Solution:

PCL-818L/HD use the first 4 digital outputs to switch the input channels on PCLD-789D. When you use the D-37 cable to connect PCLD-789D to PCL-818L/HD, you need to set the 4 jupmers (JP1/JP2/JP3/JP4 on PCL-818HD, JP8/JP9/JP10/JP11 on PCL-818L) to S sides. This will make the 4 digital outputs being used with D-37 connector. The default settings are at D sides, and these 4 digital outputs are used with the 20-pin D/O connector.