Products Mentioned

MAX3292 Production
The MAX3291/MAX3292 high-speed RS-485/RS-422 transceivers feature driver preemphasis circuitry, which extends the distance and increases the data rate...
Datasheet
MAX3292 on Analog.com

MAX3291 Production
The MAX3291/MAX3292 high-speed RS-485/RS-422 transceivers feature driver preemphasis circuitry, which extends the distance and increases the data rate...
Datasheet
MAX3291 on Analog.com

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Valid values for the MAX3292 preemphasis setting resistor (Rpset)

		            
		

			

									
					    Yuya
					
							


			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Mar 18, 2024 
				
			
			
		
    
		
     
				
		
    
		    		    
		    
		    		      		            		        
    Category: Hardware
    
    		        		            		    
    Product Number:  MAX3291  
    
    		        		    		    	
			
    Hello
    I'm considering MAX3291/3292.
    

    In slow bands such as 48kbps, Rpset becomes a large value such as 12MΩ. Is this setting valid?
    Please let me know if there is a way to enable the pre-emphasis function in slow bands.
    

    Best regards,
    

    Yuya
    
									
			        
		
    

				
						
        

    
    Tags: 
                                            
                 interface and isolation
                
                                                    
                 hardware
                
                                                    
                 RS-232 / RS-422 / RS-485 / RS-562
                
                                                    
                 MAX3291
                
                                                    
                 MAX3292
                
                                        
                                            
                
                
                                
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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Mar 19, 2024 4:24 PM
            
                                    
					
      
	
      

		
		
      
					
      Hi Yuya,
      

      I would be happy to help with this question.
      

      The minimum input current that should be available on the PSET pin is 110uA. As you can see on the Preemphasis vs. RPSET plot in the datasheet, preemphasis may not work when using resistor values that restrict the current into the PSET pin to less than this value.
      

      While there are some recommendations in the datasheet for using this part at low data rates, even the typical operating plot does not show operating conditions much above a preemphasis interval of 2us. For comparison, a 48kbps data rate would need a preemphasis interval of around 20us, as you have likely noted.  I think you would have difficulty making this work with this part, even with very low supply noise.
      

      Unfortunately, all of our transceivers with preemphasis look to be limited in the same way. 
      

      Thanks,
      

      Shasta
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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                on Mar 20, 2024 8:27 PM
            
                                    
												
						in reply to ShastaThomas
					
									
        
	
        

		
		
        
					
        Hi, Shasta
        

        Thank you for your reply.
        

        Does this mean that the pre-emphasis function of 2 us or more cannot be used?
        I will check if I can communicate at 48kbps without using the pre-emphasis function.
        

        Best regards,
        

        Yuya
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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        •  Analog Employees  
        
            
						
                            
            
            				
            
                on Mar 25, 2024 2:46 PM
            
                                    
												
						in reply to Yuya
					
									
        
	
        

		
		
        
					
        Hi Yuya,
        

        The below chart on the datasheet has showed the pre-emphasis is not guaranteed when it is greater than 1us, data rate less than 1MHz.
        

         
        

        Best,
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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