In most cases, a gate or trigger signal – a voltage that suddenly rises from usually 0 volts to a high level such as 5 or 10 volts – indicates the start of a new note in a modular synth. However, some systems – such as the original Moog modular – used an s-trigger instead, which was a wire that was shorted to 0 volts ground, like the closing of a switch wired to ground. You cannot interconnect these two systems without some form of conversion between the two, which can be as simple as a special cable.
For more in the subject, I refer to you to Moog format specialist Synthesizers.com.
Similar:
shorting trigger