P9-17,18 no output.

I connected P9_17, 18 (i2c-0’s SCL, SDA) to an oscilloscope. When I launch command “i2cdump -f -y 0 0x70”, I could get response correctly. But I can’t see the waveform on the oscilloscope. Can any expert give me a hint?
Thanks.

P9_17/P9_18 are i2c1_scl/sda...

sudo sh -c "echo 'BB-I2C1' > /sys/devices/platform/bone_capemgr/slots"

then:

i2cdump -f -y 1 0x70

Regards,