When a typical two-conductor footswitch (latching or momentary) is inserted into one of these jacks and the switch is engaged, the device is expecting to see something that was disconnected become connected, or vice versa. When you engage the switch, you're physically changing something in that part of the circuit to being either connected or shorted to ground.
In modular synth terms, it might be tempting to think of a signal that goes from +5V to 0V to be like saying "Turn off". And while 0V might indeed be related to the device's internal ground reference, it isn't the same thing as physically shorting a connection to ground. So your drum machine might not be responding to your modular synth as you'd expect because it's looking for one thing and you're giving it another.
The trick then is to differentiate between two voltages that we designate as "on" and "off" and convert one of those electronic states into the physical state of being grounded. This can be accomplished with an electronic switch, provided it includes a common jack that connects to one or the other of a pair of I/O jacks. I haven't seen many of these around in the modular synth world, but the Q128 Switch from synthesizers.com is an example of one.
(If you know of others, tell me about them, and I'll list them here!)
[If you're wondering about V-trig vs. S-trig, check out the note at the end of this post.]
Taking the foot out of switch
One of the switch's I/O jacks is connected to common when the control signal is high, and the switch's other I/O jack is connected to common when the control signal is low. Only one or the other is connected to common at a time.
If a ground plug -- a simple device that even someone with the most modest of soldering skills can make (see more on this below) -- is inserted into the switch's common jack, anything else connected to that jack is also grounded. Thus it's possible to force a physical connection to ground using a control signal, just like pressing a footswitch.
Consider the Roland CR-78 CompuRhythm (photo of its backside below). A tap on a momentary footswitch plugged into the drum machine's START/STOP jack starts the CR-78, and another tap stops it. That's because a tap on the switch briefly shorts the jack to ground. Some other part of the circuit is watching for that state change, and starts or stops the CR-78 accordingly. The VARIATION and WRITE jacks work the same way.
Using a switch like the Q128 mentioned above, the same thing can be accomplished by:
- Connecting the Q128's A jack to the CR-78's START/STOP jack
- Inserting a ground plug into the Q128's COMMON jack
- Sending a CV pulse to the Q128's CONTROL jack
The CONTROL jack's "normal" state (no pulse) is low, which means A is not connected to COMMON (the B jack, which we're not using, is connected to COMMON instead; but what's more important is that A is not connected to ground). Sending a pulse from the modular or DAW to the Q128's CONTROL jack causes it to go high for the duration of the pulse, momentarily connecting the A jack to COMMON (ground). The CR-78 sees that state change, and it starts playing (or stops, if it was already playing).
Side note: Re-inserting cables can leave the CR-78 in a stuck state. If it stops responding, usually pressing the START/STOP button once will clear it up. If not, reinsert the Q128 CONTROL cable. If you're sending CVs from your DAW, sometimes reinserting the CONTROL cable needs to be done while the DAW's transport is running. So far I've only encountered this quirky behavior with the CR-78.
It's worth noting that if you insert an oscilloscope between the Q128's A jack and the CR-78 expecting to see the control pulse reflected there, you won't see it -- the scope trace will just sit there at 0V. Behind the scenes though, the leg of the CR-78 circuit to which the A jack is connected will short to ground when the Q128's CONTROL jack goes high, and the CR-78 will respond accordingly.
DIY ground plug
It's exceedingly easy to make one. Grab a 1/4" mono (2-conductor / Tip-Sleeve) plug -- the same type used for your modular patch cables. I like to use the Switchcraft 280, but there are others, like the Neutrik NYS225. The same concept applies to 1/8" (3.5mm) patch cables used on Euro systems; a good candidate being the Neutrik NYS226.
Unscrew the housing, and you'll see two metal terminals, probably sheathed within a plastic tube. The tube is an insulator to prevent exposed wire within from touching the housing, which (if it's metal) is part of the Sleeve's connection to ground. Since we intend to short the tip to ground for this plug, the insulator isn't relevant. You can leave it in there when you're finished, or take it out.
The shorter of the two terminals connects to the plug's Tip. The longer terminal has a pair of metal strain-relief flanges at the end used to squeeze and hold the cable, and it's connected to the plug's Sleeve. The goal is to fuse these two terminals together so that the Tip is directly connected to the Sleeve (see the red line in the diagram below).
About V-trig vs. S-trig
You may have heard about a special cable that converts voltage gate (V-trig) signals to switch trigger (S-trig) signals (if not, there's a good explanation here). If you have one of these cables and can deal with the Cinch-Jones plug, it may work for your device. Keep in mind though that such a cable is intended to interface between synthesizers that use these opposing trigger types. It's essentially solving the same problem discussed here, but even so the conversion cable may not work with every non-synth instrument.
I tested the conversion circuit in the cable against a variety of devices with footswitch jacks using various trigger sources from my modular and my DAW, and I found that it did not work more often than it did. While that may have more to do with the particular devices I tested than with the converter itself, it's worth saying that the method described in this post has worked with every device I've tried -- even when mixing and matching modular formats in between source and destination.