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MAX22028 Recommended for New Designs
The MAX22025–MAX22028/MAX22025F–MAX22028F compact isolated RS-485/ RS-422 transceivers provide 3.5kVRMS of digital galvanic isolation between the cable...
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MAX22028 on Analog.com

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Pullup-pulldown resistors over A and B pins

		
	
				

			

									
					    dylancristiani
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on May 2, 2024 
				
			
			
		
    
		
		                    	    
    
        	    
    Thread Summary
    
                The user inquired about the MAX22028 RS485 pull-up and pull-down resistors, noting that a 10kΩ resistor setup results in a 70mV idle differential voltage, below the specified 200mV threshold. The final answer explained that the receiver's threshold can vary between -200mV and +200mV, making the 70mV idle state potentially unreliable. A more robust design should use 1kΩ resistors to ensure the idle voltage is >200mV, aligning with the receiver's '1' state.
                        	    
    
                
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    Category: Datasheet/Specs
    
    		        		            		    
    Product Number:  MAX22028 
    
    		        		    		    
    
		    		    
		    			
    
			    
    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
    
			
    
			
									
		                             
                        

    
    
                                            
                 Datasheet/Specs
                
                                                    
                 Digital Isolation Technology
                
                                                    
                 Isolated RS-485
                
                                                    
                 MAX22028
                
                                
            
    
    
    
		
    

				
				        
        		
    
			
						
									
												
			
			
                        
                                    
                        
                        
                        
            
			            
						
						
			
			
			
			
		
    

        
        
        
    
					
    
			
    


    
















    
	
    
	
    
					



			








	
	
	
	



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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on May 2, 2024 11:57 PM
            
            
						
			            		
      
		
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      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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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on May 2, 2024 11:57 PM
            
            
						
			            		
      
		
										0
					
	
      

		
		
      
		
      
                                
                                                            
      
					
      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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