Is there any problem with using direct LVDS connections between two Altera devices (Cyclone IV) residing on different boards (without external transceivers)? We are facing strange behavior that is reflected by low impedance (few ohms) between the LVDS signal (usually the negative line) and GND. The relevant signals are sometimes unstable and sometimes not functional at all.

Ideally there should not be issue interfacing since both are same device family and with similar LVDS IO standard. Have you had a chance to check on the electrical values ie VOD, VID, VCM at the TX and RX of both devices to see if still within the specs?

I think checking the eye diagram using a scope would be useful to tell what is wrong with the eye.

There are no issues connecting two FPGAs directly, on the same or different boards, using LVDS, providing you've terminated the lines as necessary. Unless you're driving a significant distance there's no need for any external transceivers. --- Quote Start --- strange behavior that is reflected by low impedance (few ohms) between the LVDS signal (usually the negative line) and GND --- Quote End --- How are you measuring this? Assuming the board(s) is(are) off then the FPGA isn't going to know that it's supposed to be driving/receiving LVDS. However, you certainly shouldn't be measuring a low impedance to GND from either trace. So, I'd be looking for a board/build issue. If your board is on and operating then you're likely to be able to detect a low impedance path to GND depending on the state of the lines - that's more or less how LVDS works. Cheers, Alex

--- Quote Start --- Ideally there should not be issue interfacing since both are same device family and with similar LVDS IO standard. Have you had a chance to check on the electrical values ie VOD, VID, VCM at the TX and RX of both devices to see if still within the specs? --- Quote End --- Thanks. We have checked the voltage values and they all seem to be within spec

LVDS problems (Cyclone IV) | Altera Community

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Altera_Forum
Honored Contributor
10 years ago
--- Quote Start ---
The two boards are not on the same chassis
--- Quote End ---
How long is the cable between these and what sort of cable is it? Is it shielded? Does it allow for a very low impedance ground path between the boards?

I'm concerned the topology of your system is causing the issue. If you've removed a device and that device still exhibits a low impedance path from a signal pin to ground then it's damaged. A poorly considered interconnect cable between the boards could be the cause.

If connecting any 'significant distance' (a bit vague, I know) you need to consider isolating the connection. There are many contributory factors as to what constitutes any significant distance, but the questions I first asked are all relevant. On top of that will be the signalling standard you're using. You also mention supplying power from one board to the other. How good is that PSU? Is it sufficiently decoupled at the far end? If it's powering your receiving FPGA then I hope it's doing it indirectly - i.e. via another regulator(s) local to that FPGA.

Cheers,
Alex

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LVDS problems (Cyclone IV)

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