You're doing all the right things asking these questions ahead of time.
If the SLC-5/04's serial port (called "Channel 0", the DB9 male connector toward the bottom of the CPU) is not plugged into anything now, you could experiment by making an adapter cable and just moving the DCS connection from the KF2 to that port. If it works with the SLC-5/04, it will also work with the SLC-5/05: their serial port functionality is identical.
My guess is that the designers were planning on connecting the DCS to multiple A-B controllers, or they had a manual that showed how to connect via a 1770-KF2 from back when A-B controllers didn't have RS-232 serial ports (before about 1988).
You can figure out the serial configuration of the 1770-KF2 from
the DIP switch settings on the module. There is no software/nonvolatile configuration, it's all hardware set.
The packets the DCS is sending to the -KF2 will have a destination "Station Number" on them, which is equivalent to the DH+ Station Number of the target device. DH+ is some old stuff; there are sixty-four possible Station Numbers but they're numbered in octal, 00 through 77.
It's likely that the SLC-5/04 is set for DH+ Station 01, and the DCS is addressing its DF1 protocol packets to DF1 Node 01. So all you have to do is configure the Channel 0 serial port to also use DF1 Node 01, so it reacts correctly to the incoming serial packets.
You can get an idea for the diversity of command code support among A-B controllers from the
1770-RM516 DF1 Protocol Reference manual in the table on pages 7-2 and 7-3.
Because the SLC is the target of the messages already, it will almost certainly support all the commands being sent by the DCS via the 1770-KF2 if those commands come through the serial port instead.
I give the DCS a <5% chance of trying to read diagnostic data directly out of the KF2, which wouldn't be reacted to by the SLC serial port and would give you some headaches.
This is the part where I mention that if you're going to do serial analysis work on legacy systems, the best money you can spend is on a
Stratus Engineering EZ-Tap.
Even if you don't also spring for a Frontline Test Equipment analyzer software suite, being able to know for sure what's on the serial line instead of squinting angrily at blinky lights is a game-changer.