Freescale MC9508 secondary processor not talking to EM357 radio

I have an XBee S2CTH (XB24CZ7UIT) module running version 4060 of the zigbee firmware. It is configured as a router. I wrote and compiled an application program in CodeWarrior and uploaded it to the module using the XModem feature in the legacy X-CTU program. It seems to start up ok and output is displaced in the Terminal window.

I setup an API Transmit frame (formulated using the XCTU Frames Generator, so it is a valid packet) and then try to send it from the application to the radio using uart_write (after initialization: radio_uart_config (115200, UART_CFG_BITS_8 | UART_CFG_PARITY_DIS)).

The packet contents are displayed in the Terminal window; however, it seems they are not sent serially to the radio. That is, uart_write appears to be sending the data out to the host on the external DOUT line instead of the internal DOUT_Radio line. This is wrong - what am I missing?

Also, the RSSI light lights up at startup when, I presume, the device joins the network. This is confirmed by the output from my xbee_modem_status_handler ISR which is called for this event.

I feel it must be a configuration issue - something fundamental, but the SDK documentation is lean. Any help here much appreciated.