Behringer's copy of Moog's Model D synthesizer has much more connectivity than Moog's original. However if you use a keyboard or Midi controller without a sync output there is no way you can trigger external modules since there is no trigger or S Trigger output on the Behringer model. There are two envelope trigger inputs on the Behringer, one for the filter envelope (FC Gate) and one for the loudness envelope (LC Gate). If you look at what comes out of these two jacks, there is a trigger output signal which idles at about 0.7V and rises up to 2.5V when triggered by the Midi controller:

Whether this is intentional or just a sneak circuit is not clear, since if you put the lightest load on these inputs the signal goes haywire:

Buffering the signal with an op-amp (a TL071 will do) to drive a NPN transistor produces a S-Trigger signal which can be used to trigger other modules which require a S Trigger input:

Scope trace 1 is the Gate in signal and trace 2 is the S Trigger output of the circuit when connected to a S Trigger module which idles at 10-12V and falls to near ground when triggered as required by S Trigger modules:

The Beringer high pass and low pass filter modules are copies of the original Moog modules. There is no Behringer 904C Filter Coupler which would give band pass and band reject functions. For some reason the A and B modules have not connectorized the control nodes, so the only option is to build your own and hook into the voltage control summing junctions. The 9 pole 3-way wafer switch once a cheap item is ridiculously expensive so I will use three rotary switches, another thing to adjust added to all those patch cables. Watch this space....