.. _txallow:

====================
Transmit Allow Timer
====================

By default, the SRS-3 will automatically key up and transmit all frames that are routed for transmission on the space link interface. This default behavior can be changed using the ``tx.allow.always`` (default ``true``) and ``tx.allow.time`` (default 0) properties.

Each second, ``tx.allow.time`` is decremented if its value is greater than 0. Before each transmission of a frame (normal or idle), the SRS-3 will check these properties and only allow the transmission if either ``tx.allow.always`` is ``true`` *or* ``tx.allow.time`` is greater than 0. If neither of these conditions are met, the frame is dropped.

To always boot in an inhibited state, the ``tx.allow.always`` property can be saved with a value of ``false`` in the boot and/or fallback stores. In the beginning of a pass, ``tx.allow.time`` can then be set to a value greater than 0 to allow transmissions during the pass. The figure below illustrates how the time can for example be set once in the beginning of a pass, or set periodically during the pass, similar to a watchdog timer. The ``tx.allow.time`` can always be set to 0 to disable the transmission as soon as possible.

.. figure:: img/txallow.svg
   :align: center
   :width: 95%

The ``tx.allow.time`` can not be stored and will always initialize to 0 on startup.
