Boss RC-5 Footswitch Guide
Intro
The Boss RC-5 Loopstation is a pedal looper with loads of features. One of these features is support for connecting an external footswitch to control additional pedal features. An external footswitch is a box with one or more buttons which can be connected to the pedal via a patch cable. This allows additional features of the pedal to be assigned to these extra switches.
For details on what specifically you can control with a footswitch, and how to configure the pedal for it, see the RC-5 Reference Manual.
Officially, Boss only ever lists their own external footswitches as being compatible with the RC-5. However, Boss' footswitches are rather large. There are many brands of generic third-party footswitches which are much more compact (and inexpensive) but it is difficult to know which ones, if any, are compatible with the RC-5.
Compatibility
The Boss RC-5 supports external footswitches with one or two non-latching normally open SPST switches. This may also be written as "SPST ON-(OFF)".
Non-latching means that the switch's state changes when you step on it and changes back when you release. It's not "sticky". These are also called "momentary" switches.
Normally open means that when you are not pressing the switch down the circuit is connected. Stepping on the switch disconnects the circuit. This is the opposite of "normally closed".
SPST means single pole, single throw. The switch has one input and one output.
Many of the inexpensive and small external footswitches available online don't list their switch type. These are almost all normally closed momentary switches. As is, these switches will not properly control the RC-5. If you plug one in you can get it to do things on the pedal but it won't be consistent with the instruction manual and it won't be usable for pedal control (each footswitch will appear to "activate" whenever you lift your foot up).
Note that for a single footswitch, any patch cable can be used to connect it to the RC-5. However, for a unit with two footswitches you must use a TRS cable. Learn about the difference between TS and TRS cables.
Options
It is possible to get an inexpensive, small form-factor, external footswitch that works with the RC-5.
1. Get a switch that is SPST ON-(OFF)
Look for a switch which specifies that it has momentary or non-latching normally open switches. These are not very common, but if you find one you like it will work with the RC-5.
2. Get a switch that is configurable
Some footswitches have additional circuitry which allows their mode to be adjusted. For example, the "NUX NMP-2 Dual FootSwitch" is switchable between latching, normally closed, and normally open. Unfortunately this also requires a power supply and is a bit more expensive than some other passive choices.
3. Get a box that you like and put SPST ON-(OFF) switches in it
You can buy a cheap footswitch and replace the individual switches in it with one that have the correct mode for the RC-5. For example, the RockStock Dual Footswitch is about as inexpensive and compact as they come and you can get it in black and white. You can then get two SPST ON-(OFF) switches from DigiKey for under $10. If you are comfortable doing the most basic soldering job which doesn't require any precision or care you can swap them out in about fifteen minutes.
(Note: this is what I ended up doing and I'm very happy with the result. It's worth considering if you really like the form factor of a device that doesn't have the right type of switches.)
You can also buy empty footswitch enclosures from a variety of big box and specialty stores.
Notes
The above is likely also true for the RC-3 and perhaps other Boss loopers but I have only personally tested with the RC-5.
If you have a MIDI setup for your board you can skip all of this entirely and just drive the RC-5 with MIDI messages.
I wanted to write this since I had a hard time finding all of this information together. After doing a bunch of research I bought a footswitch that I thought would work and it was still wrong. Based on that, plus a whole bunch of product reviews, it seems like others might have similar questions. Hopefully it helps someone.