The aim was to design new drivers and devices, which were compatible with each other and also backwards compatible with the Standard Parallel Port. The benefits of Parallel data transference over serial is that it flows along 8 wires as opposed to four, which increases the speed of data, however it still doesn't compare to the speed of USB, which is one of the main reasons why I believe that all printers and scanners should be upgraded to use the USB interface, as it will increase our productivity by de-limiting bottlenecks in the system.
See Appendix (D) for a diagram of electrical and mechanical information of the Centronics interface. Serial Interface The Serial port is considered to be one of the most basic external connections to a computer; the serial port has been an integral part of most computers for more than 20 years. Although many of the newer systems have done away with the serial port completely in favour of USB connections, most modems still use the serial port, as do some printers, PDAs and digital cameras. Few computers have more than two serial ports.
The term serial comes from the fact that a serial port "serializes" data. That is, it takes a byte of data and transmits the 8 bits in the byte one at a time. The advantage is that a serial port needs only one wire to transmit the 8 bits (while a parallel port needs 8). The disadvantage is that it takes 8 times longer to transmit the data than it would if there were 8 wires. Serial ports lower cable costs and make cables smaller. Before each byte of data, a serial port sends a start bit, which is a single bit with a value of 0.
After each byte of data, it sends a stop bit to signal that the byte is complete. It may also send a parity bit. Serial ports, also called a COM port (which stands for communication), are bi-directional. Bi-directional communication allows each device to receive data as well as transmit it. Serial devices use different pins to receive and transmit data -- using the same pins would limit communication to half-duplex, meaning that information could only travel in one direction at a time. Using different pins allows for full-duplex communication, in which information can travel in both directions at once.
Serial ports rely on a special controller chip, the Universal Asynchronous Receiver/Transmitter (UART), to function properly. The UART chip takes the parallel output of the computer's system bus and transforms it into serial form for transmission through the serial port. In order to function faster, most UART chips have a built-in buffer of anywhere from 16 to 64 kilobytes. This buffer allows the chip to cache data coming in from the system bus while it is processing data going out to the serial port.
While most standard serial ports have a maximum transfer rate of 115 Kbps (kilobits per second), high speed serial ports, such as Enhanced Serial Port (ESP) and Super Enhanced Serial Port (Super ESP), can reach data transfer rates of 460 Kbps. An important aspect of serial communications is the concept of flow control. This is the ability of one device to tell another device to stop sending data for a while. The commands Request to Send (RTS), Clear To Send (CTS), Data Terminal Ready (DTR) and Data Set Ready (DSR) are used to enable flow control.
For example if you have a modem that communicates at 56 Kbps. The serial connection between your computer and your modem transmits at 115 Kbps, which is over twice as fast. This means that the modem is getting more data coming from the computer than it can transmit over the phone line. Even if the modem has a 128K buffer to store data in, it will still quickly run out of buffer space and be unable to function properly with all that data streaming in. With flow control, the modem can stop the flow of data from the computer before it overruns the modem's buffer.
The computer is constantly sending a signal on the Request to send pin, and checking for a signal on the Clear to send pin. If there is no clear to send response, the computer stops sending data, waiting for the Clear to send before it resumes. This allows the modem to keep the flow of data running smoothly. Again although serial is used by such devices as modems, mice, PDA's and Data capture devices, it is still slower than USB and is also prone to bottlenecks, as data is transferred. For this reason we again recommend that all of these serial devices be upgraded to USB.