I am making an RS485 master device. The prototype is able to successfully transmit and receive from a Modbus slave, using 2-wire (half duplex) mode. Full duplex operation is planned, haven't tried it yet.
Does the MAX489 require any pull up/down biasing resistors? I can find no mention in their docs about this, nor specifics about 'fail safe' biasing.
Edit – actually it does sort of mention, there is a pull up for an open network condition. Does this suffice or is a pull-down also needed?
Thanks.
Best Answer
The datasheet says on the first page:
(Please note that "input" means both differential input pins.)
Table 2 shows the same. So you don't need the biasing resistor for this receiver if you do not have termination resistors. If any other receivers on the bus do not have this feature, or if you have termination resistors (which force a zero differential voltage if no transmitter is driving the bus), the bus still needs biasing.
There are receivers that guarantee a high level when the differential voltage is zero; those would not need biasing in any case. The MAX489 is not one of those.
On a receiver without fail-safe feature, you would use two resistors: a pull-up resistor to force the non-inverting input high, and a pull-down resistor to force the inverting input down. Due to the differential receiver, this would be interpreted as a high level; a differential receiver would not work with a single pull resistor, because the other input would be floating, or still at the same level.
If the fail-safe feature is implemented with resistors, then the specified behaviour would imply that there are two resistors, one on each pin. But the datasheet does not mention any pull-up/-down resistors; how the fail-safe feature is actually implemented does not matter to you.