For some reason I was thinking the second schematic was using a direct connection to the Atari SIO port and the signaling you gave was confusing me. I can see that it likely is RS232 instead. Now it makes more sense.
I also originally thought the second schematics was for SIO, but once I thought about it realized that RS232 serial was more likely. It might even be able to work with SIO using different firmware.
From the instruction sheet provided the device you have appears to be the parallel to parallel version, the top schematic Richard posted, which adds a buffer to an existing parallel printer interface.
Edited by BillC, Tue Aug 8, 2017 1:47 PM.