I would suggest the following items.
1. Change your "A" variable to a byte instead of a word since asynchronous serial ports transmit and receive ASCII characters which are bytes not words.
A var byte

2. Check the manual on HSERIN and use the timeout and label options with your command to help debug the issue.
Maybe something like this....

MAIN:

HSERIN 100, timesup, [A]
LCDOUT $FE, 1
LCDOUT $80, "A: ", Dec A
PAUSE 1000
GOTO MAIN

timesup:

LCDOUT $FE, 1
LCDOUT $FE, $80, "Timed Out!"
PAUSE 2000
GOTO MAIN

END


Regards,