Serial and Parallel Ports - How a Computer Works (2015)

How a Computer Works (2015)

11. Serial and Parallel Ports




Two ports are provided on the back of the system box to connect external devices. Printers, modems, scanners connect to the ports. The serial and parallel ports are now being superseded by USB.

Serial Port


Data is fed to the Serial Port.

Data is fed to the I/O controller chip. Data is converted into a serial stream and connected to the serial port.



The serial port is used typically to connect an external modem to the computer. Data bits are sent one bit after the other on one line. This is slower than the parallel port.

A 9 way D type plug on the back of the computer connects to a 25 way D connector on the modem.

Parallel Port


The parallel port enables data bits to travel over eight data lines to a printer The parallel port is a large D type connector on the back of the computer. A multi core cable connects this connector to a similar connector on the printer. A printer is the most often connected peripheral to the parallel port. When the printer is switched on a signal is sent to the computer. The select line signal tells the computer that the printer is online and ready to receive data. The parallel port has largely been replaced by USB for connecting printers.



When you send a document to the printer data bits are loaded onto lines 2 to 9. Once the first data packet has been sent the computer sends a strobe signal on line 1 to the printer. This signal lasts for one microsecond and lets the printer know data is ready to be read into the printer. Once the data is read in a signal on line 10 from the printer acknowledges receiving the data sent through lines 2 to 9.

Parallel Data


Data is fed to the Parallel port.

Line 11 from the printer tells the computer when it is too busy handling data to receive any more. When the computer receives this signal no further data is sent to the printer until the signal is cleared.

Line 12 sends a signal to the computer when the printer runs out of paper.

Line 15 is a general error line used by the printer to tell the computer an error has occurred such as jammed paper etc.

Line 16 is a signal from the computer that resets the printer to its original power up state.