Digital I/O
NI USB-6501 vs FPGA
Hi Zibi,
To answer your first question, first you should know in what state is the IO on FPGA without configuration. I'm not familiar with Altera devices, but as is described at Altera Forum, all unconfigured IO should be in tri-state (high impedance), so I believe nothing should happen with USB-6501.
However, if you want to directly connect USB-6501 to your FPGA chip, be careful with what logic you are connecting where. USB-6501 uses TTL (0-5V), while Altera chips I found were operating usualy with lover voltage logic like LVDS, LV PECL, LVTTL. Therefore, check the logic to communicate properly, and not to damage your FPGA chip!
To answer your second question. If you connect input to input, nothing should happen beside you will not have any input data. If you connect in to out, nothing should happen againg until the voltage levels (logics) are compatible. In fact, connecting input to output is what you wish to do. The only problem can arise if you connect output to output. This doesn't apply just for this specific case, but it is general, not recommended connection as it might damage your devices. If there is no difference in potentials (same voltage on both outputs),you should be ok.
However, connecting outputs is not recommended! If you need to connect them anyway and you want to be sure nothing will happen, I would start thinking of additional circuity to limit current, for example. But my recomendation is the same as you can find in all documentation, not to connect output to output.
Regards,
Martin
Thank Martin
Of course, all unconfigured IO are in tri-state, but when we don't earlier programme device FPGA. Because, by using one output port and one input port, I would like to use USB-6501 card to communication with my device and control same pin. So, by using software (Quartus) I am programming two port: 8-bit output and 8-bit input. I directly connect these ports to USB card. Important is that when I turn on my device these ports now are always configured. In second hand, after turn on (connect to USB port) of USB card: “At system startup and reset, the hardware sets all DIO lines to high-impedance inputs. The DAQ device does not drive the signal high or low. Each line has a weak pull-up resistor connected to it.”. So I have connection: FPGA(out) – USB-6501(tri-state); FPGA(in)- USB-6501(tri-state). If I well understand when I turn on my FPGA and USB card, nothing should happen 
.
Next I can create task and configure port on USB card but I will be careful that right onfigured IO. In this case I have connection: FPGA(out) – USB-6501(in); FPGA(in)- USB-6501(out). So everything is all right. If last my suggestion are true?? My Altera chips is operating usually with 3.3 voltage logic so I will be to use same bus buffer (HC541). Or better will be to apply same digital isolators ??
Regards Zibi
Hi Zibi,
Since I'm not expert in Altera chips, and I have checked theirs datasheets just briefly, I'll suppose you are right with power up states of your FPGA chip.
Inspite of the state of IO on your FPGA chip, I can say that your first statement is correct. USB-6501 is in High-Impedance state at startup (USB-6501 User Guide and Specification, pg. 16, Power on States), therefore it doesn't matter whether you connect output or input to it 
.
Regarding to your second question, not just voltage levels are important itself. What you have to care about is what logic you use on one, and what logic you use on other side. Therefore, I would use some Logic Translator (like TTL to CML or PECL or LVDS ....). I would double check with Altera what they recommend, as I believe the chip itself will be way more sensitive than USB-6501 
.
Best regards,
Stefo
