Solution:

We found the improper DO behave of port 0 while the "multiplexer daughter board" is adopted. When you output DO to port 0 or port 1, the port 1 will not work properly. Besides, in such application (the "multiplexer daughter board" is adopted), the high nibble of port 0 can not be used anymore. You also can find those information at page 104 of electronic User's Manual. Following information just for your reference. The PCLD-789 Amplifier/Multiplexer allows up to 16 differential analog input channels to be multiplexed into one analog input (A/D) channel of a DAS card. The PCLD-779 and PCLD-770 allow for total isolation of up to eight multiplexed channels. When used with multiplexer boards, the first four D/O channels of the DAS card are used for scanning/selecting analog input channels of the PCLD-770, PCLD-779 or PCLD-789 one channel at a time. Because of the digital output allocation when using these expansion boards, the first byte of D/O channels (0-7) on the DAS card are no longer available for standard digital output. Each PCLD-770/779/789/889 must occupy its own A/D channel.