Hi Ed,
Here's a link to the SSA324 user manual
https://www.rtsintercoms.com/br/rts/file?i=97659&lg=engThis unit will convert 2 4W intercom channels into 2 2W systems, which then need a power supply to run RTS beltpacks, like a BP325. You mentioned earlier you had an existing RTS system, so I'm guessing you just need to interface to the ATEM.
Simply put, you need to connect the talk and listen ports of the ATEM (XLR on the 2M/E models, or BNC AES on the Studio converters) to the SSA324, and then the 2W port of the SSA324 to the RTS system power supply (PS-8, PS-10, PS-31, etc).
Specifically, connect the terminal block on the back of the 324 like this...
1- pin 2 of female XLR for ATEM talkback out
2- pin 3 of female XLR for ATEM talkback out
3- pin 2 of male XLR for ATEM talkback in
4- pin 3 of male XLR fro ATEM talkback in
If you have an ATEM 2/4 M/E with XLR talkback, this is where you connect the 2 XLRs wired above. If you want to connect to a studio/talkback converter instead, you will have to convert these XLR to AES audio on BNC using something like a Flying Cow AES/Analog converter and connect to the proper loop BNCs on the back of the BMD converter.
The J101 XLR on the 324 is meant to connect to your RTS power supply. The PS connection and wiring varies by model, which one do you have? Then depending on your model the setting of the switch S101 might come into play.
After all of these connections are made, you need to "null" the 324 for best performance. you do this by plugging a headset into the front of the 324, pressing the TEST button, and adjusting the Hi-Med-Low pots until a minimum of test tone is heard in the headset.
Given how well the ATEM intercom works (NOT!) you may feel that all of this is not worth the trouble. We have tested the ATEM with a similar 2W>4W converter so we know this setup is correct and functional given the quality/limits of the ATEM. In our case we use a completely separate intercom system if that tells you anything...