EK-LM3S9B92-B echo on uart1 problem

Hi,
I'm working with IAR compiler to EK-LM3S9B92, and trying to convert uart_echo sample from uart0 to uart1 (PORTC_6 PORTC_7).
but it is not working (the same for uart0 PORTA_0 PORTA_1 is OK). I'm connecting the above ports to external RS232 Transceiver to my PC.

following code:

int main(void)
{
SysCtlClockSet(SYSCTL_SYSDIV_1 | SYSCTL_USE_OSC | SYSCTL_OSC_MAIN |
SYSCTL_XTAL_16MHZ);

SysCtlPeripheralEnable(SYSCTL_PERIPH_UART1);
SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOC);

IntMasterEnable();

GPIOPinTypeUART(GPIO_PORTC_BASE, GPIO_PIN_6 | GPIO_PIN_7);

UARTConfigSetExpClk(UART_BASE, ROM_SysCtlClockGet(), 9600,
(UART_CONFIG_WLEN_8 | UART_CONFIG_STOP_ONE |
UART_CONFIG_PAR_NONE));

//
// Loop forever echoing data through the UART.
//
while(1)
{

while(ROM_UARTCharsAvail(UART_BASE))
{
//
// Read the next character from the UART and write it back to the UART.
//
UARTCharPutNonBlocking(UART_BASE, UARTCharGetNonBlocking(UART_BASE));
}


}
}

Thanks in advance.