Dan for the win! Thanks Dan it was indeed a problem with the Baud variable being declared an int in the Arduino sketch. It would have been nice to get a compiler warning in this case for a dummy like me. 

To double-check, are you sure the problem is not in the Arduino sketch? Python exhibiting the same behavior tips to that IMO. A quick search yields a similar issue due to using an int for larger baud values:

https://arduino.stackexchange.com/questions/67568/serial-port-fails-with-speeds-greater-than-19200

> On Jun 27, 2025, at 2:51 AM, Jim Ruxton <jim.ruxton@gmail.com> wrote:
>
>
> I'm not clear how this works. Because if I change the baud rate it definitely stops any communication between the laptop  and the Arduino so baud rate must have some meaning in this case. I also tried using a python program I wrote and got the same behaviour  of working up to 19200 then stopping so I don't think it's an issue with Pure Data. Maybe a cable issue?
>>
>> Note that Arduino is usually (or always?) connected through an USB device
>> which emulates a serial device (hence the comport).
>>
>> In this case the baud rate has no meaning (the baud rate is only used by
>> physical UARTs).
>>
>> Le jeu. 26 juin 2025 à 00:06, Antoine Rousseau <antoine@metalu.net <mailto:antoine@metalu.net>> a
>> écrit :
>>
>> > With the latest comport 1.3 (available on Deken) you can set the baud
>> > rate  up to 4000000, if your system allows it. See
>> > https://git.iem.at/pd/comport
>> >
>> > Le mer. 25 juin 2025 à 22:53, Alexandros Drymonitis <adrcki@gmail.com <mailto:adrcki@gmail.com>> a
>> > écrit :
>> >
>> >> How did you find such a limitation? You can use it with a 115,200 baud
>> >> rate just fine on any OS (including Linux).
>> >>
>> >> On 6/25/25 23:50, Jim Ruxton wrote:
>> >> > I'm using the comport object to speak to an Arduino. I'm finding the
>> >> > fastest baud rate I could use was 19200.  I'm on Linux. Would anyone
>> >> > know if this is a limitation of the object in Linux or if there is
>> >> > something else going on. I'd like to send data faster if possible.