Reset message to Arduino from Max?

my advice would be to use 32u4 based micros as HID devices instead of serial.
There are several reasons for that, not only becuse serial connection seems to break from time to time, in first place on windows,
but also to be able to distinguish devices
if multiple are used at same time.

To allways get THAT micro into THAT serial object in Max
no matter in which USB port, slot, HUB, order etc they get connected is not a trivial task.
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I have used many 32u4 based boards with modified USB descriptors
as Joystick devices on both Mac and Windows.
joystick mode is better because keyboards and mice are not allowed as HID devices in Windows OS.

Back to your request ...
Resetting serial on 32u4 boards would be possible if you insert
while(! Serial);
in void setup()
which causes native USB Arduinos to run setup routine
when one reopens serial port.
Same as other USB to serial like CH340 or ftdi devices do.
---
But the question is why really your micro stops working,
i2c buss not working means complete stall of the main loop
which is reading it.
Are you sending too much and too fast data ?
In that case restarting Aurduino would not help much.
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anyway, here is link to one of many posts I posted over the years to deal with multiple boards:

https://cycling74.com/forums/rename-serial-ports-using-arduino

I would wote against midi mode due to several restrictions, but use joystick !!!
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P.S. If you decide to go that way, I would post newer instruction for boards which work well at least till last IDE 1.8.19. I don't use Arduino 2 because I don't run any OS that supports that version. In short - there is no need to fiddle with any usb descriptors etc for joystick variants, only to add new definitions in bords.txt file.

hi Source Audio, and thanks for replying...

first -
>But the question is why really your micro stops working, i2c buss not working means complete stall >of the main loop which is reading it. Are you sending too much and too fast data ?

I did wonder that. My concern is that if the loop runs too slowly, the "touchkeys" will feel unresponsive. (This is perhaps made a little more complicated because I'm using proximity rather than touch - the copper sensors are underneath a sheet of 2mm acrylic and another 1mm foam to reduce impact.)
So I reduced the baud rate to 19200, and increased the loop delay to 100ms (from 50ms), and am letting it run now to see if it still stops working (might take a while).
(By the way, the yellow LED I was referring to is the "data send" LED, but you probably figured that out).

>Resetting serial on 32u4 boards would be possible if you insert while(! Serial);in void setup()

The current sketch has while (!Serial); delay(100);
(I'll post the Arduino code in a separate post, in case you want to look at it)

>To allways get THAT micro into THAT serial object in Max

I haven't had a problem with that (yet! AFAIK!). I have a menu listing all of the available serial ports attached to the serial object in each patch that uses it ( the menu is refreshed when the patch is loaded). So I can either manually select which device is read by each [serial] or, in the case of permanent installations, where I know which USB port each device is connected to, use a text file read on load up which contains a list of port names to distribute to each of the relevant sub patchers.

Using [hi]...
I've had a poke around the Arduino examples, and loaded the Keyboard and Mouse libraries, but haven't managed to find something that I can add to my sketch which shows me how to send the sensor data via an HID connection.
I have modified the sketch so that the board shows up as a HID device, though haven't tried more than one connected keyboard yet. Also a little concerned by the warnings that I need a physical "connection on/off" switch. I don't really want to add any additional complications for teachers using the permanent installation!
If I select a device in [hi], that is sending either keyboard or mouse data, but I haven't set that up as an input device in the (Mac) system settings, does that mean that any data that comes over that connection _won't_ affect the mouse cursor?
And presumably the data stream (Filt0: 12 values; Filt1: 12 values etc) is just output from the [hi] object as a stream of labelled data?
Sorry if this is a bit confused - I'm somewhat out of my comfort zone here!)
Can you point me to a sketch which shows sending data via HID, that I can copy into my existing sketch?

thanks
David (Arduino sketch follows)

It is more important to know if you only use on/off states
or send raw sensor data.
Which makes HUGE difference.
If only on/off states, all would be quite easy.
You set threshold, then send only when state of one sensor changes.

I would avoid using key or mouse HID.
Current arduino joystick lib suports 32 buttons,
11 analog inputs and hat switches.

P.S.
That can be modified for more buttons - to match your 36 touch senors.

Here's the sketch for the 3 mpr121s (touch sensors) connected via the first three ports on a TCA9548A multiplexer to an Arduino Micro. There may well be unnecessary bits of code in here. Also, I've only just added the Keyboard.h related commands , which has renamed the available device in the [serial] menu, and allowed the micro to show up in the [hi] menu.
Note, in void(loop)
for (uint8_t t=0; t<3; t++) - t<3 is because there will only ever be a max of three attached touch sensitive keyboards.
the definition of pinscanArray is at the bottom of the sketch

#include "Wire.h"
#include <Adafruit_Sensor.h>
#include <Adafruit_MPR121.h>
#include <Keyboard.h>

#define TCAADDR 0x70
#ifndef _BV
#define _BV(bit) (1 << (bit))
#endif

Adafruit_MPR121 cap = Adafruit_MPR121();
Adafruit_MPR121 cap2 = Adafruit_MPR121();
Adafruit_MPR121 cap3 = Adafruit_MPR121();

int sensorsAt[] = {-1, -1, -1, -1, -1, -1, -1, -1};

void tcaselect(uint8_t i) {
if (i > 7) return;

Wire.beginTransmission(TCAADDR);
Wire.write(1 << i);
Wire.endTransmission();
}

// Keeps track of the last pins touched
// so we know when buttons are 'released'
uint16_t lasttouched = 0;
uint16_t currtouched = 0;

void setup()
{
while (!Serial); delay(100);

Serial.begin(19200);
while (!Serial) ;
Keyboard.begin();

Wire.begin();

pinScanArray(); //scan multiplexer for connected sensors, place in array

tcaselect(0); // TCA channel for sensor1
cap.begin(); // use the default address of 0x77

tcaselect(1); // TCA channel for sensor2
cap2.begin(); // use the default address of 0x77

tcaselect(2); // TCA channel for sensor3
cap3.begin(); // use the default address of 0x77

}

void loop()
{
for (uint8_t t=0; t<3; t++) {

int x = sensorsAt[t];
if (x != -1) //check to see that a sensor is connected to the muliplexer port
{

tcaselect(x);
//following code from Adafruit sketch for the mpr121
{currtouched = cap.touched();

for (uint8_t i=0; i<12; i++)

// reset our state
lasttouched = currtouched;

Serial.print("Filt");Serial.print( x ); Serial.print(": ");
for (uint8_t i=0; i<12; i++) {
Serial.print(cap.filteredData(i)); Serial.print("\t");
}
Serial.println();
if (Serial.available())
Keyboard.write(Serial.read());
// put a delay so it isn't overwhelming
delay(100);
}
}
}
}

void pinScanArray() {
for (uint8_t t=0; t<8; t++) {
tcaselect(t);
Serial.print("TCA Port #"); Serial.println(t);

for (uint8_t addr = 0; addr<=127; addr++) {
if (addr == TCAADDR) continue;

Wire.beginTransmission(addr);
if (!Wire.endTransmission()) {
Serial.println(t); Serial.println(addr,HEX);
sensorsAt[t] = t;
}
}
}
for (int i = 0; i<7; i = i + 1) {
Serial.println(sensorsAt[i]);
}
}

I’m sending raw data, as it’s _much_ easier for me to handle/parse the data in Max (where I mostly know what I’m doing) than in the arduino (where I don’t have an embedded feel for how the code works).
also, because it’s useful to be able to change the threshold on the fly, depending on whether kids are playing the boards with a finger tip or their whole hand.

here is simplest joystick code

#include "Joystick.h"

Joystick_ Joystick(JOYSTICK_DEFAULT_REPORT_ID,JOYSTICK_TYPE_GAMEPAD,
36, 0, false, false, false, false, false, false, // 36 buttons activated , hatswitch & 6 axis set to 0 / false = deactivated(true = activated)
false, false,false, false, false); // Rudder, Throttle, Accelerator, Brake, Steering, set to false = deactivated

int SW1; int exSW1; int SW2; int exSW2;
void setup() {for (int pin = 2; pin < 8; pin++){pinMode(pin, INPUT_PULLUP);}
Joystick.begin();}

void loop() {
int SW1 = digitalRead(2); if (SW1 != exSW1) {Joystick.setButton(0, 1-SW1); exSW1 = SW1;}
int SW2 = digitalRead(3); if (SW2 != exSW2) {Joystick.setButton(35, 1-SW2); exSW2 = SW2;}
delay(10); // slow down if not needed this fast
}

-------
it uses 36 button addresses, only 1st and last are added to
main loop, just to show how to address them. (0 - 35)
all you need to do is to assign each of your 36 readings to button number.

Can you tell me the name of the joystick library you use? I can't see anything called Arduino Joystick or plain Joystick in my Library Manager.
...
Found it- - this one, presumably
https://github.com/MHeironimus/ArduinoJoystickLibrary

https://github.com/MHeironimus/ArduinoJoystickLibrary/

Thanks for that. I'm going to have to think about it for a while to figure the rest out and how it might fit together.

Does the Joystick library just send the captured values over USB as is? There's no equivalent of Serial.print needed?