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AM26C31: Problem with simple transmit/receive circuits.

Daniel Milliser
Prodigy 30 points
Part Number: AM26C31
Other Parts Discussed in Thread: AM26C32

We have been using AM26C31 Driver / AM26C32 Receiver setup for many of our applications, but this new one has me stumped.     

We are trying to run them in/out of a VME x64 backplane (J2 "UsrDef" pins specifically), with less than 2ft of twisted pair wire between the individual J2 card slots.   

What I am seeing is that the 1Y/1A line is stuck low on about 3/4 of our chips.  The 1Z/1B pair is functioning as expected. 

Additionally, on those 3/4 lines, only 1 pair of the 14 "Y" outputs are reporting an incorrect logic output on the Receiver side.  all of the other pairs report the logic output I would expect, given matching inputs.

Basic schematic attached below.  Nothing earth-shattering:

  • +1
    TI__Guru 53555 points

    Hi Daniel,

    It sounds like there may be a short to ground on some of the non-inverted bus lines causing them to remain at ground potential. It makes sense that some of the data could still propagate in this state due to the inverting channel remaining functional. The reason for this is likely the fail-safe design of the receiver interpreting a differential of ~0V (when both lines are near ground) as a logic high. The remaining non-functional pair may not be able to drive the inverting line low enough to reach this fail-safe state and thus maintains a logic 0 output. 

    Has there been an attempt to swap out the line drivers/receivers in the faulty equipment to determine if the problem is potentially caused by damaged devices? If the failure persists when new devices are swapped in, it's likely that the short is occurring elsewhere in the system. Let me know if it appears to be device damage and we can further investigate what conditions may have caused them to fail. 

    Let me know if you have any other questions in the meantime. 

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    Prodigy 30 points

    That was exactly it, thanks for putting me on the correct path.   The circuit card designers overlooked the Address and Data lines that spilled-over to the J2 connector (B-row) specifically :  B23-B30 

    VME64x Bus P2 Pin Outs and VME64 Pinout Signal names (interfacebus.com)

    All of the positive lines giving us fits were tied to one of those pins.  :)