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RS-232 Standard

Summary Of The Scope Of The RS-232 Standard

The RS-232 standard doesn't define everything nor does it address every conceivable situation. There are three basic categories of issues related to RS-232:

1. Some engineering specifications are absolutely explicit.
   • Voltage levels:
     Voltage is referenced to a zero-volt ground and ranges from +15 volts to -15 volts.
   • Baud rate values:
     A baud is a signal transition from negative to positive, or positive to negative. In an RS-232 interface there is one binary character encoded during each baud. The term "baud" is often used to mean "bits per second". For the RS-232 standard, it's true that 1bps=1baud, but that's not true for other interconnection technologies. Notably, when a modem transmits onto the phone line at, say, a data rate of 1200 bits per second it is encoding two bits during each signal transition; two bits per baud.
   • Shape of the signals that represent '1's and '0's:
     If an engineer were using an oscilloscope to observe the signals on an RS-232 line, they would have to comply with a specific set of constraints.
   • The specifications for the interface circuitry:
     The chipset that transmits the signal is called an RS-232 line driver. The line driver must send a signal that meets the requirements of the standard and that is exactly what the receiver circuitry is expecting. The circuit design parameters for the transmit and receive components are completely specified.
   • The purpose for each wire used in the cable:
     There are 25 wires described in the RS-232 standard. The purpose of each wire is explicit. Moreover, not all 25 wires are required to make a simple connection between two devices. Your personal data assistant (PDA) or even your wristwatch may have a tiny two-wire RS-232 interface that allows you to backup and restore data. Based on the requirements of the interface design, as few as two of the 25 lines may be implemented.
   
   
2. There are some general guidelines allowing certain issues to be flexible.
   • The method to start and stop the flow of data:
     Two fundamental control signals are provided that indicate the operational state of the RS-232 device and the willingness of the device to receive characters. We might find one implementation that requires both signals be asserted before data is sent. In another case, a designer may decide that if a device says it's ready to receive characters then it must be in an operational state. In this case only one of the control signals will be monitored. There are other control signals that indicate that the phone is off the hook and a dial tone is being received; that the phone is ringing; and more. The method used by a particular implementation to start and stop the flow of data is constrained by the available choices but not specified in absolute terms.
   • The method to coordinate the sender and receiver:
     Coordination includes the recognition that a new character has arrived and that the character transmission has stopped. Also, the baud rate used between the transmitter and receiver must be coordinated. As with the flow of data, there are a variety of options available but the implementation selects the specific method.
   
   
3. Some considerations have no specifications in the standard.
   • The way that characters are represented by bits:
     RS-232 defines the specifications for encoding, transmitting, receiving, and decoding "characters" of 5, 6, 7, or 8 bits in length. If these characters are ASCII, or EBCDIC, or graphics, or a bit-map representing a fax document, the RS-232 standard doesn't know, or care about it.
   • The type of connector used to attach the cable ends:
     Originally a 25 pin connector (called a DB-25) was the most popular. This connector is still in wide use today; mostly on the backs of external modems, printers, or other peripheral devices. A more recent development (well, since the mid-1980's) has been the introduction of the 9 pin (DB-9) serial connector now found on almost all desktop and notebook computers. Even more recently is the proliferation of PDA's (personal data assistants) with their very simple (often 2-wire) interfaces using tiny phono plugs or other proprietary connectors. The RS-232 standard doesn't require any particular connector.