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Visitor vladimir50
Visitor
6,929 Views
Registered: ‎03-25-2011

RS-422 connection

I have a board with RS-422 driver powered by 5v. Can I connect it directly to SPARTAN3AN differential input? The driver is SN65C1167 from TI, datasheet attached. Driver's output swing is 3v around 1.8v, so the voltage applied to FPGA's pin is between 0.3 and 3.3, which is acceptable for single ended connection. However, LVDS33 standard defines input differential voltage as 600mV max. The question is - what definition should I follow, single ended or differential?

 

Thanks in advance

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5 Replies
Professor
Professor
6,926 Views
Registered: ‎08-14-2007

Re: RS-422 connection

Normally I would not recommend driving RS422 directly into an FPGA input because of the

large common-mode voltage range.  However if you control the source of the signal you

might be able to use the LVDS input standard using a resistor network rather than a simple

termination resistor.  The data sheet seems to imply that the driver's common mode voltage

point can be affected by the load.  If you have 1.2V available on the board (I think you should for

the S3 core supply) you could make a simple network with four resistors, one from each

pin of the FPGA to +1.2V and one from each pin of the FPGA to the pin of the driver.  The

resistors form a simple divider and help to center the swing near the LVDS common mode

piont.  You might need to experiment with values, but I would start with 11 Ohms for the

resistors to 1.2V and 39 Ohms for the series resistors to the driver.  All resistors should be placed

as close as possible to the FPGA.

 

-- Gabor

-- Gabor
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Teacher rcingham
Teacher
6,923 Views
Registered: ‎09-09-2010

Re: RS-422 connection

I would recommend using a RS-485 receiver IC with a power supply matching that of the Spartan3AN. I'm confident that Maxim and other companies will make one.

Note that RS-485 is an improved version of RS-422.

------------------------------------------
"If it don't work in simulation, it won't work on the board."
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Visitor vladimir50
Visitor
6,923 Views
Registered: ‎03-25-2011

Re: RS-422 connection

Thank you for the quick responce.

The problem is that I have to connect 14 differential chanels, and this is a lot of resistors, comparing to potential zero:-)

Can you provide me with some reference schematic of Spartan's IO port?

 

Thanks again

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Historian
Historian
6,917 Views
Registered: ‎02-25-2008

Re: RS-422 connection


@vladimir50 wrote:

Thank you for the quick responce.

The problem is that I have to connect 14 differential chanels, and this is a lot of resistors, comparing to potential zero:-)

Can you provide me with some reference schematic of Spartan's IO port?

 

Thanks again


You're better off just getting the proper RS422/RS485 receivers.

----------------------------Yes, I do this for a living.
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Highlighted
Newbie dxzhu
Newbie
6,549 Views
Registered: ‎11-07-2012

Re: RS-422 connection

Is it possible to use FPGA to serialize several RS-422 or RS-485 signals with different clocks? Also, do we need to reprogram the FPGA if one of the incoming clocks changed?

 

Thanks!

 

Daniel

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