BLVDS_25 @ SPARTAN3

System where the differential bus Lines outside terminated at both ends with 100 Ohm. When the Spartan3 drives the lines, I measure a Differential Voltage of about 2V in Amplitude. That means, I get 1 Volt of Differential Output Voltage from my BLVDS Pins even its specified as 250 - 450 mV in Spartan3 Datasheet. Do someone know why do i get such a high voltage?

You get such a high voltage because you didn't bother to read any data sheet ever printed that includes the BLVDS I/O standard.

This is a full-swing 2.5 V signal that explicitly requires specific series/parallel terminations like the early LVDS drivers in FPGAs that didn't have proper current sources.

Read the data sheet, bask in the glory of the illustrations that show the required termination networks. Heck - these were so common that Bourns even produced special resistor network packs so designers wouldn't have to use those "bulky" discrete resistors.

I hope your design can be hacked up easily. Good luck.

Metin,

Bus LVDS is not a standard. It is a specialized hack to create a multi-point bus with LVDS like signals.

Yes, it is supported, but it requires resistors both at the transmit end, and the receive end.

Bus LVDS also can not operate anywhere near the speeds that regular LVDS can.

It is unfortunate that someone took a really good standard, like LVDS, and then hacked it to do the multi-point bus job. The fact that it still has "LVDS" in its name should not fool you: it is not a standard, but a solution that has to be customer engineered for every bus connection topology (resistors all change value when you change the drops).

Austin