when the PC start up and shut down, the uP board will halted."

1) What about in between start-up and shut-down?
2) Does it occur always or sometimes?
3) Is 8051 board an already tested product?


If (3) is true,
(a) There may be problem in your RS-422 hardware
(b) Your PC is not properly grounded. If not grounded, a PC has 115V AC (if power input is 230V) on its chasis/ground.


Good Luck

Regards

IJAZ