[41south]

The other thing to look out for if directly connecting a sensor to the bus extension will be the pullup resistors. The TX board already has those and in my experience most breakout type modules also have them - so they end up in parallel. It appears that best practice is to have a set of pull-ups in each section of the bus, therefore reinforcing the need for a second bus extension chip at the sensor end of the extended I2C bus.

In fact taking another look at the circuit, if I am correct, no sensors should have the pull-ups fitted when the resistors R7 and R8 are fitted. The Adafruit SHT31-D module for instance has 10k resistors on board, which will end up in parallel with R7 and R8

[werk_ag]

... It appears that best practice is to have a set of pull-ups in each section of the bus, therefore reinforcing the need for a second bus extension chip at the sensor end of the extended I2C bus.

That is true. The Extended I2C port on the TX board as designed to be used with the Soil/Leaf interface, which includes the second I2C bus extender chip to complete the circuit. In this case, pull up resistors are present at both sides of the circuit, as it should be.