highly-parallel highspeed connection between two FPGA boards

[Maurice Branson]

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

 

[MM]

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

 

[John_H]

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.

 

[dave]

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?

 

[David L. Jones]

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.

I'd second HDMI, certainly worth looking at. I just added HDMI to a
Virtex-4 myself.
The Virtex-4 doesn't have the TMDS signaling used on the real HDMI
interface, but you can use LVDS or whatever over the standard
connector.

The Type A connector has 4 pairs, and the Type B has 7 pairs per
connector.

If you are short on space, the HDMI Mini connector has just been
released.

Dave.