Serial Interface

ZentriOS provides a flexible serial interface. It supports various serial protocols and modes, and is easy to use for both humans and machines alike. ZentriOS supports two serial protocols, UART and SPI.

UART Protocol

A single or dual UART is available for use by the host MCU. If a host can support two UARTs, one UART may be used for control and data communication, and the other used for debug logging. If a single UART is available, control, data and debug logs (if enabled) are interleaved.

SPI Protocol

SPI provides a higher speed communications interface for higher bandwidth applications such as audio and video. Full support for a serial-SPI communications interface will be added in an upcoming release of ZentriOS.

Sending Commands and Streaming Data

A ZentriOS device connects a host system to a network via Wi-Fi.

First the ZentriOS device is configured to connect to the network and to other hosts on the network. Then the ZentriOS device allows the local host to communicate with other hosts by exchanging data.

To exchange data with other hosts, you can stream data transparently between the Wi-Fi interface and the local serial interface.

Alternatively, you use ZentriOS commands to mediate the exchange of data.

ZentriOS commands can be issued via:

The local serial interface and the remote terminal are mutually exclusive: connecting a remote terminal disables the local serial interface to the command parser.

The local serial interface can either stream data, or send commands and receive responses. It cannot do both simultaneously.

Note: to guarantee that no characters are dropped when using the local serial interface, uart.flow must be turned on. This applies at any baud rate.

The HTTP RESTful API interface is available at all times, if the HTTP server is running and the RESTful API is enabled.

Access to the ZentriOS command parser is determined by:

Serial Bus Mode

Serial bus mode concerns the local UART serial interface.

A module running ZentriOS can operate in two distinct local serial bus modes, determined by the bus.mode variable:

ZentriOS commands can also be issued via the local serial interface when in STREAM mode:

The diagram below shows a schematic representation of access to the command parser via the serial interface and the Wi-Fi interface.

Note that this diagram illustrates access to the command parser only. Access via ZentriOS commands to Wi-Fi ports and other module features is unaffected by serial bus mode or remote terminal status.

Command configuration and protocols are as described in Command Mode and Command Protocol, whether commands are issued via the UART in COMMAND mode or COMMAND breakout, via the remote terminal, or via the HTTP RESTful API.

In STREAM mode, you can stream data between the UART and the Wi-Fi interface, as described in Stream Mode.

As the STREAM mode + Remote Terminal case in the above diagram suggests, you can stream data to a remote host, and at the same time issue commands to the ZentriOS device via the remote terminal or the HTTP Server RESTful API. A command issued to the command parser blocks commands from other interfaces until it is completed.

The state diagram below shows the transitions between serial bus modes.

Rebooting to COMMAND mode

To reboot to COMMAND mode, set the bus.mode variable to command, save, and reboot:

set bus.mode command

COMMAND breakout from STREAM mode

To issue commands in STREAM mode, send the breakout character sequence specified by the This is by default $$$.

Alternatively, set up a GPIO to enter command mode by enabling the GPIO function.

On switching from STREAM mode to local COMMAND mode, ZentriOS responds with Command Mode Start.

You can then issue any ZentriOS commands.

You can set bus.mode to command and save, so that on the next reboot the ZentriOS device starts in local COMMAND mode.

When you have entered local COMMAND mode from STREAM mode via a breakout sequence or GPIO, you can return to STREAM mode by issuing the exit command.

Rebooting to STREAM mode

To switch to STREAM mode from command mode, set the bus.mode variable to stream and reboot:

set bus.mode stream

Exit from COMMAND Breakout

When you have entered local COMMAND mode, or remote COMMAND mode, from STREAM mode via a breakout sequence or GPIO, you can return to STREAM mode by issuing the exit command. ZentriOS responds with Command Mode Stop. For example:

Sequences and CommandsDescription


Break out of STREAM mode
Issue some ZentriOS commands

Return to STREAM mode

Using the Remote Terminal

When the remote terminal is enabled and connected via telnet, the local command interface is disabled. However the UART can still stream to the Wi-Fi interface if the module is in STREAM mode. When a ZentriOS device is in COMMAND mode and a remote terminal is connected, disconnecting the remote terminal switches back to COMMAND mode. You can then issue commands via the local serial interface.

Enabling and Connecting the Remote Terminal

Enable the remote terminal with remote_terminal_enabled:

set remote_terminal.enabled 1

Then connect a remote device via telnet to the ZentriOS device. The default port is 2000. The host IP or name depends on the ZentriOS configuration.

For details, see the Wi-Fi Remote Terminal application note.

Command Mode

Command mode is the primary mode used to configure ZentriOS. Understanding how command mode works, and the options provided, is the key to mastering the use of ZentriOS. The serial interface is designed to cater for humans and machines alike, and the interface is configurable to suit the needs of both.

Note: When controlling ZentriOS via an MCU, we suggest using machine mode, described below.


In command mode, ZentriOS and the ZentriOS device effectively provides a slave interface to the host. The host is the master and ZentriOS is the slave. The host initiates all transactions which follow the format of "host issues a command, ZentriOS provides a response". ZentriOS commands are used by the host to read and write ZentriOS variables, send control information, and send and receive data across network connections.

The variable system.cmd.mode is provided for convenience to make it easy to switch between human mode and machine mode. Setting system.cmd.mode conveniently changes the value of each of the following variables to configure the desired command mode.

Human Friendly Command Mode

By default, the serial interface is configured to be human friendly with the following settings. See the system variable documentation.

set system.print_level all       -> Turn on all debug & informational prints
set system.cmd.header_enabled 0  -> Disable a response header
set system.cmd.prompt_enabled 1  -> Turn on the user prompt
set system.cmd.echo on           -> Turn on echo to see what you're typing

Machine Friendly Command Mode

To configure the serial interface for machine friendly operation, use the following settings. Detailed information about these settings is available in the system variable documentation.

set system.print_level 0         -> Turn off all debug & informational prints
set system.cmd.header_enabled 1  -> Enable a response header (described below)
set system.cmd.prompt_enabled 0  -> Turn off the user prompt
set system.cmd.echo off          -> Turn off character echo

Command Protocol

ZentriOS commands are formatted as shown below. <command name> <arguments>\r\n where:

In the case of the get and set commands, the arguments are in the form: <variable name> <arguments> where:

The command is terminated by a carriage return and newline character (\r\n).

Response Format

Responses from ZentriOS follow the format shown in the text below.

<response data>

where ...

Response Error Codes

  0 = Success
  1 = Command failed
  2 = Parse error
  3 = Unknown command
  4 = Too few args
  5 = Too many args
  6 = Unknown variable or option
  7 = Invalid argument
  8 = Serial command line buffer overflow
  9 = Bounds error, command specific, bounds of the command were exceeded

Log Format

If system.print_level is greater than 0, a log header together with one or more informational debug logs may be returned in addition to a response header and response data when a command is issued. Log headers follow the format shown in the table below.

<log message>

where ...

Stream Mode

As depicted in the following diagram, stream mode provides a transparent connection between a serial port and a network stream such as a UDP or TCP client or server. It is perfect for implementing applications such as a wireless serial port.

Bytes or characters sent from the host to a serial interface are transparently pushed by ZentriOS to a network stream via a wireless interface. Conversely, bytes or characters received by a network stream (from a remote server) via a wireless interface are transparently pushed by ZentriOS to a serial interface connected to the host. A wireless serial port is a typical application that uses stream mode.

Stream mode is used primarily by host applications that can handle asynchronous data (with the optional use of hardware flow control). It is perfectly suited to applications that only need to use a single network stream at any one time.


The simplest way to setup a ZentriOS device to use Stream mode is to do all the ZentriOS configuration first using command mode, then reboot into stream mode. Configuration involves just a few simple steps: - Setup a wireless and network interface to use with stream mode e.g. wlan interface and TCP client. This typically requires setting up the wireless interface and network stream to auto-connect, or enable connection using a GPIO control pin. - Set the serial bus.mode to stream. - Then save and reboot the module.

Both the wlan and softap interfaces have an auto-start on bootup capability using the wlan.auto_join.enabled and softap.auto_start variables. The TCP server has auto-start capability using the tcp_server.auto_start variable. Network clients can be started using a GPIO or manually by breaking out of stream mode as described below.

Breaking out of Stream Mode

When the serial bus is in Stream mode, commands are disabled and every character sent to the serial port is automatically forwarded to a network stream. There are two ways to break out of stream mode and enable command mode entry temporarily. The first method is to send a character sequence, configurable with the, after 3 seconds of inactivity on the serial interface. By default, the stream break out sequence is $$$. The other option available to break out of stream mode is to use a GPIO configurable with the variable. When you're done entering commands, return to stream mode by simply issuing the exit command.