Solution:

Basically, due to those 16 bits DIO were DI with DO at the same pin, so the initial status was set as LOW. You can read the default status via program DEBUG under MS-DOS Prompt. That's why our customer met the phenomenon. Please set the pin as HIGH to overcome this problem. Moreover, the description on the User's Manual just enlighten us the connection when we connections the signal with other TTL devices (while set the signal as DIGITAL INPUT). Following information just for your reference. C:\Windows>debug - i 303 00 <=== the default status. The voltage between pin-1 (or pin-2, pin-3, ..., pin16) and pin-18 is around 0V. -o 303, ff <=== Now the voltage between pin-1 (or pin-2, pin-3, ..., pin8) and pin-18 is around 5V. -i 303 01 <=== When I add a pull-up resistor between pin-1 and pin-18.