FPGA and RS422

[Rune Christensen]

Hello

Is it possible to connect a RS422 differential signal directly to a FPGA
instead of using a converter from RS422 to TTL voltage levels?
Would it be possible for a FPGA to create a RS422 signal?

Is there any information about this on the internet?

Thanks
Rune Christensen


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[Jim Granville]

Rune Christensen wrote:

Hello

Is it possible to connect a RS422 differential signal directly to a FPGA
instead of using a converter from RS422 to TTL voltage levels?
Would it be possible for a FPGA to create a RS422 signal?

Is there any information about this on the internet?

Possible, yes. Practical, maybe not.
Most FPGAs have differential IP modes, so could accept RS422 IPs,
with the caveat, WITHIN their common mode range.
For true RS422/485 devices, that is around -7V..+12V, which allows
ground movement between ends of the cable.
FPGA pin common mode is much less than this, but you could extend it
with a resistor pad (which also attenuates the wanted signal )
Then there is ESD protection. Most separate RS422/485/CAN transcievers
have higher tolerance to ESD, than FPGA.

RS422 output needs sufficent drive current, and balanced drive, but
that is within the capability of FPGA.
For short-haul, and within-the-box signaling, this could be do-able,
but perhaps not for connection to 100m cables in a factory...
-jg

 

[Jason Berringer]

To further that thought, if you use a transformer you can remove the common
mode votages, and bias them yourself on the isolated side of the
transformer. This is documented in Crystal semiconductor products since
digital audio uses RS 422 for transmission. See CS8415 data sheets and app
notes. A couple of transient voltage suppressors as well and you're in
business.


"Jim Granville" <no.spam@designtools.co.nz> wrote in message
news:%nTPc.1606$zS6.195491@news02.tsnz.net...

Rune Christensen wrote:
Hello

Is it possible to connect a RS422 differential signal directly to a FPGA
instead of using a converter from RS422 to TTL voltage levels?
Would it be possible for a FPGA to create a RS422 signal?

Is there any information about this on the internet?

Possible, yes. Practical, maybe not.
Most FPGAs have differential IP modes, so could accept RS422 IPs,
with the caveat, WITHIN their common mode range.
For true RS422/485 devices, that is around -7V..+12V, which allows
ground movement between ends of the cable.
FPGA pin common mode is much less than this, but you could extend it
with a resistor pad (which also attenuates the wanted signal )
Then there is ESD protection. Most separate RS422/485/CAN transcievers
have higher tolerance to ESD, than FPGA.

RS422 output needs sufficent drive current, and balanced drive, but
that is within the capability of FPGA.
For short-haul, and within-the-box signaling, this could be do-able,
but perhaps not for connection to 100m cables in a factory...
-jg

 

[Rune Christensen]

Thanks for the information

I think I will go for the interface chip it looks like the most simple
solution.

Cheers
Rune

"Jason Berringer" <look_at_bottom_of@email.com> skrev i en meddelelse
news:jmXPc.21006$Jq2.927116@news20.bellglobal.com...

To further that thought, if you use a transformer you can remove the
common
mode votages, and bias them yourself on the isolated side of the
transformer. This is documented in Crystal semiconductor products since
digital audio uses RS 422 for transmission. See CS8415 data sheets and app
notes. A couple of transient voltage suppressors as well and you're in
business.


"Jim Granville" <no.spam@designtools.co.nz> wrote in message
news:%nTPc.1606$zS6.195491@news02.tsnz.net...
Rune Christensen wrote:
Hello

Is it possible to connect a RS422 differential signal directly to a
FPGA
instead of using a converter from RS422 to TTL voltage levels?
Would it be possible for a FPGA to create a RS422 signal?

Is there any information about this on the internet?

Possible, yes. Practical, maybe not.
Most FPGAs have differential IP modes, so could accept RS422 IPs,
with the caveat, WITHIN their common mode range.
For true RS422/485 devices, that is around -7V..+12V, which allows
ground movement between ends of the cable.
FPGA pin common mode is much less than this, but you could extend it
with a resistor pad (which also attenuates the wanted signal )
Then there is ESD protection. Most separate RS422/485/CAN transcievers
have higher tolerance to ESD, than FPGA.

RS422 output needs sufficent drive current, and balanced drive, but
that is within the capability of FPGA.
For short-haul, and within-the-box signaling, this could be do-able,
but perhaps not for connection to 100m cables in a factory...
-jg

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Outgoing mail is certified Virus Free.
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