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Connecting multiple PCA9617A port B

Question asked by David Gomez on Jan 22, 2020
Latest reply on Jan 27, 2020 by Jozef Kozon



My question concerns the ability to connect multiple port B's of two different PCA9617A devices to the same bus. 


I my application I have a single master device (an LPC1769 MCU) that must communicate with two slaves. The slaves need to communicate back to the master; however, they do not need to communicate with each other. 


It is recommended by the datasheet that in a star configuration such as this the master be connected to the port A sides and the two slaves be connected to the port B side. However, in my application, the slaves connected to the B side are connected to a moderately long cable (~0.5m) and have an Output Low Level Voltage that is 0.4V max. I am concerned that in some marginal cases, a slave may not be able to pull the voltage below the 0.4V the port B side needs to trigger a low output. 


This would be resolved by using the port A side with the slave; however, this would then require two port B sides to be connected to the single master. 


What are all the concerns with having multiple port B sides connected to the same bus? It seems to me there is only one reason why you wouldn't want to do this: the port B output of one PCA9617A can only bring the I2C bus voltage down to 0.54V - 0.6V which would not register as a low voltage to the other PCA9617A.


However, in my application, I do not need the slaves to be able to communicate with each other and therefore, I do not care if the low from one PCA9617A does not register. The 0.54V - 0.6V low voltage will still register with the MCU as a low and the MCU will still be able to send data through to the slaves. 


Are there any other problems that could occur if two port B sides are connected? Otherwise it seems like I would be fine to do this. 


Thank you,