I have an MSP430F6659 connected to a ISO1541 via I2C with 1.5K pull-ups. The I2C bus is through wire instead of PCB trace so I'm able to swap out identical PCBs with the ISO1541 ICs.
One of these boards with the ISO1541 works just fine. I can see the clock and data signals in the scope. However, when I connect the other one and try to send a byte with the same MSP430, the signal remains flat at 3.3V.
Doesn't the master have a say as to whether or not the signal goes low? Can any of the slaves prevent the master from bringing the line low?