Pullup-pulldown resistors over A and B pins

Question

Hi sirs i've a question regarding the MAX22028 pullup and pulldown over A and B rs485 pins: according to the datasheet i can read: "the most important factor when sizing these resistors (PUP and PDWN) is to guarantee that the idle voltage on the bus (VA-VB) is greater than the receiver input threshold (+200mV for the MAX22025-MAX22028, -50mV for the MAX22025F-MAX22028F)..."; in the board i'm managing, only the RXA driver is used, and the TXA pin is NC; the PULLUP and PULLDOWN resistors are 10K that leads to a VA-VB differential idle value of about 70mV; while in the datasheet it states that the minimum idle delta must be 200mV, i can see that the receiver is able to receive data: i'm wondering if i'm misunderstanding something. Thank you in advance

ericb · Accepted Answer

Hello, 
 The receiver has a threshold voltage which determines if the output is 1 or 0. This threshold value may be any value between -200mV and +200mV and can vary between parts as well as drifting over time or temperature. Thus when VA-VB is between -200mV and +200mV, the receiver's output of the MAX22028 is undetermined. It may be 1 or it may be 0. To prevent this the biasing network of the pull-up, pull-down and bus terminating resistors is used to assure that the idle state of VA-VB is > +200mV and remains 1. 
 Its possible, that with your idle value of 70mV, the receiver is correctly recognizing the idle state and this would mean that the receiver's threshold is below 70mV. Since this could change for various reasons, a more robust design would set the idle voltage to be >200mV. 
 
 Eric)

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Pullup-pulldown resistors over A and B pins