Looking into buying some MAX485 modules. The module that I'm looking at has the below schematic and I wonder what the function of R5 and R6 is.
Looking at the MAX485 datasheet, those resistors are not there in the typical operating circuit.
Questions
Thanks in advance.
1- Sets levels when no wires are connected
2- Yes, they can be removed, it does not affect operation.
3- I have the same doubt
@LarryD, I wasn't planning to. You may assume each device has its own power supply so no need to run a Vcc. For the test setup it will all be powered from the same PC.
In an RS485 bus idle state - i.e. no node is driving the bus - the receiver output can be undefined - generally resulting in the UART picking up random data. Fitting R5 and R6 places the bus in a known state in this scenario. You only need one node to have these resistors fitted.
As long as 1 node has them, then you don't need them fitted on other nodes.
Yes, you can mix 3v3 and 5v on the same bus.
The 3v3 nodes should use a MAX3485 or equivalent.
There's a good app note from Analog Devices (AN960) here: AN-960: RS-485/RS-422 Circuit Implementation Guide | Analog Devices
Note that the MAX485 has a built in fail safe feature to detect an open circuit so the biased resistors are not needed. The datasheet says:
The receiver input has a fail-safe feature that guarantees a logic-high output if the input is open circuit.
However, if you mix RS485 devices, check which ones have a built-in fail safe mode as any that don't will detect random data.
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