1) Try to post source code as code. Note the "Insret Program Code" button when you do make posts. This will make the code look like:

void Timer2(void) interrupt 5 using 1 {
    Tick --; 
    if (Tick == 0) { 
        if (DIR2 == STRAIGHT) { 
            switch (STATE2) { 
                case 0: 
                    P27 = 1; 
                    P15 = 1; 
                    P0 = 0x09; 
                    P1 = 0x29; 
                    Tick = 30; 
                    STATE2 = 1; 
                    break; 
                case 1: 
                    P0 = 0x0A; 
                    P1 = 0x2A; 
                    Tick = 30; 
                    STATE2 = 2; 
                    break;
                ...

 

As you can see, this is more readable than the inlined code you posted.

2) One thing here that worries me is your if (DIR2 == STRAIGHT) statement. Why don't define enough states to separate forward and backward direction and stopped? Then the switch statement will know what the next step will be. And note that it might be advantageous to you to keep track of which state you stop your motor in, depending on your requirements for holding torque.

3) What more are you doing in your interrupt, or what more are you using your "Tick" variable for. If all you use it for is to count down from 30 to do the next step, then you do not need to run this interrupt with 1kHz speed, but could use a longer interval such as 2ms, 3ms, 5ms or 15ms instead.

As I did write in an earlier post: If your ISR is always enough faster than 1ms that it leaves enough time for your main loop, then it should not affect the serial communication. Or is your ISR using a global variable larger than one byte that is also used by the serial code, and where the serial code may pick up a half-modified variable sometimes? Note that your processor can't atomically read or write variables larger than one byte so great care is needed when sharing larger variables between an ISR and the main loop.

Please post the full source for the ISR, and the variable declaration for all variables used by the USR together with info if any of the variables are shared with the main loop.