Ports, Serial Ports, RS-232c Serial Port Communication

RS-232c Serial Port Communication

Serial port communication on PCs follows the RS-232c standard, which specifies how information and commands travel over nine wires in the serial cable. The UART chip controls all RS-232c serial communication.

Hardware flow control is accomplished by the UART chip using two of these wires:

One wire to stop data

One wire to start data

Hardware flow control a message to stop or start data flow need only travel from the receiving PC to the receiving modem.

The cable uses nine wires and can have either a DB25 (25-pin) or a DB9 (9-pin) connection at either end. The table below lists the purpose of each of these nine wires and the pin connection for both the DB9 and DB25 connectors at the computer or DTE end.

Notice the acronyms for the modem lights in the third and fourth columns. On a PC, serial port communication is asynchronous, meaning that data flow does not stay in sync with a clock. The RS-232c standard includes pin outs used to synchronize data transmission with the receive and transmit clocks, but, since PCs don’t use this feature, only nine pins are used, making possible the more convenient 9-pin connection for PCs rather than always requiring a 25-pin port.

The two pins that are used for hardware flow control in the above table are RTS and CTS. In most cases, RTS and CTS stay on the entire time a communication session is active.

Between the two modems, carrier (indicating that the line is still open) from each modem must also stay up for communication to take place.

When the receiving computer wants to stop receiving data, it drops the RTS signal. Its modem responds by dropping the CTS signal and deactivating the line. When the receiving computer is ready for more data, it raises RTS, and the receiving modem responds by raising CTS and activating the phone line. (“Dropping” and “raising” signals here are terms used by communications technicians. They mean that a signal is either stopped or started, respectively.)