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

Conclusion

The Hayes Command Set was the modem language of the world. It was simple, powerful, and easy to steal (oops, I mean adopt).

Even modern LTE and 5G modems still use AT commands today.

Key Ideas Table

Reference Links

• Hayes Microcomputer Products (Wikipedia)
• AT Command Set (Wikipedia)
• Dial-up Modems (Wikipedia)
• 56K Modems (Wikipedia)
• AT Command List (Reference)