LVDS - Transciever Failes on Unconnected Rx

[shs1611]

Hi,
I am facing a peculiar problem. One of the designs I am working on, has an LVDS interface for data transmission. These cards are to be externally connected through a back plane. Each of these cards can be independently tested. Now when i power on one card to test its functionality, the LVDS tx'r is powered on & enabled. The second card is not connected. Once the testing is over, say in about 10 - 15 minutes time, we interconnect the 2 cards, but no communication takes place. After a lot of probing, we figured out that the LVDS tx'r is dead. The IC was replaced, and the same issue persisted for the second round of testing as well.

The third time, we left both the cards connected, but did not power the second card. In this case, the IC was not damaged and the design worked perfectly.

Now, does this mean that the tx'r should not be enabled (there is an explicit enable pin) without it being connected (termination is on the receiving end). Throughout the LVDS spec, it is said that termination is only to avoid reflection. So, is it ok to leave the transmitter enabled & its output floating.?

Please do reply

Thanks

 

[PaloAlto]

You can use your LVDS transmitter not being terminated without any damage at all. Of course there will not be lvds transmission, instead it will behave like a kind of CMOS buffer. You should see by the way a small current consumption per tx without the termination. Say 1mA instead of 4mA.

 

[shs1611]

You can use your LVDS transmitter not being terminated without any damage at all. Of course there will not be lvds transmission, instead it will behave like a kind of CMOS buffer. You should see by the way a small current consumption per tx without the termination. Say 1mA instead of 4mA.

Hey, thanks for your reply.
But it still remains a mystery why it has failed then. I thought one of the FET's in the output stage might have been forced to sink current in reverse direction, which caused it to fail. So what could the possibility.?
The chip had an enable pin for the LVDS txr and that was enabled during power on without the Rx'r being connected. Now we have moved that enable pin logic to be driven from the Rx'r side, through the back plane, so it is enabled only when the connection is established. This has solved our problem. But just a lil curious to know if Tx will fail if outputs are left floating.

Also, the 644 recommendation says that it is not recommended to use the transmitter unterminated. Quoting from the National Semiconductors' Application Note AN 971 (Point to point configuration), transmission media must be terminated at far end by 100 ohms, line to line. I thought there might be some implication.

Thanks again for your reply. [/b]