backplane for power, ethernet, serial and usb data

[godg]

Hi,

I’m trying to build a midterm system between a costly and highly custom VPX and a platform where each board interfaces with one another by cables.
for example:
imagine I have an off the shelf mini-itx motherboard, a switch, and 2 network video encoders
my idea is to have a back plane that would supply power, serial communication (if required) and network. I would have to build custom fixtures for each board and cables to interface between the board and the back plane but globally  the total number of cables will be reduced.

i have searched the internet for answers, mainly regarding to high speed connector and pcb layout but no answer was found.
Could someone please help me? any input is welcome

[uncle_bob]

Hi,

I’m trying to build a midterm system between a costly and highly custom VPX and a platform where each board interfaces with one another by cables.
for example:
imagine I have an off the shelf mini-itx motherboard, a switch, and 2 network video encoders
my idea is to have a back plane that would supply power, serial communication (if required) and network. I would have to build custom fixtures for each board and cables to interface between the board and the back plane but globally  the total number of cables will be reduced.

i have searched the internet for answers, mainly regarding to high speed connector and pcb layout but no answer was found.
Could someone please help me? any input is welcome

Hi

How fast is the network connection?

How fast is the video?

You *could* be talking about data rates in the 10 MHz range. It is also possible (though unlikely) you could be dealing with 22 GHz. Your cards will dictate how fast the connections will be.

Bob

[Rerouter]

What standard connections are you using presently, and what power levels are involved,

As you gave an example of an ITX motherboard, does it need stuff like sata for storage routed out, or simplified to a network or usb boot
Is it 100mb , 1Gb, 10gb, etc networking you need, is it via ethernet, or other means, (to get an idea of impedances and frequencies)
What video connection(s) are you planning to use, VGA is more forgiving than HDMI for instance,

And how much power do you need to carry, you may need a twin plug back plane, one for high current power and one for I/O to keep costs down,

[godg]

thks uncle_bob

network connection would be maximum gigabit Ethernet.
video is normal ntsc /pal and  if its not to costly i would choose SD video.
serial communication frequency is not high (rs232, rs485, i2c)
usb may present some problems because it needs more shielding to solve interference problems no?

[godg]

thks rerouter

sata storage will not be routed, it would require for me to deal with a lot of new and unknown issues i think, due to frequency issues.. right?
yes I’m talking about the network, usb com and uart communications (rs232 rs485)
for Ethernet its would be maximum 1Gb
the other com I’m worried about is usb
I’m planning on vga, as u said i imagine hdmi being a lot more complicated

i was planning to have a dual connector back plane. one connector for I/O and one for power.
this boards don’t use a lot of current but I’m planning to add a motor controller on the future (30 amps)

[Rerouter]

USB if your ok with low speed will happily pass through a 50 ohm back plane connector,
Ethernet, 100Mbit is far more forgiving, but if you think your up for it, gig should be possible,

VGA component and composite generally use 75 ohm transmission line,
USB uses 44 Ohm terminated circuits,
Ethernet uses 50 Ohm...

I would say pick 50 ohm as your standard, as they are the higher speed circuits,

RS232 and RS485 are so slow by comparison that it doesnt matter to them,

[uncle_bob]

Hi

I would back up a bit.

Coaxial insert connectors for 50 ohm stuff are not at all cheap. High speed shielded balanced connectors (for Gigabit Ethernet) also are not very cheap. Doing up an impedance matched board to handle all this sort of thing is ... not cheap. You also need the switching for all of the high speed stuff. You can't just put 50 ohm or Gig E lines on a bus. They need to come into the proper "gizmo" to terminate and switch the signals.

Are you *sure* this will save you money?

Bob

[godg]

Hi bob,
thanks, my main reason for this is not to save money. it’s a feature I’m trying to implement on a project.

my initial idea was to use a multipin connector (edge card) that would support gigabit Ethernet. my thought is that if Ethernet is supported most of the other data connections would be. maybe I’m wrong...

this post has a second point to: i don’t know if some sort of passive electronics is required to maintain the signals.

[uncle_bob]

Hi bob,
thanks, my main reason for this is not to save money. it’s a feature I’m trying to implement on a project.

my initial idea was to use a multipin connector (edge card) that would support gigabit Ethernet. my thought is that if Ethernet is supported most of the other data connections would be. maybe I’m wrong...

this post has a second point to: i don’t know if some sort of passive electronics is required to maintain the signals.

Hi

Ok if cost is not going to stop this dead. Here's what I would suggest:

Pick a vendor, Samtec is one https://www.samtec.com. There are a number of others. They have a *lot* of connectors. The issue isn't finding one, it's sorting out which of the many to use. Cost quickly gets into the mix.

Q strips are one alternative:

https://www.samtec.com/connectors/high-speed-board-to-board/mezzanine/qstrip

They will handle just about any high speed signal you will ever run into with current boards. There is no reason for picking them other than they are the first one on the page . You might get away with routing the 50 ohm stuff over it. A more classic approach would be to add another connector to the mix that has coaxial inserts in it. Power may or may not work out with the high speed connectors. You could add a dedicated power connector or possibly get that done on the connector that has the coax inserts.

So, what is the result: You have three connectors on the edge of your PC board. They mate up with three connectors on the backplane. It's slightly more exciting to layout. It's not much different than a single connector from the "easy to use" side.

Bob

[godg]

thks bob i will search a bit more and follow ur advice