Toyota AD820 - nothing on LCD display when power on

Hi,

I just buy an old Toyota AD820 (not the A model).
When switching the power on, nothing appears on the LCD display -except an '_' on left at the upper line and the red LED light on.
No button responds.

Nothing writtent in the user manual I have (35 pages) about such trouble, nor a way to 'reset' the software or test it.

Because i'm just new about this machine, somes questions :

Does have the computer to be connected through serial port to enable the 820 to start ?
Any thing to check ?

Thanks in advance for your advices.

PS : and if someone is interrested by the Manual (pdf), let me know.