Saturday, August 16, 2008

RS232 module

The RS232 standard is one of the oldest physical communication standards in computer world. The standard defines low-cost serial communication in a robust way where bits are sent sequentially on a copper line. It was originally defined for connecting devices such as computers, terminals and printers to modems. This equipment is connected through their serial port. Nowadays, the computer to computer link with a so-called null modem cable is commonly used.
Communication as defined in the RS232 standard is an asynchronous serial communication method. The word serial means, that the information is sent one bit at a time. Asynchronous tells us that the information is mot sent in predefined time slots. Data transfer can start at any given time and it is the task of the receiver to detect when a message starts and ends.
With synchronous communication, a clock or trigger signal must be present which indicates the beginning of each transfer. The absence of a clock signal makes an asynchronous communication channel cheaper to operate. Fewer lines are necessary in the cable. A disadvantage is that the receiver can start at the wrong moment receiving the information. The RS232 standard specifies that the voltages on the wire for sending logic 0 are from +5V to +15 V. The voltages for sending logic 1 are from -5V to -15V. Most micro controllers are not capable of generating these voltages. So to connect microcontroller to a true RS232 device, you need to convert the TTL voltage of 0V to +5V into voltages between -10V and +10V.

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