Advanced Serial Data Communications

In the serial communications world one of the most enduring specifications is RS232

. Indeed, many personal computers (PCs) still have at least one RS232 port, even with the popularity of Universal Serial Bus (USB) ports. It is partly this longevity that has created a market for converters to interface between RS232 and other serial interfaces e.g. RS422 and RS485.

RS232 serial ports usually have a 25-pin (DB-25) male connector, although most of the pins are not required and a 9-pin (DB-9) male is sufficient. (Note that many vendors supply 9-pin to 25-pin converters.) In addition, although the standard recommends a maximum distance of just 50 feet, with suitably shielded cable it's possible to reach about 10,000 feet, albeit at the slower rate of 19,200 bps.

It's instructive to review why a 9-pin connector is adequate for most serial data scenarios. The key point is that only three wires are required to communicate data one 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 data.)

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 data stream to indicate when data transmission is about to begin (on a per-byte basis) and when it has finished. (Note that in some cases, a special 'parity' bit is also used within the data stream to help improve data integrity and error checking.) This is not applicable with RS485 and RS422.

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 data is being transmitted.

Before the functions of the key 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.)

From the DTE's perspective the transmit data (TD) pin is where data transmitted and the receive data (RD) pin is where it is received. However, note that the receiving DCE will receive this data on its TD pin when using a straight-through cable.

The other significant RS232 data pins are RTS (request to send), CTS (Clear to Send), DTR (data terminal ready), DSR (Data Set Ready), CD (Carrier Detect) and RI (Ring In). The latter two are rarely used in today's modems and will not be discussed further.

The use of RTS/CTS is relevant when hardware flow control is deployed between the DTE and DCE devices. These pins are used in combination so that, for example, when the DTE raises the signal on the RTS pin because it has data to send, the DCE at the other end will raise its CTS pin if it is ready to receive.

And what of the DTR/DSR pins? Although originally intended as an alternate hardware flow control mechanism, they are not much used any more.

by: Daniel Lorenz

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