Posted on August 15, 2023 in Technology

Parallel ports: A Brief History

Parallel and serial ports used to be everywhere, and now they’re more or less limited to ancient printers and old iPads. What happened to them?


Serial ports came first, but just barely. Serial in this context means that the information is processed one stream at a time, which the receiving device will then have to stack up to read. For example, printers: a serial connection on a printer would give the printer the ASCII data one bit at a time, and it’s up to the printer to stack up the bits to make the words.

But if the data’s being transferred one bit at a time, why does it need so many pins?

On a computer, each pin on a serial port does something different – some regulate the out- and in-put speed, some are purely for grounding the connection, and some are responsible for transferring the requests for data between the computer and the peripheral it’s connected to. And each peripheral has different needs! A mouse or a CNC is going to need more information about the data than a printer or a bar code scanner. There might also be a parity pin, which ensures the data sent is correct. They came in all sorts of shapes and sizes, from circular 7-pins to trapezoidal 25-pins for motherboards.

Serial ports are actually faster (now) than parallel ports because the data’s transferring one bit at a time. If you can make the bit transfer faster, then the entire serial port speeds up with it, because data transfer speeds are basically arbitrary! Serial ports could keep up with computers as they improved. Parallel ports have to be sure that their data’s being received all at the same time: if one pin can’t be optimized any more than it already is, then that pin holds back the speed of the data for the other data-transferring pins. If the data doesn’t make it at the same time, then the computer doesn’t know how to interpret it. Imagine receiving parts for an IKEA chair out-of-order and being told you had to start assembling it now even though you don’t have the legs or screws yet.


Parallel ports actually appeared at about the same time as serial ports, and allowed for multiple streams of bits (the ‘parallel’ part) instead of just one. The port was feasible in the 1970s, but the first commercial parallel port appeared on IBM printers, in the early 1980s. Printers were where they found most of their use. The pins sped up printing by presenting the ASCII (a character library that uses sets of binary characters to represent letters) to the printer all at once, instead of serially.

However, parallel ports came with a couple of problems. They couldn’t match a serial port’s speed, once bit-cycle-times shot down, and the three major companies attempting to use them for their printers came up with different protocols for each operating system, so everything had to be double checked for compatibility.

Where’d They Go?

As said before, the USB has taken over much of the parallel port’s turf, and where USB is inconvenient, network printing rules supreme. There’s not much space left for these parallel pin plugs out in the wild. They’re still around – people still need access to legacy machines no matter the industry or time – but they’re not usually on regular, consumer electronics anymore.

And yet, they aren’t extinct. Serial ports still exist on old or simple tech that can’t take high speeds and still function, things like scientific equipment, or stenotype machines. Because the transfer’s tightly regulated, serial ports avoid overloading the tiny computers inside these several-thousand-dollar instruments.

Universal Serial Bus (or USB) plugs use similar tech, just highly compressed and much faster. USBs are also transmitting data serially, hence the ‘serial’ in the name. Parallel ports may have been left behind, but serial’s sticking around. If you look at the inside of the actual connecting piece, you’ll still see pins, albeit different ones than the kind serial connectors used to use.

 Serial ports represent a major breakthrough in data transfer tech, and they’ve stuck around to this day!