The lowly serial cable. It’s been around forever, and despite what you might think, it might be around a for a long time to come. But why in an industry hell-bent on speed and iteration would we still have a place for a technology so old it predates what we might consider a “computer”? To answer that, we need to look exactly what a serial cable is—because it might not be exactly what you think—and that’s just one reason this old tech is still essential to this day.

What is a serial cable?

As you might expect, serial cables connect two devices to enable data transfer from one to another via a serial communication protocol, a method of data transfer that involves sending data sequentially, one bit at a time. Conversely, its evil twin parallel communication involves transmitting multiple bits of data at the same time (several streams simultaneously).

Serial communication, the cables that enable it, and their connectors have evolved a lot over the years, but at the core, the concept behind it is still the same.

The interesting history of the serial cable (quickly)

Did you know that the Recommended Standard 232 (RS-232) serial cable, the one that you can buy right now, is over sixty years old? We can hear you gasp in amazement (or horror) that such old tech is still in use today. Canned foods often have a longer shelf-life than computer hardware. But they didn’t have computers in 1960 as we know them today, so why did they need a serial cable?

The serial cable was originally created to enable electromechanical teletypewriters to communicate with modems. Teletypewriters eventually gave way to computer/data terminals, which needed to connect peripherals like printers and screens, so just kept on using serial cabling. It worked fine, it was easy to install, and there was no reason to change it.

A Teletype machine and an Acoustic Data Coupler 300 Modem. That’s 300 bps. Source: University of Missouri - St. Louis
A Teletype machine and an Acoustic Data Coupler 300 Modem. That’s 300 bps. Source: University of Missouri – St. Louis

When MS-DOS based personal computers began their inevitable rise, manufacturers used the same standard— which wasn’t much of a “standard” so much a set of plugs that fit together—so PCs users could connect to (buy) existing equipment in the market.

Fun fact!

On MS-DOS, you could only run one program at a time. Yes, I know it sounds painful, but it was definitely a step-up from teletypewriters. ISA Cards were used to connect peripheral cards to the motherboard. The problem was that these cards had no way of telling the computer what resources were required, and the computer had no way of assigning resources to these add-in cards. Essentially, the user had to know what their PC contained, and ensure that these devices didn’t conflict with one another, or it was crash time. You had to be quite a bit more technologically savvy to use a computer than today.

What do you want to do today?

When Microsoft delivered Windows 3.1 unto the world, everything changed. Finally, you could run multiple programs simultaneously (if they were well-behaved). Programs could now interact with system device drivers that were in control of communicating with the PC’s hardware via, you guessed it, serial ports. Of course, users still had to inform Windows of the resources the add-in cards required by entering data into the System in the Control Panel… but we just called that “installing drivers.”

As time went on, there was a gradual transition from simple ISA to more powerful PCI cards. Eventually, serial ports were improved to allow for more efficiency. Even so, RS-232 ports continued to be used in personal computers until the late 1990s, when other standards, like USB, began to replace them.

That doesn’t mean the RS-232 serial cable has become obsolete. No spoilers, networking veterans!

Types of serial cables

In reality, there are a lot of cables and ports that fit the “serial” definition, and can be categorized based on the interfaces and communication standards, including what you might think of when you think of serial cables, the catchy-named ones like RS-232, RS-422, and RS-485. But it also includes some you might not, like fiber channel, IEEE 1394 (aka FireWire®), and even USB.

Time for a quick review of each cable (bookmark this for a quick reference in the future)!

Side note

There’s a lot of variations on serial cables. I mean, here is just a small list of different types of RS-232 cables alone! With that in mind, we’re focusing on the main standards that make up the types.

What’s a RS-232 serial cable?

The oldest and most popular of the “classic” serial cables first developed in the 1960s. An RS-232 serial cable supports transmission speeds of a whopping 9,600 bps out to a length of 15 meters, and are essentially point-to-point connections. Featuring asynchronous communication capabilities, they also have hardware and software control, along with parity checking.

A significant number of old computer devices and factory automation systems still use RS-232 interfaces even today, making a serial cable an invaluable tool.

What’s a RS-422 serial cable?

The next generation of the original, the identical-looking RS422 serial cable employed some fancy electrical engineering innovations to enable connections across multiple points, and far greater distances than the RS232 cable. These types of serial cables support speeds as high as 10 Mbps at 12 meters. They can also transmit data as far as a maximum of 1.2 km, but the transfer rate drops off significantly.

This warhorse comes in a lot of variations, here's the 9 and 25-pin versions.
This warhorse comes in a lot of variations, here’s the 9 and 25-pin versions.

What’s a RS-485 serial cable?

An RS-485 serial cable is generally used when a single controller needs to control up to 64 devices in a bus topology. It uses three-way transmissions but is still compatible with RS-422 interfaces as it uses the same differential signaling.

RS-485s are still in use today in all manner of “noisy” environments, like industrial applications, and some avionics systems.

RS-485 with a RS-232 adaptor. Source: Core Electronics
RS-485 with a RS-232 adaptor. Source: Core Electronics

What’s a Fiber Channel serial cable?

First developed in 1988, Fiber Channel serial cables support high-speed data transfer and are used mainly in applications that require the fast and lossless transfer of data, such as connecting data storage to servers in data centers.

While Fiber Channel runs mainly on optical fiber cables when it comes to data centers, it can also be used with copper cables. Transfer rates vary but can be rated up to 128Gbps.

Source: SFCables.com
Source: SFCables.com

What’s a FireWire (IEEE 1394) serial cable?

Dubbed “FireWire” by Apple (actually an Apple, Panasonic, and Sony co-development, but that’s another article), the serial cable also known as IEEE 1394 High-Speed Serial Bus enables high-speed data transmission. This serial bus supports asynchronous and isochronous (steady and recurring, like the counting of a clock) applications.

These cables are primarily used for applications that require high-speed data transfer, including video streaming and connecting peripherals to computers such as disk drives, graphics cards, and high-speed scanners.

FireWire is also notable for being able to carry power as well as data, and is available in copper, fiber optic, or CAT 5 versions.

Editor’s note: remember SCSI drives? FireWire was intended to be the serial replacement for that slower, parallel transfer protocol.

Source: PrimeCables.com
Source: PrimeCables.com

What’s a USB cable?

At the risk of insulting everyone reading this, USB is the format that eventually beat out FireWire, and is largely the standard in most consumer electronics for both power and data transfer. You probably have at least one in your eye line right now, they are that ubiquitous today. That “S” in the middle? Stands for “serial”.

USB borrowed a lot of features from FireWire, like power transfer. USB, in particular USB-C, has also gone through a lot of iterations since 2014, each one adding higher transfer rates and more discrete charging options. Currently, USB-C has data transfer rates topping out around 2.4GB/s.

A man on a scooter looks out over a cliff to the ocean
Trying to find the right USB cable for my printer.

But that’s not right!

Yes, we know that serial cables and ports usually refer to cables and hardware compatible with R232 and related standards. However, USB and FireWire cables transfer data as a serial stream, technically making them serial cables. So, a comprehensive answer to the question, “What is a serial cable?”, should include USB and FireWire cables.

What are serial cables used for now?

If we’re talking serial cables in general, they’re used for pretty much the same thing as the past 60 years. Remember, USB cables are also serial cables.

However, if we’re referring specifically to our classic friend the RS-232 and related serial communication standards, you’d be surprised how much equipment still employs these types of ports and cables.

RS-232 is still quite common in commercial and industrial applications. For example, almost all commercial routers and switches include standard serial ports, so you can configure them before you’ve set up any of the network ports.

Tip: always carry a standard serial cable or two of varying lengths with you when you’re doing anything related to a physical network set up!

A data terminal from 1970
“The RS-232: Not just for DEC VT-100 data terminals anymore!” Image: Wikipedia

Yes, networks have evolved, but why fix something that’s not broken and is still totally reliable? There’s a lot of legacy equipment still in use today for the simple fact that it’s reliable and doesn’t justify a pricey replacement. And there’s a good chance that legacy equipment still has a serial port somewhere.

So, no matter how you define them, serial cables are still widely used today.

Technically, personal computers still use serial cables too. Most storage devices use serial ATA cables to connect to the motherboard. What is a serial ATA cable? The serial ATA cable allows high-speed communication via two pairs of conductors. It replaced the parallel ATA that operated at a much lower frequency.

Why would you never want to be without a serial cable at work?

As a networking company, we’d be remiss if we didn’t drive home the point that it’s really important to have a serial cable with you all the time. You’re gonna need it. Serial cables might be basic, but they’re still one of the most reliable ways to access essential hardware physically.

Before you suggest doing it via the network, you first have to set that network up. And what will you do if the network is down? Ask it nicely to do your bidding? Massage it into compliance?

Serial is a quick and easy way to achieve router and switch configuration. In fact, with some networking equipment, it’s the only way. Serial cables also make it easier to fix problems, especially if you have automated configuration backup.

So, instead of getting frustrated and causing even more delays and downtime, wouldn’t it just be easier to make sure you always have a serial cable on you? After all, it’s always better to be safe than sorry.

  1. Ryan LaFlamme Avatar

    I can see at least seven USB cables in my eye line, right now.

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