msp430 2553 串口通信问题

我想实现 单片机和电脑串口 通信，就是用串口助手发一串字符给单片机，然后单片机接到了再发给电脑。现在问题出在，我想将单片机接收到的字符串放在数组里，没放进去不知道为什么，，，求教大神，，，不胜感激


#include "msp430g2553.h"
unsigned char string1[];
unsigned int i;
unsigned char flag,flag1;
void main(void)
{
WDTCTL = WDTPW + WDTHOLD; // Stop WDT

while(1)
{
BCSCTL1 = CALBC1_1MHZ; // Set DCO
DCOCTL = CALDCO_1MHZ;
P1SEL = BIT1 + BIT2 ; // P1.1 = RXD, P1.2=TXD
P1SEL2 = BIT1 + BIT2 ; // P1.1 = RXD, P1.2=TXD
UCA0CTL1 |= UCSSEL_2; // SMCLK
UCA0BR0 = 104; // 1MHz 9600
UCA0BR1 = 0; // 1MHz 9600
UCA0MCTL = UCBRS0; // Modula tion UCBRSx = 1
UCA0CTL1 &= ~UCSWRST; // **Initialize USCI state machine**

IE2 |= UCA0RXIE; // 开收中断
_EINT();
// __bis_SR_register(LPM0_bits + GIE); // Enter LPM0, interrupts enabled

if(flag==1) //接收完毕
{
flag=0;
IE2 |= UCA0TXIE; // 开发中断
}

if(flag1==1) //发送完毕
{
flag1=0;
IE2 &= ~UCA0TXIE; // 关发中断
}
}
}
// Echo back RXed character, confirm TX buffer is ready first
#pragma vector=USCIAB0RX_VECTOR
__interrupt void USCI0RX_ISR(void)
{
while (!(IFG2&UCA0TXIFG)); // USCI_A0 TX buffer ready?
do{
string1[i++]=UCA0RXBUF; //接收
}
while(string1[++i]!='');
flag=1;

}

#pragma vector=USCIAB0TX_VECTOR
__interrupt void USCI0TX_ISR(void)
{
do{
UCA0TXBUF = string1[i++]; // 发送
}
while(string1[++i]!='');
flag1=1;

}