The modbus_set_error_recovery() function shall set the error recovery mode to apply when the connection fails or the byte received is not expected. The argument error_recovery may be bitwise-ored with zero or more of the following constants.
By default there is no error recovery (MODBUS_ERROR_RECOVERY_NONE) so the application is responsible for controlling the error values returned by libmodbus functions and for handling them if necessary.
When MODBUS_ERROR_RECOVERY_LINK is set, the library will attempt an reconnection after a delay defined by response timeout of the libmodbus context. This mode will try a infinite close/connect loop until success on send call and will just try one time to retablish the connection on select/read calls (if the connecton was down, the values to read are certainly not available anymore after reconnection, except for slave/server). This mode will also run flush requests after a delay based on the current response timeout in some situations (eg. timeout of select call). The reconnection attempt can hang for several seconds if the network to the remote target unit is down.
When MODBUS_ERROR_RECOVERY_PROTOCOL is set, a sleep and flush sequence will be used to cleanup the ongoing communication, this can occurs when the message length is invalid, the TID is wrong or the received function code is not the expected one. The response timeout delay will be used to sleep.
The modes are mask values and so they are complementary.
Its not recommended to enable error recovery for slave/server.