Inside the Hayes Command Set for Modems

Ah, modems—the magical boxes that let us scream data at each other over phone lines. Before fiber optics, before Wi-Fi, before the world decided dial-up was slow as a turtle in molasses, there was the Hayes AT Command Set.

Want to make your modem dance? You spoke Hayes!

A Brief History of Hayes Modems

Hayes Microcomputer Products basically invented the modem command set that became the industry standard. Founded in 1977, these folks figured out that users needed a way to control their modems without flipping switches like they were launching the Apollo program.

Thus, the AT command set was born.

The Syntax of Hayes Commands

And so much more!

Things You Can Do with Hayes Commands

• Dial a number (ATD555-1234)
• Answer a call (ATA)
• Hang up (ATH)
• Check connection speed (ATI)
• Set speaker volume (ATL1 to ATL3)
• Enable/Disable auto-answer (ATS0=1 or ATS0=0)
• Configure flow control (AT&K3)
• Check signal strength (AT+CSQ)
• Force a protocol (V.32, V.34, V.90, V.92, etc.)

Weird and Unusual Hayes Commands

Some modems let you:

• Play DTMF tones (AT+VTS=3)
• Switch to fax mode (AT+FCLASS=1)
• Enable voice mode (AT+VSM)
• Send SMS (on GSM modems) (AT+CMGS)
• Spy on a phone line (Not that we’d recommend it!)

Duplex & the “Seeing Double” Issue

Ever seen double characters when typing on an old modem? That’s half-duplex mode for you!

• Full-duplex = Sends and receives simultaneously. No echoes.
• Half-duplex = One direction at a time. You see your own typing echoed back.

If your cable was wrong or your settings were borked, everything you typed echoed back like a ghost.

Advanced Hayes Commands & 56K Modems

When 56K modems became a thing (V.90, V.92), you got new commands:

• AT+MS=V90,1 (Force V.90 mode)
• AT+PQC=1 (Enable Quick Connect)
• AT+PCW=1 (Enable modem-on-hold)

These allowed faster connections and modem-on-hold, which let you answer a call without dropping your internet.

Other Modems That Used the Hayes Set

The Hayes AT command set was so good that every modem manufacturer stole it:

• USRobotics (they added proprietary tricks)
• Zoom (cheap but used the same commands)
• Rockwell/Conexant chipsets (in everything)
• 3Com (because USR got bought by them)
• Lucent/Agere (yes, they had modems too)

Did Hayes Like This?

Absolutely not. They tried to sue everyone but lost. By the time they realized their command set was being used everywhere, it was too late. Companies took their idea and ran with it.

Hayes vs. the Competitors

Modern Uses of the Hayes Command Set

The Hayes AT command set is alive and well—despite dial-up modems having gone the way of the dodo.

From cellular modems to IoT devices, AT commands continue to provide a simple way to control modems, thanks to their flexibility and ease of use.

Let’s take a look at how the Hayes command set evolved and where it still lurks in the digital wilds today.

The Evolution of the Hayes Command Set

Born in the 1980s, the Hayes AT command set standardized modem communication.

When dial-up died out, you’d think AT commands would have followed. Nope!

Instead, they evolved and became the backbone of many modern communication protocols.

The Evolutionary Timeline:

• 1980s: Hayes invents the AT command set for dial-up modems.
• 1990s: Cellular modems adopt AT commands for GSM control.
• 2000s: IoT devices and industrial hardware start integrating AT commands.
• 2020s: Even 5G modems still support AT commands for control functions.

Where Are AT Commands Used Today?

1. Cellular Modems & 5G/LTE Devices

Modern cellular modems—whether 3G, 4G, or 5G—support AT commands for control and diagnostics. Examples include:

• Quectel, Telit, Sierra Wireless, and Huawei modems use AT commands to control calls, SMS, and data connections.
• AT+CGATT=1 (Attach to a cellular network)
• AT+CMGS="+123456789" (Send SMS)
• AT+CSQ (Check signal strength)

2. IoT and M2M (Machine-to-Machine) Communication

IoT devices still rely on Hayes-style AT commands for communication with cellular networks. Example use cases:

• Remote monitoring devices
• Industrial automation
• GPS tracking systems

Popular IoT cellular modules using AT commands:

• SIM800 / SIM900 GSM modules
• ESP32 LTE shields
• NB-IoT (Narrowband IoT) devices

3. Satellite Modems

Did you know that some satellite communication systems use AT commands? Yup!

• AT+SBDI (Iridium satellite short-burst data transmission)
• AT+GSN (Retrieve satellite network ID)

4. Embedded Systems & Microcontrollers

AT commands are still a lightweight way to control modems from microcontrollers. Devices like:

• Arduino + GSM shields
• Raspberry Pi cellular modules
• ESP8266 Wi-Fi modules (Some AT-based variants!)

5. USB Cellular Dongles

You can control most USB cellular modems with AT commands via serial terminals:

• Huawei E3372
• Sierra Wireless MC7455
• ZTE MF823

Try this:

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screen /dev/ttyUSB0 115200
AT+CGMI   # Get manufacturer info
AT+CGMM   # Get model name
AT+CSQ    # Signal quality

The Connection Between Hayes Modems & Cellular Modems

Is there a direct lineage between dial-up modems and cellular modems? Absolutely!

• Both use AT commands. Cellular modems evolved from dial-up standards, keeping the Hayes-style syntax.
• Both use serial interfaces. Traditional modems used COM ports, while cellular modems use USB-to-serial bridges.
• Both rely on status responses. A CONNECT response in dial-up is like a +COPS (Carrier Operator) response in cellular.

Conclusion

The Hayes AT command set didn’t die—it evolved. From dial-up modems to cellular and IoT devices, it remains a universal way to control network connections. If you’re tinkering with cellular modems, satellite systems, or IoT gadgets, you’re still speaking the ancient yet surprisingly modern language of Hayes!

Key Ideas Table

Reference Links

• Hayes AT Command Set (Wikipedia)
• GSM AT Commands (Reference)
• Quectel AT Command Guide
• Huawei USB Modem AT Commands
• Hayes Microcomputer Products (Wikipedia)
• AT Command Set (Wikipedia)
• Dial-up Modems (Wikipedia)
• 56K Modems (Wikipedia)
• AT Command List (Reference)