DSR (Data Set Ready)
7	RTS (Request to Send)
8	CTS (Clear to Send)
9	RI (Ring Indicator)

Signal Specifications

RS232 works on the two-way communication that exchanges data to one another. There are two devices connected to each other, (DTE) Data Transmission Equipment & (DCE) Data Communication Equipment.

The Data Terminal Equipment (DTE) wants to send data to Data Communication Equipment (DCE). The following signals are used.

        1.     DTR (Data Terminal Ready)

·        The DTE alerts the DCE for communication by sending this signal.

        2.     DSR (Data Set Ready)

·        The DCE send this signal to the DTE to inform DTE that DCE prepares itself for the communication.

        3.     RTS (Request to Send)

·        Once DTE receives the DSR signal, it prepares itself for the data transfer.

·        DTE send the RTS signal to DCE to inform that it is ready for the data transfer.

         4.     DCD (Data Carrier Detect)

·        In response to RTS, DCE tries to make contact with the remote MODEM.

·        If the carrier is detected it informs the DTE about it through the DCD signals.    

         5.     CTS (Clear to Send)

·        Once a stable contact is established with the remote MODEM, the DCE sends the CTS to DTE.

·         Due to this transmission begins.

Interfacing PC to 8051 using RS232 standard.

·        The PCs are based on 8086, 80286, 80386, 80486 and Pentium microprocessor.

·        Generally PCs have two COM ports, COM1 and COM2 . Both the ports have RS232 type connection.

·        The COM1 is used for mouse and COM2 is used for connecting devices like MODEM.

MAX232

The Max-232 IC is started by the maxim integrated products in 1987. It is an integrated circuit which converts the signals from the RS232 serial port to the proper signal which are used in the TTL compatible digital logic circuits. The MAX232 can convert the signals like RX, TX, CTS, and RTS and it is a dual driver/receiver.

The MAX-232 IC is an integrated circuit which consists of 16 pins and it is a resourceful IC mostly used in the voltage level signal problems. Generally, the MAX-232 IC is used in the RS232 communication system for the conversion of voltage levels on TTL devices that are interfaced with the PC serial port and the Microcontroller. This IC is used as a hardware layer converter like to communicate two systems simultaneously. The image of MAX232 is shown below.

The MAX232 consist of 16 pins with 3 major blocks. The power supply of this integrated circuit is 5V which is compatible with most of the embedded system. In the three major blocks, the first block is a voltage doubler by using the IC switched capacitor techniques are used to make the voltage doubler. If once the voltage is doubled, then the second block is converted to +10V and      -10V. The main function of voltage level is converted in the 3rd block which consists of the two transistors and 2 receivers. The following diagram shows the construction of the MAX232.