Does the M251 support PWM or High Speed Counter?

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No.
There is no TM2 or TM3 with fast output for PWM or HSC
The alternative would be to get an OTB distributed IO module or a M241 controller.
The M241 has built in fast outputs that can be used for PWM.

OTB part number:
OTB1C0DM9LP CANopen
OTB1E0DM9LP Ethernet TCP/IP
OTB1S0DM9LP Modbus serial