NI PCIe-8372

[GraemeP]

Hi

I'm pretty much lost as where to post this question, but this looks to be the closest forum.

I have a PCIe-8372 4-lane cablePCIe PCIexpress card. It is connected via cable to custom hardware. When operating in 4-lane mode the system (NI board, cable, and our custom board) work fine.

If we lower the lane width to x1 on our board (simple FPGA re-programming), then the host PC will not 'see' our hardware and the PCI-8372 does not even attempt to negotiate a connection.

Is this normal behavior? Must the 8372 always be operated in x4 mode?

Thanks.

[Roberto_DS]

Hi GraemeP,

Yes this is a great place to pose questions relating to anything NI as it will be picked up by one of our application engineers in that region. Alternatively you can also call into your regional NI office and set up a service request to speak with us on the phone/ through email.

With regards to your question about PCIe-8372 4-lane set up.

The connection you have is x4 cabled PCI Express link that will be used with this specific MXI-Express x4.

This link is a dual-simplex communication channel comprised of four low-voltage, differentially driven signal pairs. The link can transmit at a rate of 10 Gbps in each direction simultaneously.

Because each lane is a differentially driven signal pair, using one lane will be able to send and receive data as you can look at it being 2 wires in each lane. However when you use a x4 configuration you are sending 4 times the amount of data as opposed to when using a single lane. This reduces the throughput drastically and the information sent will not be enough to register on the other end. 

x4 ensures that the data is transferred in full and as fast as possible avoiding bottle-necking in the process.

I hope that this helps.

Kind Regards

Roberto

National Instruments AE

[GraemeP]

Thanks for the comments.

So can you confirm that this card and a 4-lane cable, will also work when the other end (our custom board) is only operating in 1-lane mode?

I am having problems getting this to work. I can see the PCIe reference clock from the 8372 arriving at our board, and our board is sending an idle data sequence to the 8372. But our board never seems to receive any data (training, or idle) from the 8372.

[Roberto_DS]

Using the  PCIe 8372 and the 4-lane cable will result in the data being transferred down the full 4 lanes from the card.

Without any more information about your custom board, working with a 1 lane configuration could mean that the three other lanes are transmitting data that is lost at the other end. 

If you can read the PCIe reference clock on your board this could mean that the single lane you are using is able to transmit that specific data through it, but the ability to transfer 4 lane's worth of information down a single lane will not be fast enough and not feasible.

If it works in a x4 lane configuration and your board communicates with it successfully i advise you to stick to this.

Kind Regards,

Roberto

National Instruments AE

[Robert__R]

GraemeP,

You should be able to connect the PCIe-8372 as x1.  I'm not sure if I've tried it, though.  One thing that could easily cause problems is improper termination of the unused lanes.  The unused receivers are supposed to be high impedance (disconnected), and sometimes disabled lanes aren't high enough impedance.  If the PCIe-8372 detects extra lanes when doing receiver detection it will cause problems.  Similarly, if your FPGA transmits anything on its extra transmitter lanes it could confuse things.

If you're careful you may be able to tape over the right pins on the cable connector to mask out 3 lanes.  The tape can fold over the connector so it covers TX on one side and RX on the other.  It's tricky to get the tape the right width and position so you don't cover additional pins, but it can be done.  Or you can try cutting the wires on your PCB and removing series capacitors.  Something to break the connection on the extra 3 lanes.

- Robert

[GraemeP]

We are trying to reduce cost of our board. We can fit a different device that supports just a single lane.

Will the 8372 work with just one lane connected?

[Robert__R]

GraemeP,

The 8372 should work with only a single lane connected.  I don't know of anything that would prevent it from working, but it's an older board so I don't recall whether I've actually tried it at x1.  I'd try prototyping on an existing board if you can to prove that it works.

- Robert