TCP Client

This application uses the ZentriOS TCP client command to open a TCP connection to a TCP echo server on the internet.

The TCP echo server, located on the internet at, simply echoes all received data back to the ZentriOS TCP client. Once the connection is established, stream_write is used to send data to the connection stream. With all data sent, stream_poll is used to check for received data on the connection stream, and stream_read is used to read data waiting on the stream.

You will only need one Zentri evaluation board for this example.


Let's get started by connecting your board to a Wi-Fi network and opening the TCP connection stream. You can simply cut and paste the commands in the following table, but DON'T FORGET to replace your Wi-Fi network name and password for the defaults.

ZentriOS Commands Description

set wlan.ssid    YOUR_AP_NAME
set wlan.passkey YOUR_AP_PASSWORD
tcp_client 50007

<- Set the name of your Wi-Fi AP
<- Set the password for your Wi-Fi AP
<- Open TCP client session

There is no need to bring up the WLAN network interface prior to opening a TCP connection, the TCP client (just like all commands that need the network) automatically connects to the network if a connection has not already been established.

An example showing what the session looks like if you entered the above commands successfully is shown below. Notice that the new TCP connection is assigned a stream handle, starting at 0 for the first TCP connection.

Session log when opening the TCP client connection

> tcpc 50007
[2014-05-01 | 12:34:45: Associating to YOUR_AP_NAME]
Obtaining IPv4 address via DHCP
IPv4 address:
[2014-05-01 | 12:34:48: Associated]
Resolving host:
[2014-05-01 | 12:34:48: Opening:]
Connecting (TCP):
[2014-05-01 | 12:34:48: Opened: 0]

Check the status of open stream before continuing. Use the stream_list command to get a list of open streams. The number in brackets appended to each line is the local port used for the TCP connection stream.

> stream_list
!# Type  Info
#0 TCPC (41713)

Writing Data

Writing data to a stream is easy using the stream_write command. Copy and paste the following commands to write data to the open stream.

ZentriOS Commands Description

stream_write 0 23
Hello from stream 0

<- Prepare to write 32-characters to TCP connection stream 0
<- Data to write

Checking for Data

To check for data on any stream (or all streams at once), use the stream_poll command as shown in the following session. If you don't want to poll for data, try assigning a GPIO to a stream to indicate when data is available. The tcp_client command describes how to assign a GPIO.

Checking for data

> stream_poll 0
> stream_poll all

Reading Data

To read available data from any stream, use the stream_read command as shown in the following session. The following example reads up to 100 bytes from the stream.

Checking for data

> stream_read 0 100
Hello from stream 0

Change Log

ModifiedChangesZentriOS Version
2015-Feb-14Changed the example to use the server1.1+