# PCIe over a short cable

For a new project I need to connect two boards via PCI express. For space reasons a custom cable assembly would be the best solution. But I'm not sure what is the best way to go here.

1. I've seen Riser-Cards with simple ribbon-cables. But when I think about differential impedances etc. these solutions do not seem to be very promising.
2. I've thought about a 2-layer Flex-PCB with a solid ground-plane. With some calculations I should be able to have the proper impedances for the high speed signals.

Is there any suggested way to handle this?

What a coincidence. I'm doing exactly that right now.

There are lots of ways to do this, but controlling the impedance is critical. The problem with using an unshielded ribbon cable is that it is hard to manage the impedance. It can be done, but finding a good cable supplier is hard, especially one that can guarantee the impedance. On the surface this looks like a good solution, but the difficulty of suppliers and stuff makes this less attractive.

Another way to do it is to use a shielded ribbon cable. I don't like this approach because the cable is expensive and terminating the shield is next to impossible.

The Flex-PCB is a good solution, but the NRE is high. If this solution is within your budget it might be the best thing.

A "quick and dirty and cheap" solution would be to use a couple of SATA cables! The impedance is right, they are cheap and widely available, and more importantly they work. Of course you will be limited in the number of signals so don't expect to get more than a PCIe x1 connection plus a reset signal (no power, JTAG, etc.).

• I've just been through this; if you can use SATA cables, do it. Everything else is very, very difficult to get right and to be able to repeat if you are able to achieve success. – akohlsmith Jul 30 '11 at 13:44
• Using SATA-cables sounds like a cool idea. I'll have to check on this. The only problem is, I need 5 lanes in total, plus two clock-pairs (one 4x and one 1x port). But maybe making a custom cable based on a SATA-cable is an option. Thanks for the info. – Nico Erfurth Jul 31 '11 at 16:00
• Note that SATA cables are solid core, and so are not good for flexing applications. But if it's static, then SATA is an excellent choice. – Rocketmagnet Aug 3 '11 at 16:44
• You can also use HDMI cables, I believe. I've seen more then a few designs in the healthcare industry use SATA though. – MadHatter Dec 2 '15 at 21:27
• Extending the SATA cable idea how about using the connectors and cables that are used for multilane SAS? – Peter Green Dec 24 '15 at 5:22

We do external PCIe at work and use the Molex connectors and cables. Molex External PCIe Not sure how much room you have but it's a great solution.

• Those connectors/cables are a good solution, but they are super expensive. The 1.64 foot cable in 30 piece qty is US$67 and an 11 week lead time according to the Avnet web site. Mouser has them for almost US$100 and 14 weeks. I'm sure these are the superior solution, but at that price... – user3624 Aug 1 '11 at 13:09