Serial port

The serial port on the Group Telepresence 3330 (GT Mini) is labeled COM. The serial port on the Group Telepresence 3351 is labeled COM 2.

The settings for the port are: “115200,8,N,1”.

The description given here presumes that you are connecting the room system to an RS-232 DTE (a PC for example).


PC end DTE signal names Room system end
2 RXD 3
3 TXD 2
5 GND 4
    Do not connect Pin 1

Sending requests over the serial connection

Each request consists of a JSON-formatted object, terminated by a newline. The object must contain another object called request, which is processed in exactly the same way as the content of the HTTP POST request above.

For example, to dial the number 1234 send the following:


To request the current call state, send:


Example responses using the serial connection

For example, if the endpoint receives the request:


It might send the response:


Which indicates success in that the endpoint has received the request and returned a response. For actions, this is usually returned immediately.

State updates work similarly, though may be returned at some point in future. If the endpoint receives a request like:

{“id”:17,”request_example”:”two”,”request”:{“action”:”state”,”filter”: “calls”,”counter”:567}}\n

The endpoint does not send a response until the call state updates. If an incoming call arrives, this is updated and the endpoint sends the response:


And an example of a failing request:


With response:

{“id”:23,”request_example”:”three”,”response”:{“error_code”:7,”error_message”:”Invalid state for action hold: no usable default call for action.”}}\n

Example serial-connection session

Below is an example session showing a request on the state of the audio and also switching the audio mute on. The example session shows you how you might use unique IDs per request. You can see that this is useful, because in the example the responses are not in the same order as the requests, but are identifiable by the ID. The example session also shows correct usage of the counter.

Send {“id”:1,”request”:{“action”:”state”,”filter”:”audio”}}\n
Receive: {“id”:1,”response”:{“counter”:385,”audio”:{“counter”:10,”mute:

Send: {“id”:2,”request”:{“action”:”state”,”filter”:”audio”,”counter”:

Send: {“id”:3,”request”:{“action”:”audio_mute”,”off”:true}}\n
Receive: {“id”: 3,”response”:null}\n
Send: {“id”:4,”request”:{“action”:”audio_mute”}}\n
Receive: {“id”:4,”response”:null}\n
Receive: {“id”:2,”response”:{“counter”:394,”audio”:{“counter”:11,”mute:t