highly-parallel highspeed connection between two FPGA boards

Hello,

designing a motherboard featuring a Virtex-4 FX140 FPGA with 24 integrated
RocketIOs (Gigabit SerDes IOs) I am now facing the problem of how to get the
signals off the board in a most space-saving and elegant way?

All 24 differential signals will be connected to some DACs outside the
motherboard to convert the signals into the analog signal domain.

In the definition of the interface I have nearly all effective degrees of
freedom. I once learned that coaxial connections are the most appropriate
way to do so in terms of signal integrity, but with 24 differential signals
and bidirectional connections that would mean at least 96 SMA connectors
(that takes very large board space and implies some skew problems due to the
differences in the line lengths).

I once used the Tyco ZDOK system (plug + receptacle) for board-to-board
connections with up to 80 parallel signals but unfortunately  the
performance was not satisfactory, so I won't be using them again. Are there
any other interface systems for the above described purpose or ways to build
a highly-parallel highspeed  connection between two boards?

I appreciate every kind of suggestion and help. Thanks a lot in advance.

Regards, Maurice

"Maurice Branson" <traubenuss@arcor.de> wrote in message 
news:469653f8$0$3834$9b4e6d93@newsspool4.arcor-online.net...
>
> designing a motherboard featuring a Virtex-4 FX140 FPGA with 24 integrated
> RocketIOs (Gigabit SerDes IOs) I am now facing the problem of how to get 
> the
> signals off the board in a most space-saving and elegant way?
>

Do you really need all 24 pairs? What is the DACs' sampling rate?

> All 24 differential signals will be connected to some DACs outside the
> motherboard

What does "outside" mean precisely?

/Mikhail 

You're seriously overthinking your problem.  Please indicate the data speeds 
you're encountering to help bound the problems in peoples' minds.

Consider the lowly HDMI interface.  These are ~1.8 Gbit/s signals that go 
over flimsy, mass-produced cables with flawless digital performance to get 
1080p signals.

Twisted pairs will get you to where you need to be.  The speed will dictate 
the eye closure over distance and you *probably* are well within the 
appropriate operating range.  For the truely demanding applications, twinax 
may be necessary but is probably overkill.

If the eye closure is a problem for your chosen transmission media and 
distance, preemphasis or receiver compensation can allow your twisted pair 
cables to provide an open eye to the A/Ds.  There are several products on 
the market that provide the transmit and/or recieve functions to allow 
multi-gigabit transmission over very cheap lines.



"Maurice Branson" <traubenuss@arcor.de> wrote in message 
news:469653f8$0$3834$9b4e6d93@newsspool4.arcor-online.net...
> Hello,
>
> designing a motherboard featuring a Virtex-4 FX140 FPGA with 24 integrated
> RocketIOs (Gigabit SerDes IOs) I am now facing the problem of how to get 
> the
> signals off the board in a most space-saving and elegant way?
>
> All 24 differential signals will be connected to some DACs outside the
> motherboard to convert the signals into the analog signal domain.
>
> In the definition of the interface I have nearly all effective degrees of
> freedom. I once learned that coaxial connections are the most appropriate
> way to do so in terms of signal integrity, but with 24 differential 
> signals
> and bidirectional connections that would mean at least 96 SMA connectors
> (that takes very large board space and implies some skew problems due to 
> the
> differences in the line lengths).
>
> I once used the Tyco ZDOK system (plug + receptacle) for board-to-board
> connections with up to 80 parallel signals but unfortunately  the
> performance was not satisfactory, so I won't be using them again. Are 
> there
> any other interface systems for the above described purpose or ways to 
> build
> a highly-parallel highspeed  connection between two boards?
>
> I appreciate every kind of suggestion and help. Thanks a lot in advance.
>
> Regards, Maurice
>
>
> 

In comp.arch Maurice Branson <traubenuss@arcor.de> wrote:
> Hello,
> 
> designing a motherboard featuring a Virtex-4 FX140 FPGA with 24 integrated
> RocketIOs (Gigabit SerDes IOs) I am now facing the problem of how to get the
> signals off the board in a most space-saving and elegant way?
> 
> All 24 differential signals will be connected to some DACs outside the
> motherboard to convert the signals into the analog signal domain.
> 
> In the definition of the interface I have nearly all effective degrees of
> freedom. I once learned that coaxial connections are the most appropriate
> way to do so in terms of signal integrity, but with 24 differential signals
> and bidirectional connections that would mean at least 96 SMA connectors
> (that takes very large board space and implies some skew problems due to the
> differences in the line lengths).
> 
> I once used the Tyco ZDOK system (plug + receptacle) for board-to-board
> connections with up to 80 parallel signals but unfortunately  the
> performance was not satisfactory, so I won't be using them again. Are there
> any other interface systems for the above described purpose or ways to build
> a highly-parallel highspeed  connection between two boards?
> 
> I appreciate every kind of suggestion and help. Thanks a lot in advance.
> 
> Regards, Maurice

What's the required signaling rate to the DAQs?

 
> 
> 

-- 
Be Happy. Don't Worry about the difference between lose and loose.. 

On Jul 12, 9:16 am, "Maurice Branson" <trauben...@arcor.de> wrote:

> designing a motherboard featuring a Virtex-4 FX140 FPGA with 24 integrated
> RocketIOs (Gigabit SerDes IOs) I am now facing the problem of how to get the
> signals off the board in a most space-saving and elegant way?
>
> All 24 differential signals will be connected to some DACs

Mass-produced SCSI internal and external cables and terminators
are easy to find.  The 68-pin cables go to 320 MHz nowadays,
and connect 27 differential pairs plus some status wires and
power wires for active terminators (which are commonly crimped
onto the internal cable end- no need to populate your board with
the terminators).

If you can drive 110 ohm wiring, and if the low-voltage differential
signal levels are the same as SCSI, your choice may be easy.

Tanks for your answers so far!

Oay, to make it more precise: I'm looking for an interconnect system 
(multi-channel plug + receptacle) for 24 differential pairs at signaling 
rates of 3 - 6 Gbps per channel.

>Tanks for your answers so far!
>
>Oay, to make it more precise: I'm looking for an interconnect system 
>(multi-channel plug + receptacle) for 24 differential pairs at signaling

>rates of 3 - 6 Gbps per channel.
>
That is an astonishingly high data rate (~70 Gbps to ~150 Gbps)! I won't
ask what it's for, as I suspect you wouldn't be able to tell us. ;-)

How far is the system architecture defined? Instead of a 'Digital' board +
'DAC' board, could you do it with a series of 'Digital + DAC' slices, even
if that meant replicating some of the functions in each slice?

On a previous project, the high potential interconnection data rates
rendered the 'obvious' approach non-viable, and so a more
communication-friendly architecture had to be developed.

On Jul 12, 6:16 pm, "Maurice Branson" <trauben...@arcor.de> wrote:
> Are there
> any other interface systems for the above described purpose or ways to build
> a highly-parallel highspeed  connection between two boards?

Look at the data sheets for PCIe card edge connectors. Some
manufacturers specify
-1.5dB bandwidth of 7GHz and more for the differential pairs.

You can use both receiver and transmitter pairs for transmitting (just
make sure you
do not plug the board into a PC). This provides you with 32
differential pairs for a 16x PCIe
connector.

Kolja Sulimma

"Maurice Branson" <traubenuss@arcor.de> wrote in message 
news:46971c5b$0$3830$9b4e6d93@newsspool4.arcor-online.net...
> Tanks for your answers so far!
>
> Oay, to make it more precise: I'm looking for an interconnect system 
> (multi-channel plug + receptacle) for 24 differential pairs at signaling 
> rates of 3 - 6 Gbps per channel.
>
>
Hi Maurice,
Dunno if this fits your requirements, but these are cheap and v. fast. Maybe 
20Gb/s.
http://www.samtec.com/ftppub/testrpt/hsc-report_sal1-bottom-entry_web.pdf
HTH, Syms.
p.s. Looks like 'bottom entry' is the way to go!

"Maurice Branson" <traubenuss@arcor.de> wrote
>
> Oay, to make it more precise: I'm looking for an interconnect system 
> (multi-channel plug + receptacle) for 24 differential pairs at signaling 
> rates of 3 - 6 Gbps per channel.

I don't know if you have already looked at them, but for things like that, I 
use Samtec connectors and cables.
http://samtec.com/high_speed_connectors/2006/si_b2b.asp
http://samtec.com/high_speed_connectors/2006/SI_C2B.asp?m=hs

Marc