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LTC1685 Production
The LTC1685 is a high speed, precision delay RS485 transceiver that can operate at data rates as high as 52Mbps. The device also meets the requirements...
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LTC1685 on Analog.com

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LTC1685 fail safe

		
	
				

			

									
					    joakimosth
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Oct 4, 2018 
				
			
			
		
    
		
		                    	    
    
        	    
    Thread Summary
    
                The user inquires about the fail-safe biasing direction for the LTC1685 RS485 transceiver, noting a discrepancy between the datasheet and AN-960. The final answer clarifies that the biasing in the datasheet (B high, A low) is specific to the SCSI protocol, while typical RS-485 configurations bias the A (non-inverting) wire to the positive supply and the B (inverting) wire to ground. For increased noise margin, the user should consider the standard RS-485 biasing as shown in AN-960.
                        	    
    
                
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    Hi!
    

    

    The LTC1685 RS485 transceiver have fail-safe feature according do the datasheet.
    

    According to your application note AN-960, the fail-safe mode is working because -30mV or more is detected as 1. and less then -0.2V is detected as 0. (However, the threshold voltages for LTC1685 according to datasheet is less than -0.3V is 0 and more than 0.3V is 1.)
    

    Assuming the same is true for LTC1685, that -30mV or more is 1 it works without any external bias of the A and B inputs.
    

    If we want to increase the noise margin, a small external bias should be applied to the A and B inputs. This is done in the datasheet in Figure 15. However, the bias seems to be in "wrong" direction. The B input is driven high and the A input is driven low. Why is it like this? Should not the A input be driven to the positive supply and the B input driven to ground as shown in Figure 9 in AN-960?
    

    Best regards
    

    Joakim
    
			
    
			
									
		                             
                        

    
    
                                            
                 RS-232 / RS-422 / RS-485 / RS-562
                
                                                    
                 fail-safe
                
                                                    
                 transceiver
                
                                                    
                 rs-485
                
                                                    
                 interface
                
                                        
                                            
                
                
                                        
                                            
                
                
                                        
                                            
                
                
                                
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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Oct 4, 2018 5:00 AM
            
            
						
			            		
      
		
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      Hi Joakim,
      

      For RS-485 in general, the supply biasing is on the A wire and the ground biasing is on the B wire, per AN-960.
      

      After reviewing LTC1685 datasheet, the diagram you referred to specifically shows biasing in a particular application that was relevant at the time the part was released. The SCSI protocol apparently uses a termination and biasing network per the diagram - i.e. 330 Ohm to supply on the "-" wire and 330 Ohm to ground on the "+" wire, with 150 Ohm in between. I'm not familiar with the reasons for this approach, but it is particular to this standard.
      

      There are other standards too that dictate particular termination networks, e.g. Profibus, but typically these have supply biasing on the non-inverting signal (A or "+").
      

      Regards,
      

      Conal
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
						
						
				
				
		
		
      
			
		
      
	
      

      

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