Mumbai University > Electronics Engineering > Sem 6 > Embedded System and RTOS

i. RS-232 (Recommended Standard number 232) was developed by the Electronics Industries Association (EIA) during the early 1960s.

ii. The maximum bit transfer rate capability and cable length:

1. RS-232’s capabilities range from the original slow data rate of up to 20 kbps to over 1 Mbps for some of the modern applications.
2. RS-232 is mainly intended for short cable runs, or local data transfers in a range up to 50 feet maximum.
3. It is a robust interface with speeds up to 115,200 baud.
4. It can withstand a short circuit between any 2 pins.
5. It can handle signal voltages as high / low as ±15 volts.

iii. Signal States

1. Signals can be in either an active state or an inactive state. RS232 is an Active LOW voltage driven interface where: ACTIVE STATE: An active state corresponds to the binary value 1. An active signal state can also be indicated as logic “1”, “on”, “true”, or a “mark”. INACTIVE STATE: An inactive signal state is stated as logic “0”, “off”, “false”, or a “space”.
2. For data signals, the “true” state occurs when the received signal voltage is between -3V to -25V, while the "false" state occurs when the voltage is 3V to 25V.
3. For control signals, the "true" state occurs when the received signal voltage is between 3V to 25V, while the "false" state occurs for voltages -3V to -25V.
4. Signal voltage region in the range >-3.0V and < +3.0V is regarded as the 'dead area' and allows for absorption of noise. This same region is considered a transition region, and the signal state is undefined.

iv. Communication Technique:

1. RS-232 is a point-to-point communication interface and the devices involved in communication are called Data Terminal Equipment (DTE) and Data Communication Equipment (DCE).
2. IF no data flow control is required, only TXD, RXD and ground line (GND) are required for data transmission and reception. The RXD pin
   of DCE should be connected to the TXD pin of DCE and vice versa for proper data transmission.
3. If hardware flow control is required for serial transmission, various control signals must be used. The control signals are implemented mainly for modem communication and some of them may not be relevant for other type of devices.
4. The request to send and clear to send signals co-ordinate between the DTE and the DCE. Whenever the DTE has a data to send, it activates the RTS line and if DCE is ready to accept the data it activates the CTS line.
5. The Data terminal Ready (DTR) signal is activated by DTE when it is ready to accept the data. The Data Set Ready (DSR) is activated by DCE when it is ready for establishing communication link. DTR should be in the activated state before the activation of DSR.
6. Data carrier detect (DCD) control signal is used by DCE to indicate the DTE that a good signal is being received.

v. RS-232 supports baud rate up to 30kbps and the commonly used baud rates are 300bps, 1200bps, 2400bps, 9600bps, 11.52kbps and 19.2kbps. RS-232 is 50 feet at highest supported bud rate.

vi. Embedded devices contain a UART for serial communication and they generate signal levels conforming to TTL/CMOS logic. A level translator like MAX-232 is used to convert the UART signals RS-232 signals.

vii. RS-232 is not suitable for multi-drop communication. It uses single ended data transfer technique for signal transmission and thereby more susceptible to noise and it greatly reduces the operating distance.

Comparison of RS232 and RS485: