Sophisticated Serial Information Communications (RS232, RS485 and RS422) 4567

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In the serial communications globe 1 of the most enduring specs is RS232. Certainly, numerous individual computer systems (PCs) nonetheless have at least 1 RS232 port, even with the recognition of Universal Serial Bus (USB) ports. It is partly this longevity that has produced a marketplace for converters to interface in between RS232 and other serial interfaces e.g. RS422 and RS485.

RS232 serial ports generally have a 25-pin (DB-25) male connector, even though most of the pins are not needed and a 9-pin (DB-9) male is adequate. (Be aware that numerous distributors provide 9-pin to 25-pin converters.) In addition, even though the regular recommends a optimum distance of just 50 feet, with suitably shielded cable it's feasible to attain about 10,000 feet, albeit at the slower price of 19,200 bps.


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It's instructive to evaluation why a 9-pin connector is sufficient for most serial information scenarios. The important point is that only three wires are needed to communicate information 1 bit at a time - a transmitting wire, a receiving wire and an earth-grounding wire. (In practice, additional wires are used to reliably send and receive the information.)

RS232 serial ports support 'asynchronous' communications. This means that no synchronizing bits are sent or received and the serial port must therefore use 'start' and 'stop' bits within the information stream to indicate when information transmission is about to begin (on a per-byte basis) and when it has finished. (Be aware that in some cases, a special 'parity' bit is also used within the information stream to help improve information integrity and error checking.)


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In contrast, synchronous communications (e.g. a parallel printer port on a Pc) uses a continuous stream of bits to allow the two devices to know exactly where the other is - this includes the use of 'idle' bits when no actual information is being transmitted.

Before the functions of the important RS232 pins are discussed, the reader should recall the terms 'DTE' and 'DCE'. For the purposes of this article we can assume that the Pc is the DTE (Data Terminal Equipment) and the dial-up modem it is connected to (for example) is the DCE (Data Communications Equipment.)

Knowing which is DTE and which is DCE is important when connecting RS232 cables. For example, connecting a DTE to another DTE device (or DCE to DCE) will require a 'null-modem' cable whereas connecting DTE to DCE will require a 'straight-through' (transmit pin maps to transmit pin, receive pin maps to receive pin etc). This is not applicable with and RS422.