sbRIO-9607 SPI communication - LVTTL "0" logic high voltage

[David_SRM]

Device used:

sbRIO-9607

sbRIO-9684

sbRIO-9687

Software:

LabVIEW 2017

Hello,

First of all some context.I have a testbench with a sbRIO-9607 with its PFGA as a controller and its processor as the real-time device, then a sbRIO-9684 as a link between the sbRIO-9607 and the sbRIO-9687, and finally the sbRIO-9687 using its I/O ports, etc. So far so good, it's a phenomenal testing platform.

Now, I have to connect it with another device via SPI communication and I have some troubles.

I'm using the NI SPI IP v1.3.0.1 package for the communication and I have been able to pull the pulse trains along the LVTTL lines of the sbRIO-9687. However, when measuring them I see that the logic voltage "1" is 3.3V and the logic voltage "0" is 1.5V, and this does not fit with what it says on the sbRIO-9607 datasheet: "output low voltage when sinking 3 mA 0.0 V minimum and 0.4 V maximum". The logic voltage "1" is fine but the logic voltage "0" of 1.5V is too high.

Could it be a high noise immunity mechanism or something like that?

I don't know what I'm doing wrong. I don't know if the problem is in the I/O settings or I have to change something in the hardware (some switch, jumper...).

Thanks in advance,
David

[David_SRM]

Hi again,

I have tested all the lvttl pins of the sbRIO-9687 and they all have the same logic "0" behaviour at 1.5V except 4 which give 0V, so I will use those for SPI communication.

But if anyone knows why this could happen I would appreciate if you could comment it, because it is a behaviour that is not reflected in the datasheets.

Thanks,

David

[TIDAD]

Hi, have you tried to connect te grounds of all sbRIO-s together?

[David_SRM]

Hi TIDAD,

Thanks for your response. No, I've not tried to conect all the grounds, but I have already checked that they are all internally connected.

In any case, I have already implemented SPI communication with the 4 pins with logic "0" at 0V, and it works perfectly.

Thanks,

David