not seeing clock on i2c bus 1


I am trying to get a cape I built working. It uses both i2c buses. i2c bus 2 (after enabling) works just like I expect. Bus 1 is not behaving anything like what I expect. I am using i2cdetect as a simple minded tester just to see the pins wiggle and do connection tests.

The i2c goes directly off the expansion pins into a NXP pca9516a with 4 buses coming out. Each bus has a 3.3kohm pull up to +5 (the chip is safe for +5 pull up even though vdd is 3.3v.) All of the buses are empty right now.

When I look at the SCL pin (they all act the same), it drops low when the program starts and never changes until the program ends. I would expect the clock to run for each address as it sends the address, register and data.

When I do the same for i2c2, it acts just like I expect and finds the three chips that are hanging off the bus.

I am at a loss as to what this can be and how to proceed. I could pull the NXP chip. but it is surrounded by RJ45 connectors and will be hard to get to.

I am new to BBBs and this is my first dance with their i2c workings.

thanks in advance for any help.