Arduino UNO Using serial port without USB Connected problem

Please does anybody have any idea what is going on? Step 1 Upload the AnalogReadSerial example to my uno. Step 2 I have connected pins TX - RX _GND to a 9 pin serial connector and with a null modem to my PC Step 3 I have open Hyperterminal and I read to my Hyperterminal screen the analogue values from arduino fine.

Problem If I disconnect USB cable ( power off arduino ) when arduino restart it doesn't send anything to my serial port. If I upload again the sketch through usb cable everything works fine I can understand what is going on!

If I upload another example such as blink after disconnect usb cable and connect again power off - on the arduino example blink works fine.

thanks in advance !!

Are you sure that it is the Arduino that has stopped sending and not Hyperterminal that has stopped responding because of the state of the Arduino pins when it is powered off ?

This happend only if i disconnect usb cable!!

This happend only if i disconnect usb cable!!

Yes, but what does that do to Hyperteminal I wonder ? Plugging USB power in again will reset the Arduino so that proves nothing.

gstavaris: I have connected pins TX - RX _GND to a 9 pin serial connector and with a null modem to my PC

You're connecting TTL logic level pins directly to an interface using RS232 signal levels? That's a very bad idea. The signal polarity is inverted and the RS232 voltage range can exceed the range that the Arduino will tolerate. For this to work, you'd need a TTL-to-RS232 converter.