In my current test configuration, I have an RS-422 interface that uses the CMOS version of the quad differential line receiver (DS26C32AM) in the destination unit under test (UUT).
During testing, I've come across a test configuration that leaves the destination UUT partially powered, despite main power being removed.
Source UUT Destination UUT Result
ON, RS-422 enabled ON, RS-422 enabled Nominal RS-422 Ops
ON, RS-422 enabled OFF Destination UUT Partially Powered On (VCC Rail Only)
ON, RS-422 disabled OFF Destination UUT Powered Off
When the source UUT is powered and its RS-422 interface is enabled and active or idling "HIGH," the destination UUT's VCC rail is partially powered even though main power to the destination UUT has been removed.
Based on this observation, there is current flow into the destination UUT's VCC rail through the RS-422 driver-receiver interface.
There are likely two possibilities for the current flow:
1. The current flows through the DS26C32AM's protection diodes and returns through chassis since the both UUT's secondary returns (VCC_RTN) are tied to chassis.
2. The current flows through the DS26C32AM's resistors that are pulled up to the VCC rail and returns to chassis as described in the first possibility.
This would be the case if the protection diodes have already failed open due to high currents in this partially powered configuration.
Questions about this partially powered configuration:
1. Is there any concern for that current flow in this anomalous test configuration could exceed the current ratings of the DS26C32AM's protection diodes or pull-up resistors?
2. If the source UUT's RS-422 driver has an output short-circuit limit, is the DS26C32AM's receiver input current limited to that amount?
3. Are there any unpowered resistance and/or diode measurements that can be made on the DS26C32AM part to determine if the part or its driver input has been overstressed?
Thanks.