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Serial Port : Serial communication takes place asynchronously, meaning that no synchronization signal (or clock) is required: the data may be sent at random intervals. In return, the peripheral must be able to distinguish the characters (one character is 8 bits in length) among the succession of bits which is sent. This is why, in this type of transmission, each character is preceded by a START bit and followed by a STOP bit. These control bits, which are needed for serial transmission, waste 20% of the bandwidth (for 10 bits sent, 8 are used to code the character and 2 are used for reception).

Parallel Port : Parallel data transmission involves sending data simultaneously on several channels (wires). The parallel ports on personal computers can be used to send 8 bits (one octet) simultaneously via 8 wires.

It seems that Parallel Ports are superior to Serial Ports as much more data can be sent per second , are there any reasons to choose Serial Ports over Parallel ports assuming you the computer/laptop has both ???

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I'll add this as a comment because I'm not entirely sure, but I believe the reason was that a serial port is error proof, while a parallel port is not. Serial is used when the communication over the port has to be error free all the time. In parallel port, a bad cable can cause lots of errors restarting the transmition from start. At least thats what I remember from the little usage I've done years ago. Not sure about the maximum speeds of both, but serial might be actually faster. – LPChip Apr 8 '14 at 14:02
Take a look at this article: Parallel vs Serial. It does a good job at explaining Serial vs Parallel. – user304064 Apr 8 '14 at 14:10
You would pick one over the other if the system only supports one or the other. This question seems broad – Ramhound Apr 8 '14 at 14:44

You're really comparing apples and oranges, because much of the hardware that connects to a PC via a serial port won't connect via a parallel port, and vice versa.

The serial port is actually controlled by a UART, as are the great majority of serial ports in devices, modems, etc. The UART handles most of the low-level details of signaling and eases load off the CPU (modern UARTS have larger FIFOs, meaning they can transfer more data per interrupt).

There is no such setup with the parallel port, basically, what you write to specific I/O ports (on x86) shows up on the pins. So any communications protocol you have to do yourself, and it will be CPU intensive. It's fine if you are programming an EEPROM under single-tasking DOS, but not really fine under a modern multitasking operating system.

As far as the overall argument whether parallel is faster/better than serial schemes, today the tendency is toward serial. With high rates of speed, it becomes troublesome to turn all 8 or 16 lines of a parallel interface to the desired state at the same time. With serial and differential signaling methods (RS-232 is NOT such a method), you can actually achieve higher speeds with the same amount of effort. This is what happened with SATA and PCIe, for example.

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