Serial Ports

nXPico M exposes one serial port to the application.

Port	Peripheral	Pins	Available
COM1	USB-CDC	USB-C	No — CODESYS gateway
COM2	—	—	Does not exist
COM3	UART1	GP4 TX / GP5 RX	Yes

Why COM2 is missing

The RP2350 has two UART peripherals. On this board UART0 is wired to the nRF9151 modem (GP16/GP17, with CTS/RTS on GP18/GP19). Opening it as COM2 would mean reassigning its pins and changing its baud rate — disconnecting the modem while the application is talking to it. The runtime therefore refuses port 2: SysComOpen2 returns an invalid handle.

This is not a pin conflict that configuration can resolve. GP12/GP13, which are COM2’s pins on the other boards, are pins of UART0 too — the same peripheral the modem holds. On nXPico M GP12 and GP13 are free as ordinary GPIO or I²C, but they cannot become a serial port.

Write portable applications against COM3

COM3 is on GP4/GP5 on all three boards. An application that uses COM3 runs on nXPico, nXPico W and nXPico M unchanged; one that opens COM2 fails to open on nXPico M.

Using COM3

Identical to the other boards — see the full example on the nXPico Serial Ports page: open with SysComOpen2 on port 3, read with ulTimeout := 0 so the PLC cycle never blocks, interrupt-driven reception.

The modem is not reachable through SysCom with any port number. It has its own I/O channels — see Modem.

No diagnostic log by default

On the other boards the runtime prints a boot log on GP0. On nXPico M UART0 is the modem’s and UART1 is COM3’s, so there is no log in the standard firmware: printf output is discarded. Diagnose through the CODESYS device log instead, which carries the runtime’s own messages — including the modem ones.

A firmware built with the M_UART_LOG_ENABLED option puts the log back on GP4 at 115200 8N1 and gives up COM3 in exchange. Ask neXo if you need that build for debugging.