Portable MiniDisc recorders from Sony usually have a wired remote control. Button presses are detected based on the resistance between pin 2 and 4.
Lots of hobby projects in the last two decades have used this simple interface to create PC-based titling and remote control solutions (I even built my own back in the day based on Lionel Pawlowski's MD COM interface, which I called FunkyMD using the PC Parallel Port).
These days, fijam's gmdrec project can do all this over USB with a MCP2221A USB-to-I2C bridge and a AD5245BRJZ50 digi pot, which is quite fancy an easy-to-use, and you can just order the hardware over at Tindie.
The connector has been measured by Maz in the gmdrec project, which includes a PCB file for the physical dimensions: Remote connectors
No dedicated "play" button. Ships with at least MZ-R900 and MZ-R909. Vol+/Vol- is done by pulling out the jog controller (that usually works as play/ffwd and prev/rew when tilted) and tilting it up/down. Left picture shows the lever pulled out, right picture shows it pushed in. The "stop" button is at the end of the remote control (the 3.5mm headphone jack is coming out of the "back" of the remote diagonally).
Ships with MZ-RH1 and possibly others. Silver and black versions.
Has a dedicated "play/pause" button and group buttons (next group, prev group).
The "group-" button has the same resistor value as the dedicated "pause" button on older units, and the "group+" button has the same resistor value as the dedicated "tmark" button on older units (the "rpt/enter" button on the RM-MC11EL has the same resistor value).
The 3.5mm headphone jack is directly at the end of the remote, aligned with the remote's center axis.
At least on the MZ-N510, with a 100kOhm pull-up resistor on the LCD data line, the device interprets them as group buttons.
Resistor values of buttons not seen on RM-MC11EL:
gmdrec resistor values
Source: Maz (Discord)
Not present on RM-MZR30MP, but understood by MZ-R30:
This recorder's behavior differs with the same resistance values when using RM-MC11EL compared to gmdrec + 100kOhm pulldown on the LCD data line.
It is assumed (but not tested) that recorders that are of the same line as the MZ-N510 or newer units behave simiarly.
Note: The RM-MC11EL doesn't have a dedicated 200 Ohm "play" button, the 3650 Ohm Play/FFwd "button" (jog "up" movement) is used as both playback and next track toggle.
|Ohms||RM-MC11EL||gmdrec + 100kOhm pulldown on LCD data|
|3650||starts playback if stopped/paused, goes to next track otherwise||goes to next track, even in paused/stopped state|
|5160||toggles pause only, does not start playback if stopped||navigates to next group|