Friday, December 19, 2008

Serial Communication - a brief

Thought Me'd just scribble a few notes about serial communication - all that Me discovered after the first experiences with booting up Beagle and getting RS232/ serial communication working. 

Serial communication, in a nutshell, is a process of sending one bit at a time, sequentially, over a communication channel, usually a computer 'bus'. Historically, this mode of communication has been used for long haul communication. Offlate, serial computer buses (like USB) have become more common, 'cause of the advantages they have over another mode of communication, the parallel communication.

The most extensive usage of serial communication in history has been to network computers using modems. A serial port on the PC was designed to be connected directly to an external modem. Thus, it is termed as "Data Terminal Equipment" (DTE) and the modem is called "Data Communications Equipment" (DCE). 

RS232 is a standard developed by the Electronic Industries Association (EIA) and other interested parties, specifying the serial interface between Data Terminal Equipment (DTE) and Data Communications Equipment (DCE). The RS232 standard includes electrical signal characteristics (voltage levels), interface mechanical characteristics (connectors), functional description of interchange circuits (the function of each electrical signal), and some recipes for common kinds of terminal-to-modem connections. The most frequently encountered revision of this standard is called RS232C. Parts of this standard have been adopted (with various degrees of fidelity) for use in serial communications between computers and printers, modems, and other equipment. 

The serial ports on standard IBM-compatible personal computers follow RS232C, and are full-duplex devices, in that they can send and receive data at the same time. They are both male connectors, as given in the pictures below. These serial ports, typically, use two separate lines for transmitting and receiving data.

The pin assignments for the above connectors in both roles (as DTE and DCE) are as follows:

RS232C, however, does not define bit rate, character framing, communication protocol, character ecnoding, data compression and error detection, which must be agreed by both sending and receiving equipment for a successful transmission. These are typically manifested as settings in the software/ serial communication clients that are used by either ends.

The most commonly found and simple usage of serial communication is found in communication using Null modem cables. In this communication, the RxD of DCE is connected to TxD of DTE and vice versa, and the common grounds (G) are connected together.

Lately, serial ports are being deprecated in the latest platforms, and USB is taking its place. However, there are USB to serial adapters widely available, that make interactions with legacy systems possible. 

Beagle, exposes a RS232 port, to which an IDC10F to DB9M adapter can be connected to expose out a serial interface. The reference manual, very cleary indicates the wiring that's required on such an adapter. Me's had some trouble regarding this, as outlined in my earlier post.

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