Straight-Through/Crossover Cable Connection

[gcp]

Dear FPGA experts,

Good day!

We want some expert advice of our present hardware  setup. The setup is to connect the ublox SARA U270 into the xilinx FPGA board. The connection is  from the DB9(RS232) of the Ublox SARA U270 into the pmod pins of the xilinx zedboard. Since it has different voltages, ublox SARA U270 is a 1.8V while the pmod pins of xilinx zedboard is 3.3V, we need to used the RS232 converter and a pmod RS232. In RS232 converter cable, we have in doubt of what to use between straight-through and a crossover cable connection. My understanding in the straight-through cable connection is good for unlike devices while on the crossover connection is good for like devices.

Based on the issues aboved, we want some enlightenment advices of the expert in this forum:

1. Is my understanding between straight-through and crossover cable connection is correct?

2. In our hardware set-up, what cable connection should we used to connect between ublox SARA U270 and xilinx FPGA board?

Thank you very much. God bless and more power!!!

 

Thanks.

 

Best regards,

Glenn

 

 

 

[jpeyron]

Hi @gcp,

I have reached out to more experience engineers to see if they have any input for you.

thank you,

Jon

[gcp]

Thank you. Hope to hear it soon.

[Bianca]

Hi @gcp,

I was trying to find out how is configured the DB9 on the SARA U270 and I saw that it didn't have a DB9 connector, which means that you are using an adapter board of which we know nothing. Out PMOD is configured as DCE, if your adapter board is also configured as DCE you'll need a crossover cable.. You'll have to check the pinout of the adapter board. I am not sure what sara is...but I saw that is some kind of cellular module,, which means that it acts as a data communication equipment (DCE), which means that the FPGA will be the terminal. 

You have to match the communication pins TX from DCE to RX of the DTE and RX from DCE to TX of DTE.

Keep in mind that the PMOD RS232 does not have the Ring Indicator (RI) conected and DCD is connected toghether with DTR and DSR and the notes from the PMOD RS232 will be no longer valid since you'll use it as a DTE, with the crossover cable.

Best regards,

Bianca

[gcp]

@Bianca

Sorry for my confusing message . We actually using the ublox EVK U-27 which includes the SARA U270 and GNSS receiver. Can you please give some comments/advice on our present setup connection?

Thanks a lot.

 

Best regards,

Glenn

[Bianca]

Hi Glenn,

First of all, you can't have 4.2V on RX because you'll burn the FPGA on the ZED. I don't know how you measured that but It doesn't seem correct. 

Second, the crossover has to make the connection between the DB9 connectors. Because you are interfacing with the PMOD not directly to the FPGA. Maybe I wasn't explicit. Since you don't have access between the PMOD and the FPGA board and make changes there, you need to match the PMOD RS232. In this case the connections you made are not correct because you have two DCE instead of one DCE and one DTE. 

RX of your adapter board goes to the TX of PMOD RS232 and the TX of your adapter board to the RX of RS232.

Best regards,

Bianca

[gcp]

Hello Ms. Bianca,

Thank you for the very informative ideas.  

Could you please check the photo below if I decode the right connection.

Thank you very much.

 

Best regards,

Glenn

[jpeyron]

Hi @gcp,

Bianca will be out of the office until next week. I would not expect a response until next week. Sorry for the delay.

thank you,

Jon

[JColvin]

Hello @gcp,

I took a look at your design, but unfortunately I'm not sure how much more help those of us here at Digilent can be for your design; none us here at Digilent have worked with the ublox EVK-U27 or a GNSS receiver, so we won't be able to offer any useful advice.

From a high level, it looks like the Tx and Rx lines between the Zedboard and the GNSS receiver are correct in terms of data direction, but I am not certain about the data direction for the UART lines between the Zedboard and EVK board since they have conflicting labels. You would also need to ensure that the voltage levels between the three boards match one another in terms of the electrical standard to make sure none of the pins are damaged on any of the boards. The main idea behind a crossover cable and a pass through cable is based on how the pins on the two host boards are hard wired to make sure that the data goes to the correct pins; since you are using individual wires to make your own connections and can assign which pins perform which function in your design in an FPGA, either cabling will work as long as the data lines are appropriately accounted for. Beyond that, there isn't much more we can offer in terms of the proper physical setup that is within the scope of the Forum. 

Good luck,
JColvin