user15630: I use  RH_RF24.h to control Si4463, and realize some simple communication between two Si4463 . But I don't know the mean of RH_RF24_MAX_MESSAGE_LEN in this example.
RH_RF24_MAX_MESSAGE_LEN is a constant ? If so , what decides it ? And if I need to sent and receive  a long  char string , how to do it ?

user10394: RH_RF24_MAX_MESSAGE_LEN is probably defined in the header file RH_RF24.h.
It doesn't matter what size it is, you should always use RH_RF24_MAX_MESSAGE_LEN rather than the number because the value might change in the library and this will break your code if you have used the number.
If you want to send a message that is larger than RH_RF24_MAX_MESSAGE_LEN then you will need to 'fragment' the message (unless the library supports that).
I would use the first byte of the message as the type of message and define one of those types as a multi part string.  Then send a number of these message and reassemble the data on the receiver. When the receiver sees a message that contains a character with the ASCII value 0 then that is the end of the string.

15630: <p>I use  <a href="http://www.airspayce.com/mikem/arduino/RadioHead/classRH__RF24.html" rel="nofollow noreferrer"><code>RH_RF24.h</code></a> to control Si4463, and realize some simple communication between two Si4463 . But I don't know the mean of <code>RH_RF24_MAX_MESSAGE_LEN</code> in this <a href="http://www.airspayce.com/mikem/arduino/RadioHead/rf24_client_8pde-example.html" rel="nofollow noreferrer">example</a>.</p>

<p><code>RH_RF24_MAX_MESSAGE_LEN</code> is a constant ? If so , what decides it ? And if I need to sent and receive  a long  char string , how to do it ?</p>
 10394: <p><code>RH_RF24_MAX_MESSAGE_LEN</code> is probably defined in the header file RH_RF24.h.</p>

<p>It doesn't matter what size it is, you should always use <code>RH_RF24_MAX_MESSAGE_LEN</code> rather than the number because the value might change in the library and this will break your code if you have used the number.</p>

<p>If you want to send a message that is larger than <code>RH_RF24_MAX_MESSAGE_LEN</code> then you will need to 'fragment' the message (unless the library supports that).
I would use the first byte of the message as the type of message and define one of those types as a multi part string.  Then send a number of these message and reassemble the data on the receiver. When the receiver sees a message that contains a character with the ASCII value 0 then that is the end of the string.</p>
