The DIU 3000 is the interface for digital voice to the Quantar. It will implement trunking talk groups, decode voice data and interface it to a wireline with tone remote to a console, and even supports crypto with SCM modules.
I found the DIU is able to output quite a bit of detailed diagnostic info via the serial port. The below is an idea of what it can do. Dumping the firmware gives some more info as well.
I can't find a schematic diagram of it. There seems to be lots of parts in one of these for what it actually does.
I think this is a DES module.