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Participant glennparaiso
Participant
1,731 Views
Registered: ‎10-11-2016

Pmod RS232 Voltages Difference

Hi,

Could someone help us. We are trying to interface a ublox SARA U270 EVK as a DCE modem into a pmod zedboard FPGA board as a DTE . We used the Pmod RS232 module just to have a direct conection to the pmod pins of the zedboard. Since, the ublox SARA U270 is a DB9 male type, we used  the Male-Male DB9 cable converter using the straight-through cable connection in order to have a coonection to the Pmod RS232 and then to the pmod pins of the zedboard. Before connecting to the pmod pins in the zedboard, we checked  first the voltage levels of the output pins in the Pmod RS232 when ublox SARA U270 is only in power (zedboard is not yet connected). Upon checking the voltage using the multi meter device, the Rx pin reading is variable from 3V to 4.20V.  Our understanding also in the pmod pins on the zedboard is that the voltage level allowed is only 3.3V. Through these readings, our undertanding is that the 3.3V pin will received the 4.20V from the ublox SARA U270 that will eventually damage to the pmod pin on the zedboard. Do we have a correct understanding on the issues? If not, can somebody enlighten us? 

Thanks.

 

Best regards,

glenn

 

 

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7 Replies
Scholar austin
Scholar
1,712 Views
Registered: ‎02-27-2008

Re: Pmod RS232 Voltages Difference

g,

 

RS232 is from ~ +/- 3v to ~ +/- 15v.  So no, no damage if RS232 connected to RS232, but yes damage if RS232 directly connected to FPGA (without a RS232 level converter, or RS232 interface).

 

Sounds like you have a RS232 PMOD interface, so that should be OK.

 

Check that you have Tx to Rx, Rx to Tx (not Rx to Rx, Tx to Tx).  A straight through cable is supposed to be what is required DCE to DTE, but the RS232 PMOD is agnostic, it may be used for DCE or DTE (it is how you wire it to the connected device).

 

It is unclear what the DB9 on the U270 is wired to do, so DCE or DTE is unlikely to be correct here.  RS232 is perhaps 50 years old now, and no one remembers Bell system modems (DCE), or terminals so wired (DTE).  Of course, those were 25 pin connectors, and since the IBM PC stated using 9 pin connectors, I suggest there is no such thing as DCE and DTE any longer (most equipment implements one or the other somewhat randomly!).

 

Austin Lesea
Principal Engineer
Xilinx San Jose
Scholar u4223374
Scholar
1,686 Views
Registered: ‎04-26-2015

Re: Pmod RS232 Voltages Difference

To extend what @austin has so correctly stated - the only way I've ever found of correctly getting a UART connected first time is to put an oscilloscope on it. That tells you both which pin is doing the transmitting (at each end) and what baud rate is being used. I'm not quite sure why GPS receivers continue to use UART, when most embedded devices have shifted over to I2C or SPI (both of which avoid the confusion with pinouts).

 

 

Regarding the incorrect voltages on the Pmod RS232 - if it's not connected to the Zedboard then it's not powered, so the behaviour is not well defined. Assuming it's the Digilent one, I'm sure that Digilent have ensured that it won't do any damage to an FPGA.

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Participant glennparaiso
Participant
1,665 Views
Registered: ‎10-11-2016

Re: Pmod RS232 Voltages Difference

@austin
Thanks. This really help us to move forward and continue the development process. Just some clarification of the Tx to Rx, Rx to Tx, I included the photo below in order you to check our present set-up and the critical voltages output. Can you please give us some comments.

 

Thanks.

 

Best regads,

Glenn

DCE-DTE-Conncetion.png
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Scholar u4223374
Scholar
1,649 Views
Registered: ‎04-26-2015

Re: Pmod RS232 Voltages Difference

@glennparaiso What is the Vcc voltage for the Pmod port? It's the top Pmod pin in your picture.

 

 

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Scholar austin
Scholar
1,631 Views
Registered: ‎02-27-2008

Re: Pmod RS232 Voltages Difference

Unplug the cable

The Tx port will be driving a voltage (pin 2 or 3 on DB9) - check each DB9, pins 2 and 3

The Rx port will not have a voltage (near 0 volts)

Then, wire TX to Rx, Rx to Tx

Ignore the PMOD voltages - they are what they are (before and after level shifter).

Austin Lesea
Principal Engineer
Xilinx San Jose
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Participant glennparaiso
Participant
1,599 Views
Registered: ‎10-11-2016

Re: Pmod RS232 Voltages Difference

@austin

Thanks. Just a clarification, which cable should I unplug? The present set-up, the power cable of zedboard and evk sara are separate. Also, in the set-up above, only the ublox SARA is plug-in. We did not plug the zedboard due to our doubt that it may damage the zedboard due to the measured voltages.

Thanks.
Best regards,
glenn
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Scholar austin
Scholar
1,583 Views
Registered: ‎02-27-2008

Re: Pmod RS232 Voltages Difference

The 9 pin to 9 pin RS232 cable,

 

Find which is TX, then wire appropriately (straight through, or crossover 2 & 3).

 

Then connect cable.

Austin Lesea
Principal Engineer
Xilinx San Jose
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