Welcome to the Forum !
I haven't used this specific DCS interface, but in my general experience you need both written documentation and careful electrical and protocol analysis to get one of these old DCS interfaces to work.
The documentation I see for that device says it's built for PLC-2, 3, and 5 controller interfaces. Remember that those have DB25 connectors, for which the pin assignments are different from the DB9 connectors on an SLC-5/0x controller.
Do you know which exact PCCC command code the SI module is using ?
It's possible the DCS is using the old PLC-2 command set. In the SLC controller, only Data File 9 can be accessed with the PLC-2 command set, aka the "Common Interface File" or "CIF".
The last time I did a DCS serial interface, I used the USB/RS232 intercept box from Frontline Test Equipment, along with their NetDecoder software. There's no doubt those tools made the difference between success and failure.
The quick and dirty way to see what the DCS is doing is to hook up the serial cable to your PC instead of the SLC-5/05 controller, and use a serial sniffer or terminal program to see what hex bytes are being sent by the DCS.