Serial communication

Asynchronous communication standards, hardware, terminology, and concepts are described here.

Serial ports are used to physically connect asynchronous devices to a computer. They are located on the back of the system unit, either integrated or using a multiport adapter, such as the 2-port, 8-port, 16-port, and 128-port asynchronous adapters.

Note: The POWER5 integrated system ports are not general-purpose, full-function serial ports. See Functional differences between system ports and serial ports for more information.

To understand the functionality of a serial port, it is necessary to first examine parallel communications. A standard parallel port uses eight pins, or wires, to simultaneously transmit the data bits, making up a single character. The following illustration shows the parallel transmission of the letter a.

Parallel communication port — Figure 1. Parallel communications port

Serial ports require only a single pin, or wire, to send the same data character to the device. To accomplish this, the data is converted from a parallel form (sent by the computer), to a sequential form, where bits are organized one after the other in a series. The data is then transmitted to the device with the least significant bit (or zero-bit) sent first. After the data is received by the remote device, the data is converted back into parallel form. The following illustration shows the serial transmission of the letter a.

Serial transmissions of a single character are simple and straight forward; however, complications arise when a large number of characters are transmitted in series as shown in the following illustration. The receiving system does not know where one character ends and the other begins. To solve this problem, both ends of the communication link must be synchronized or timed.