SN65HVD08 Open Circuit in receiver

Hello Greg,

The SN65HVD08 has "Receiver Failsafe" biasing to handle invalid bus states caused by Open Bus conditions such as an open connector, or an Idle Bus that occurs when no driver on the bus is actively driving.  This is accomplished by offsetting the input receiver thresholds so that the indeterminate range, where the output could be sampled either as a high or a low, does not include the zero volts differential level.  The device will output a High on the R pin when the differential voltage on the bus is more positive than -10mV, and will output a Low on the R pin when the differential voltage on the bus is more negative than -200mV.  However the voltage range in between -200mV and -10mV is the "indeterminate range" where the device could output either a High or a Low depending upon a number of factors and the exact point of transition will be device specific. 

If I understand you correctly, in the case you are observing the R output as Low when there is an Open bus, you do not have the 120 ohm termination resistor connected to the bus and there is nothing to set the voltage on the A and B pins or provide a current path between them to drain any voltage potential that may exist on the bus pins from capacitance.  Have you measured the actual differential voltage on the A and B pins during this state when there is no connection and the R pin should be a High?  It could be that there is a negative differential voltage on the bus pins and without a current path between A and B, or a driving condition on the bus to set the differential voltage to a voltage more positive than -10mV, the device could be working properly.  Could you measure the voltages on the A and B pins under the various setup conditions (one board with open connection but no termination resistor, one board with open connection with 120 ohm termination resistor, two boards in High Z mode with no termination resistor, etc.) and note the value of the R pin under those conditions?  This will help determine when the R pin is going to be a High or a Low and what might be needed to do in your application.

You could also reference the following App Notes for additional information on RS485 Receiver Failsafe Biasing.

http://www.ti.com/lit/an/slyt324/slyt324.pdf

http://www.ti.com/lit/an/slyt514/slyt514.pdf

Regards,

Jonathan