/UART of C167 is not generated

Hello,
I am trying to get an interrupt signal from C167 internal UART, which normally should be generated when there is a signal input at RX.
I send out a command MVAR(), which generate TX output signal and jump the pin TX to RX, in order to get RX signal. Then I use oscilloscope to catch at /UART (active low) pin but it's always "high" logic.
That mean even there is RX input to the UART, there is no receive interrupt. Does anyone has idea how can I get it ? I would be very appreciate for your help.

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