200Mbps link between FPGAs

It's been a while since I looked at FPGAs, but back when I was looking the faster ones they offer LVDS signalling.

"This (2001) standard originally recommended a maximum data rate of 655 Mbit/s over twisted-pair copper wire, but data rates from 1 to 3 Gbit/s are common today on high quality transmission mediums"

--
Bill Sloman, Sydney

I was convinced that the FPGA LVDS should not leave the PCB, but it turns out that you are right, there are good transceiver chips, like the SN65LVDS051. Some folks seem to be using only the FPGA built-in transceivers, but it is way outside of my comfort zone.

Problem solved; thank you, Bill!

Best regards, Piotr

Straight LVDS should be OK over 2m of CAT5/6 cable. At 200 MBPS, you could do it brute force, clock+data in each direction.

To go a little farther and faster, SFP modules + SERDES are great.

--

John Larkin   Highland Technology, Inc   trk 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

"The ANSI/TIA/EIA-644-A (published in 2001) standard defines LVDS. This sta ndard originally recommended a maximum data rate of 655 Mbit/s over twisted

-pair copper wire, but data rates from 1 to 3 Gbit/s are common today on hi gh quality transmission mediums."

LVDS is already an overkill.

Why waste money on an even more extravagant over-kill?

Two metres isn't far, and he doesn't need isolation.

--
Bill Sloman, Sydney

I'd go LVDS directly between FPGAs, but if you want an LVDS-LVDS buffer, FIN1101 is fast (about 1 ns) and cheap.

--

John Larkin   Highland Technology, Inc   trk 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Unfortunately, the LVDS common mode voltage is quite limited, so the Tx and Rx grounds should be within +/- 1 V from each other.

Care is needed when wiring DC power and ground feeds between cards especially if there are some inductive loads fed from the same DC power. The spikes in the ground return could be quite large.

With a self-clocked modulation scheme, Ethernet magnetics (for isolation) should keep up with the requirement. You can get four parallel streams at 125 MHz with long (100M) wires, so a short link can probably handle the 200 Mbps task with two pairs (send and receive).

I presume availability of logic modules for modulation, demodulation, and FIFO functions?

ill

t

standard originally recommended a maximum data rate of 655 Mbit/s over twis ted-pair copper wire, but data rates from 1 to 3 Gbit/s are common today on high quality transmission mediums."

What we did with a Taxichip link - which wasn't quite that fast at 125Mbit/ sec - was to use a transmission line transformer to provide isolation (main ly to block any ground link through that path).

It was just a twisted pair of enamelled wire wound onto an RM6 coil former. I started off with subminature coax, but checked to see if twisted pair wo uld work and it did. The Taxichip link used an 10/8 code which avoided data packets with a high DC content.

A simple common mode choke would block spikes.

--
Bill Sloman, Sydney

whit3rd wrote in news: snipped-for-privacy@googlegroups.com:

You can use plain old CAT6 OR get specialized CAT7 or even newer stuff that is not in the main channel yet if the wire is ever an issue. But we used it and made our own comm link code.

And you can get shielded ENET wire too. Cheap, so make two runs and put each direction on one and only use the tighter twisted pair.

whit3rd wrote in news: snipped-for-privacy@googlegroups.com:

Also at such a short hop, you could easily fabricate your own twisted pairs and shield them individually. and bundle that up inside a nice piece of braided jacket.

Whooo! That stuff is not cheap, but it sure is the top grade stuff.

22ga SPC teflon is Silver plated copper and "over-twist and relax" technique back to less twist keeps the twist memory so small turns do not yield an opened up twist segment.

snipped-for-privacy@decadence.org wrote in news:qu7fd5$mol$ snipped-for-privacy@gioia.aioe.org:

They have the good heat shrink too!

fredag den 27. december 2019 kl. 16.36.03 UTC+1 skrev John Larkin:

I do 200Mbps "SPI" on three pairs in a HDMI cable, the far end is just LVDS receivers and a pair of shift registers. Double data rate with one shift register on each clk edge, an aligned and inverted clock from another LVDS receiver with the pair swapped

Even CAT5 and CAT6 twisted pair might be an overkill pf two metres.

It might be worth trying ribbon cable with every second wire as a ground (or well decoupled power link).

Crimp connectors are cheaper than RJ45.

--
Bill Sloman, Sydney

Sounds like the Unibus cable from the 1970's, a few meter ribbon cable at most, unipolar TTL transmitters and receivers, quite a lot of ground wires, but not every other. This was in the days when computer cycle times were around 1 us. Quit a lot of problems with long cables and ground potential issues between equipment powered from different mains sockets.

If you are going to use ribbon cables at 200 Mbit/s, at least use differential signals (such as LVDS), not unipolar with every other wire grounded. Not as good noise performance as twisted pairs, but much better than any unipolar system.

Test EMC characteristics with an old (900 MHz) 2G mobile phone just close to the cable.

If that's found to be underperformiing, twisted ribbons are available, but more than twice the price

--
  Jasen.

Especially the GSM phone is a nice RFI generator, since it uses TDMA and hence a high peak to average power ratio. Do the testing in a room with weak base station field (e.g. in a cellar), since this will force the mobile phone handset into maximum power.

Bill Sloman wrote in news: snipped-for-privacy@googlegroups.com:

What? One per conductor? Nope. Not cheaper.

RJ45 terminates with cheap, easy crimp connectors.

8 terminations, one crimp. Cheaper than 8 or even 4 indicidual crimp connectors... on labor alone.

Better, there may still be SCSI cables commercially available; those were for LVD, have terminator options as well as shielded connectorized cables with lots of twisted pairs. It's not clear that the drivers (which were all bidirectional) were fast enough for the task at hand, but the cables should do it, neatly, with 110 ohm termination.

This variant has nine pairs and lots of power/handshake lines

There is no need to look for a place with a weak signal. Just make lots of short calls. The handset will transmit at full power for the first few seconds of each call before it negotiates a new power level.

John