'AT' (Hayes) Serial Modem Operation Manual (Draft)


Unlike the VT100 terminal, the modem does not try to faithfully emulate any particular original device.

Instead it is designed to provide an MVP (minimum viable product) providing a modified Hayes 'AT' compatible command set for connecting over Wi-Fi via TCP/IP sockets with an optional Telnet protocol layer.

Although later standardized by the TIA/EIA, in the early days of dialup modems the 'AT' command set of the Hayes SmartModem was copied by other modem makers and became a de-facto standard or rather a convention, as other modem manufacturers took the liberty to alter and extend the 'AT' command set. The same applied to the so called 'S' Registers where settings and configuration are made and saved to non-volatile storage (NVS).

See Hardware Configuration for details on connecting from a UART on your system to the modem.

'AT' Commands


The original 'AT' command set was strictly in upper case. This is because the bit sequence of the ASCII values for 'A' and 'T' have a specific property that enables autobaud detection of the connection to the data terminal equipment (DTE).

  • The modem only responds to 'AT' commands in upper case.


  • Commands are terminated by <CR> (carriage return, <ctrl>-m, 0x0D, decimal 13) usually generated by the Enter or Return key on your keyboard.
  • Commands can by edited, before pressing <CR>, using <BS> (backspace, <ctrl>-H, 0x08 decimal 8) to erase the previous character entered. You may need to configure the terminal to generate <BS> when you press the Backspace or <- key on your keyboard.


The 'AT' command processor is based on a finite state machine (FSM). If you type anything that is not recognized by the rules of the FSM you will immediately see an Error message.

'AT' Command Help

Typing AT$ when connected to the modem will result in the following 'AT' command summary being returned by the modem.

Modem AT Commands


  • All commands must be prefixed with AT except for the immediate command A/.
  • Commands can be concatenated into a single command string, except in the cases where the command accepts all the remaining input up to the <CR> as part of the command.
  • See the table below for examples.


The maximum length command string that can be entered is 80 characters.

Each command is explained in the following table.

'AT' Command Summary Table

Command Description Comments/Examples
AT 'AT' test AT entered on its own will respond OK and tests that command processor is responding
A/ Repeat last command (immediate) This immediate command is not prefixed by AT but repeats the last command entered as soon as A/ is entered ie. it does not wait for a <CR>
$ Help Responds with the 'AT' Command Help see above
I or I0 Information zero Responds with the modem model string VT132 'AT' Modem
I1 Information one Responds with the modem firmware version string eg. V1.0.0
I2 Information two Responds with the modem firmware build chain version string eg. V4.0-1234
Z Reset modem Soft resets the modem. Reloads settings and 'S' Registers from NVS. Closes open TCP/IP connections. But does not disconnect from the Wi-Fi Access Point (AP) if connected
&F Restore factory defaults Return the current settings and 'S' Registers to factory defaults. But does not save these to NVS
&W Write settings to NVS Writes the current settings to NVS. This takes approximately 10 seconds, during which the modem appears frozen. The modem will respond OK when the write is complete
D Dial (TCP/IP connect) The full form is ATDhostname:port where hostname is either the IPV4 address in nnn.nnn.nnn.nnn notation or the DNS name of the host. The :port is optional and specifies a numeric TCP/IP port number for the connection, the default is 23. There are a number of possible error conditions explained below under Wi-Fi Connection Results. If the connection is successful the modem will enter Data Mode
+++ Escape Sequence When connected and in Data Mode the Escape Sequence will return to Command Mode. The +++ Escape Sequence is only recognised if surrounded by a guard interval of 1 second ie. there must be a one second pause before and after entering +++ or it will not be recognised but simply treated as part of the data stream
O Return Online When connected and in Command Mode ATO will return to Data Mode. If not connected the modem will respond NO CARRIER
H Hangup When connected and in Command Mode ATH will close the connection. If a connection is closed the modem will respond HANGUP
&A Enable Answer mode Unless AT&A is entered the modem will not respond to requests for incoming connections. See Answer Mode below for details
A Answer When an incoming connection request is indicated by the modem responding RING the request can by manually accepted with the ATA command. If the connection is successful the modem will enter Data Mode
Sn Select current register Select register n as the current 'S' register
? Query current register The modem will respond with the value of the current 'S' register. If no register has been selected the default is zero (0). As commands can be concatenated it is normal to append this to the Sn command to query a specific register eg. ATS15? will query the value of 'S' Register 15
=r Set current register Sets the value of the current 'S' register to r. If no register has been selected the default is zero (0). As commands can be concatenated it is normal to append this to the Sn command to set a specific register eg. ATS15=1 will set the value of 'S' Register 15 to 1
&K or &K0 Disable CTS/RTS flow control The 6-pin modem header provides CTS/RTS lines. AT&K0 disables their operation. This is the default
&K1 Enable CTS/RTS flow control The 6-pin modem header provides CTS/RTS lines. AT&K1 enables their operation
+W? Query Wi-Fi Access Point (AP) connection status Responds with the status of the Wi-Fi connection as a string. The possible responses are listed below under Wi-Fi Connection Results
+W=sss,ppp Join Wi-Fi Access Point (AP) Connects to the Wi-Fi access point with SSID - sss and password - ppp. The modem responds OK immediately but the connection happens asynchronously so the status of the connection should be checked with AT+W? or AT+W$. The SSID and password are immediately stored in NVS regardless of whether they are valid or the connection was successful
+W$ Show Wi-Fi IP address Responds with the IP address allocated to the modem from the AP by DHCP. If the connection has not completed or has been unsuccessful the IP address returned will be
+W# Show Wi-Fi MAC address Responds with the MAC address of the modem. No connection is required, the MAC address is always available
+W+ Reconnect to Wi-Fi AP Reconnects to the AP using the SSID and password stored in NVS. If the modem is already joined to the AP, the connection is first disconnected and then reconnected
+W- Disconnect from Wi-Fi AP Disconnects from the AP
+B? Query Baud Rate Responds with the current baud rate configured for the connection to the DTE
+B=n Set Baud Rate Sets the current baud rate to a supported value closest to n. The supported speeds are 4800, 9600, 14400, 19200, 38400, 57600, 115200. After the command is entered there is a 5 second delay before the baud rate is changed to allow changes to be made on the DTE before the modem responds. The new baud rate is immediately stored in NVS
+T? Query Telnet TERM environment variable Responds with the current string value of the TERM environment variable that will be used by the Telnet protocol layer when negotiating the TERMINAL-TYPE option. If no value has been set, or has been cleared, the default value vt100 will be used
+T=ttt Set Telnet TERM environment variable Sets the current string value of the TERM environment variable to ttt. The Telnet protocol layer will use this value when negotiating the TERMINAL-TYPE option. If the command includes no value and is given only as AT+T= then the TERM environment variable is cleared and the default value will be used in future. The new value is immediately stored in NVS.

'S' Registers

There are a total of 51 'S' Registers reserved in the modem, S0 to S50.

Most of them are undefined and unused. The defined registers are set with default values but some registers are ignored as their purpose is typically not useful, this is indicated in the table below. Changing the value of a register that is ignored will have no effect on the operation of the modem.

'S' Registers originally held only 8-bit (byte) values. In this implementation registers are 16-bit.

Defined 'S' Registers

Register Description Range Default Value Used
S0 Number of rings before Auto-Answer 0–255 (0 = never) 0 Yes
S1 Ring Counter 0–255 rings 0 Yes
S2 Escape character 0–255, ASCII decimal 43 ('+') Ignored
S3 Carriage Return Character 0–127, ASCII decimal 13 (Carriage Return) Ignored
S4 Line Feed Character 0–127, ASCII decimal 10 (Line Feed) Ignored
S5 Backspace Character 0–32, ASCII decimal 8 (Backspace) Ignored
S14 TCP/IP Port number for Answer Mode 0-65535 23 Yes
S15 Telnet Protocol for Data Mode 0 - disabled, 1 - enabled 0 Yes
S16 Negotiate Telnet SGA 0 - Won't/Don't, 1 - Will, 2 - Do, 3 - Will/Do 3 Yes
S17 Negotiate Telnet ECHO 0 - Won't/Don't, 1 - Will, 2 - Do, 3 - Will/Do 3 Yes
S18 Negotiate Telnet BIN 0 - Won't/Don't, 1 - Will, 2 - Do, 3 - Will/Do 0 Yes
S19 Negotiate Telnet NAWS 0 - Won't/Don't, 1 - Will, 2 - Do, 3 - Will/Do 3 Yes
S20 NAWS Negotiate Columns 0-255 columns 80 Yes
S21 NAWS Negotiate Rows 0-255 rows 24 Yes
S22 Negotiate Telnet TERMINAL-TYPE 0 - Won't/Don't, 1 - Will, 2 - Do, 3 - Will/Do 3 Yes
S39 CTS/RTS Flow Control 0 - disabled, 1 - enabled, set by AT&K command 0 Yes

Wi-Fi Connection Results


Wi-Fi cannot be started when the terminal is in 30 line mode due to the limited amount of internal memory in the EPS32.

If you attempt to (re)connect the Wi-Fi when in this mode you will receive the response: WIFI ERROR

Return the terminal to 24 or 25 line mode and you will be able to (re)connect the Wi-Fi.

Dial Responses

The following table details the responses from the modem for the Dial command ATDhostname:port

Response Reason for Response
NO DIALTONE when no Wi-Fi connection has been established with an AP
ALREADY IN CALL when a connection is already established ('Dialed' or 'Answered') with another host
ERROR when no hostname is provided
NO ANSWER when a TCP/IP socket can't be opened with the hostname:port
CONNECT when a TCP/IP socket is opened with the hostname:port

Query Wi-Fi Responses

The following table details the responses from the modem for the Query Wi-Fi Access Point (AP) connection status command AT+W?

Response Description
WIFI NOT STARTED when no Wi-Fi connection has been attempted since power-on or hardware reset
WIFI IDLE if Wi-Fi status is queried during a connection attempt
WIFI NO SSID when no AP with the given SSID/password is found following the AT+W+ or AT+W= commands
WIFI CONNECTED following a successful connection to an AP with the AT+W+ or AT+W= commands
WIFI CONNECTION LOST following a lost connection with the AP
WIFI DISCONNECTED following an unsuccessful connection attempt, or a successful disconnection with the AT+W- command

Telnet Protocol

The Telnet protocol in normally considered an optional layer when establishing TCP/IP socket connections with remote hosts to operate as a Network Virtual Terminal (NVT).

Some hosts will provide a more functional service if Telnet is negotiated.

The Telnet daemon telnetd I have on MacOS won't accept a connection if Telnet is not negotiated.

When Telnet is not enabled, TCP/IP socket connections are 8-bit clean. With Telnet enabled only one value, 0xFF known as the IAC byte (Interpret As Command) is overloaded to indicate that the next byte is a Telnet command. Telnet must negotiate binary mode (BIN) to transmit binary files.


If connecting to a remote c-Kermit server ie. under c-Kermit on a remote host enter the command set host /server * 8080

DO NOT enable the Telnet protocol as you are not acting as an NVT in this use-case and c-Kermit doesn't expect Telnet.

Telnet Options Implemented

The modem implements the following minimum set of Telnet options:

Each Telnet Option is negotiated in via a request/response exchange described as Do/Don't (request) and Will/Won't (response). Trying to understand how these work for each Option usually requires reading the RFC (linked in the list above) and extreme patience and experimentation.

Usually you either want an Option completely On (Do/Will) or Off (Don't/Won't).

Setting the supported Options and their default values are defined via specific 'S' Registers detailed above.

In summary the defaults are:

Option Default Description
SGA Do/Will SGA is required for the NVT to work character by character and not in linemode
ECHO Do/Will tba
BIN Don't/Won't To operate as an NVT, binary mode is not required. File transfer protocols like KERMIT and XMODEM do their own binary encoding
NAWS Do/Will The remote host can learn your terminal windows size in characters, the default is 80 x 24 set in S20 and S21 respectively
TERMINAL-TYPE Do/Will The remote host can learn your terminal type, the default is vt100


TERMINAL-TYPEs must be known by the remote system to be recognised.

When connecting to telnetd on MacOS I use vt100+ from the terminfo database which provides support for color over and above the standard vt100 terminal type, making text applications like htop work as expected and in color.

Enabling Telnet Protocol

Telnet protocol is not enabled by default.

  • To enable the Telnet protocol you must manually set 'S' Register S15 to 1 with ATS15=1
  • To disable the Telnet protocol you must manually set 'S' Register S15 to 0 with ATS15=0

The Telnet protocol is applied to both outgoing connections 'Dialed' with ATD and incoming connections 'Answered' with ATA or Auto-answer.

Telnet Protocol Framework

The Telnet protocol implementation in the modem is not fixed, but is instead based on a library libtelnet. There is further information about Telnet and Telnet Options on the GitHub repository for the library.

The library provides a framework for implementing any Telnet Option. So if there are Options that you feel are important beyond the minimum set already implemented let me know, providing a compelling use-case, and I will consider adding them.

Answer Mode

Listening for incoming TCP/IP socket connections is not enabled by default.

  • To enable listening for incoming TCP/IP socket connections you must manually enter AT&A to Enable Answer Mode.
  • Answer Mode will remain enabled, and can only disabled by an ATZ Soft Reset, hardware reset or power-cycle.
  • Incoming TCP/IP socket connections will cause the modem to respond with RING, repeated every three (3) seconds.
  • As each RING occurs the Ring Counter in S1 is incremented by one (1).
  • The user can Answer the incoming call at any time with ATA and the modem will accept the TCP/IP socket connection and enter Data Mode.
  • If the Number of rings before Auto-Answer is set in S0 to a number greater than zero (0 = never) and S1 is greater-then-or-equal to S0 the modem will Auto-answer: accept the TCP/IP socket connection and enter Data Mode.
  • If Telnet protocol is enabled by ATS15=1 then the Telnet Protocol will be negotiated with the remote host after the modem enters Data Mode.
  • The user can Hangup an incoming call by sending the Escape Sequence +++ (with guard times) to return to Command Mode and then sending ATH to Hangup.
  • A Hangup ATH will reset the Ring Counter in S1 to zero (0).
Last Updated: 10/16/2020, 11:33:38 AM