FE-5650A serial interface

[Crosswalkersam]

Hi everyone,

I just received a FE-5650A rubidium reference in the mail .
I read a lot online about what these units can do, and appearently it should be possible to set their output frequency via serial as is shown here:
https://www.qsl.net/zl1bpu/PROJ/Ruby.htm
http://www.leapsecond.com/u/parker/ParkerIntroRFS-PPCP.pdf (page 89 and following).

I opened up my unit, and the PCB looks completely different: Instead of a PIC + some AD9830 I am greeted with FPGAs, CPLDs and a MAXIM DS80C323.

My first Idea was: The DS80C323 has 2 serial pairs in its package, one of which is even connected to a NL27WZ125 line driver / buffer (U412).

However, I am unable to make the unit respond to my 'S \CR' query. Does anyone have experience with these (newer?) versions of the FE-5650A and can tell me how to get it working?

Many thanks!

[Crosswalkersam]

Edit: It apperas that the output of that MAXIM chip is directly connected to pin 8/9 of the DB9 connector (through the line driver).
That lines up with this datasheet, where pin 8/9 are dedicated as 'RS232 TX/RX'.
https://www.morion-us.com/catalog_pdf/RB_58xx_REV_9-Blue.pdf
So my intuition about that chip was right.

I am still unable to interface with it though.