Hi Expert,
There are several app notes and blogs mentioned that the fail-safe resistor will has negative impact for the number of node in RS485 network. The network only require one-pair fail safe resistor. My customer use the same schematic for all nodes as shown below:
What is the equivalent resistance using following schematic?
Every node has fail-safe resistor, how this impact maximum number of node?
Thanks.
Zhou
Hi Zhou,
thanks for sharing the schematic! And yes there is plenty of material offered by TI about RS485 and I do understand it can be overwhelming sometimes.
The device that your customer is using has a 1/8 Unit Load and can support up to 256 nodes on the bus. The fail safe resistors are pretty handy in terms of defining the output of the receiver when malfunctions or idling state happens, but you'll need to calculate it, since it represents an additional common mode load to both buses A-B.
Now to your questions:
- On the single bus, the equivalent resistance would be almost the same value as R7 or R8, that is 3.3Kohm
- The fail resistor adds to the common mode load which for RS485 Standard must be smaller than 375ohm. In order to stay below this value, I recommend to calculate the maximum number of nodes that can be connected onto the bus. You can get to this value simply by dividing the Unit Load by the equivalent input of each node.
For more details on this topic I really suggest you to read this easy app note: Failsafe resistor calculation.pdf
For more info about how the unit load is defined by the EIA-485 Standard you could read the following :
Another interesting blog about fail safe bias network:
As always, if you have any question please feel free to ask.
I hope this clarifies your thoughts.
Best regards,
Adrian
