Moving Crio Modules out of the Crio.

[Tom Line]

Ok, here's a fun one for those who are knowledgeable about the inner workings of the Crio.

A week or so ago we had an interference fit on some parts on the robot, and wanted to move the modules around in the Crio. Of course, in a couple cases you can't really move them, so we jumped to another option we thought would be workable.

We picked up a couple dB15 connectors, and soldered together a 2 foot jumper cable. We tested this out with both an ohm-meter, then ran current and checked with an am-meter to insure that none of the soldered connections were shorted or close to shorted. It checked out perfectly. We used twisted pair cat6 cable to reduce any possible noise.

However, upon unplugging the analog crio module from the crio and placing the jumper between the crio and the analog module to connect the two, the analog module stopped returning battery voltage (or any other measurement for that matter).

Does the Crio or the analog module look for a specific resistance before they start talking to eachother? Is there some other hardware reason (perhaps intentional) that you cannot do this?

We even 'guessed' that perhaps the outside housing on the db15 was another ground, so we soldered a wire from one housing to the other - that didn't change anything.

Thoughts?

[Alan Anderson]

The communication between the cRIO backplane and the plugin modules uses very very very fast signals. If you pair up signals inappropriately, you will introduce noise that can disrupt the connection. If you make the extension too long, you will introduce time delays that can disrupt the connection. If you don't pay close attention to transmission line effects, you will create signal echoes that can disrupt the connection.

Bottom line: don't expect it to work.

[Joe Ross]

I would place the cable between the analog module in the cRiO and the analog breakout.

[juchong]

The cRio communication protocol is essentially SPI, but it is still very fast. I'd be careful as to not cause too many reflections or introduce too much noise. If you manage to find a DB-15 cable that's very well shielded and of good quality, you can probably get away with it, although it might cause very odd problems for your programmers. I wouldn't mess with it if I were you.

[Trent B]

Like others I think it is an area you don't want to likely mess with, you are probably better off making a cable between the breakout and the module (as this works on the digital) and I am pretty sure all the D/A Conversion logic occurs in the module then it simply forwards this data to pins on the breakout. Also I would much rather have several hundred dollar modules (list price) surrounded by a magnesium chassis and let the breakouts get broken if something hits them.

[EricVanWyk]

Originally Posted by Tom Line

We picked up a couple dB15 connectors, and soldered together a 2 foot jumper cable. We tested this out with both an ohm-meter, then ran current and checked with an am-meter to insure that none of the soldered connections were shorted or close to shorted. It checked out perfectly. We used twisted pair cat6 cable to reduce any possible noise.

Using twisted pair is great for differential signals (like ethernet or CAN), but isn't good for unrelated signals.

You might have better luck with pre-assembled cables, but I wouldn't hold my breath. As everyone else has said, you are playing with some darn fast signals.