So I don’t remember exacty how all this works, it’s been a few years since I touched any of this code. But a quick check suggests the scheduler (or so called background tasks in PROS) only needs to run to deal with transmit data, moving from internal buffers to an area the vexos cpu can deal with. For input, IIRC, read char is looking for bytes in a fifo being filled by the vexos cpu so you could probably call from an ISR.