[FRC Blog] myRIO Expansion Port - What's the Deal?

[FrankJ]

Originally Posted by compwiztobe

FIRST probably won't allow any custom components in the power pathway. Only official KOP relays and speed controllers.

Rather, an active device is one that modifies the signal sent to the Talon or Spike (or Jag or Vic). If the connection to a speed controller is not directly from the roboRIO pins or MXP headers, through passive devices (PCBs, bumpers, etc.), to the speed controller signal port, then the intermediate device has to be approved in advance.

I have long since stop trying to predict what the GDC will do. But speed controllers & relays fit into the blogs definition of an active device. The proposed rule says one active device. NOT one active intermediate active device connected to a second active device. The blog has an approval process for active devices. Part of the approval process seems to be that you will need the resources to make the device commercially available to all the teams. So the approval bar starts off pretty high.

[Aren Siekmeier]

Originally Posted by FrankJ

I have long since stop trying to predict what the GDC will do. But speed controllers & relays fit into the blogs definition of an active device. The proposed rule says one active device. NOT one active intermediate active device connected to a second active device. The blog has an approval process for active devices. Part of the approval process seems to be that you will need the resources to make the device commercially available to all the teams. So the approval bar starts off pretty high.

I can see where you're getting this reading, but look at the other two options for controlling a motor or servo. It says directly connected to PWM pins or connected through passive devices, which obviously doesn't mean no speed controller/relay at all. The intent seems to be that all actuators are still controlled by the approved controllers (Talons, Vics, Jags, and Spikes), while the signals to these controllers can be supplied in the 3 ways described (direct connection, passive connection, or one approved active device).

But I agree both that the wording doesn't say this exactly and that we can't predict all of the GDC's intent or future actions.

[ice.berg]

Does anyone know what kind or type of connector is used? Its a 17x2 pin connector, but not sure on anything else of it. Trying to find a source for a female plug to adapt to it. Thanks

[Joe Ross]

Originally Posted by ice.berg

Does anyone know what kind or type of connector is used? Its a 17x2 pin connector, but not sure on anything else of it. Trying to find a source for a female plug to adapt to it. Thanks

The blog links to the NI MXP developers guide at https://decibel.ni.com/content/docs/DOC-39578 which gives several mating connectors.

[FrankJ]

I think "passive" in First terms would include extension headers for the pins, wires, pcb traces, etc. Anything that does not modify the pathway. You make a daughter board that covers all the pins, but passively passes through the used pins. perhaps with a locking PWM connector that has the power & ground so you use a conventional PWM cable. Without allowing for the in the rules, an inspector somewhere would not let you do that.

The wording is the way it to allow for servos & VEX motors which take a direct PWM input well as conventional brushed motors & controllers.

[crake]

Originally Posted by SoftwareBug2.0

Maybe this is a stupid question but why do I need to use a SPICE simulation to figure out what the ports are doing? Why can't I just have a table with drive strengths and speeds and such?

Well you don't HAVE to use anything. There are a lot of tools out there for creating schematics and layout. The NI tools however offer that, plus a lot more, including simulation. If you're going into electrical engineering (particularly analog design), SPICE is a really valuable tool.... and you get it for free! Also the template for the board outline is already loaded for you.

And why do they assume that your circuit will be flat on the roborio and tell you clearances for stuff under where they think your PCB will be? Can't they just let you figure it out from the mechanical drawings?

Yes, you can, and calling out backside clearance does not replace the need to do a good job on the stack up analysis and mechanical tolerance calculations. However it does remind people that the default board position offers limited backside space, and if you want backside components, or long through-hole leads, etc. you will need to move to a taller connector.

[ice.berg]

Originally Posted by Joe Ross

The blog links to the NI MXP developers guide at https://decibel.ni.com/content/docs/DOC-39578 which gives several mating connectors.

Appreciate it, Thanks!

[Alan Anderson]

Originally Posted by compwiztobe

I can see where you're getting this reading, but look at the other two options for controlling a motor or servo. It says directly connected to PWM pins or connected through passive devices, which obviously doesn't mean no speed controller/relay at all.

Servos do connect directly without an intermediate speed controller. So do some servo-like motors, though those might not be legal actuators.

[DonRotolo]

Originally Posted by FrankJ

So something like an Talon or an spike would be an active device. If you wanted to use a beefier relay than a spike, that would be an active device & would require submittable & approval.

Please bear in mind that "active device" and "ACTIVE DEVICE" are two different animals.

The former encompasses an awful lot of things
The latter is defined as something that fits onto the MXP and needs FIRST pre-approval.

Carry on...

[SoftwareBug2.0]

Originally Posted by crake

Well you don't HAVE to use anything. There are a lot of tools out there for creating schematics and layout. The NI tools however offer that, plus a lot more, including simulation. If you're going into electrical engineering (particularly analog design), SPICE is a really valuable tool.... and you get it for free! Also the template for the board outline is already loaded for you.

Well certainly; I don't have to use the MXP port at all. And if National Instruments wants to provide a simulator and a board layout tool then that's fine too. But these things shouldn't have to be connected. Perhaps there's a link someplace I've missed but it looks like if you want any more than the voltage range and intended use you must use SPICE. For example, how many amps can I draw on each of the pins? What are the supported speeds of the serial port? These should be easy to spell out in text. I should not have to run a simulation to find out.

I agree that SPICE is useful. It's just not clear to me how the MXP port would cause you to use SPICE if you'd been given a proper data sheet. If I'm speccing a filter a simulation is great to go along with the graph for what the cutoff is supposed to look like. But in the MXP port there should't be anything interesting analog wise: everything is either digital, straight to an ADC, or straight to a DAC. And I doubt that the digital stuff is fast enough that the use of SPICE is warranted.

Originally Posted by crake

Yes, you can, and calling out backside clearance does not replace the need to do a good job on the stack up analysis and mechanical tolerance calculations. However it does remind people that the default board position offers limited backside space, and if you want backside components, or long through-hole leads, etc. you will need to move to a taller connector.

Do people actually screw this up? There is something to be said for making documentation accessible but please assume that high school students have mastered the ruler.

[EricH]

Originally Posted by SoftwareBug2.0

Do people actually screw this up? There is something to be said for making documentation accessible but please assume that high school students have mastered the ruler.

All. The. Time. It's really awkward when you forget to account for the thickness of a structural member in design and have to account for it in hardware with a sawzall. Been there, done that. Ever since, when working that sort of project, it's been "Did I account for ALL of the intermediate stuff when making this length?"

The other factor, of course, is that it's quite possible to mess up numbers with the mechanical drawings. (Trust me--even with a good drawing, sometimes the only thing saving your bacon is some really good measuring to interpret the drawing.) By saying that X needs Y distance of clearance, they are taking an aspect of the drawings that may or may not be clear to the average EE/programmer and saying "Pay attention to this. You need to take this into consideration. The drawings are less idiot-resistant than we'd like to be able to make them."

[MrBasse]

Originally Posted by SoftwareBug2.0

Do people actually screw this up? There is something to be said for making documentation accessible but please assume that high school students have mastered the ruler.

Thats a bold assumption. Being a high school drafting and shop teacher has shown many things to me, most importantly is that rulers and fractions elude even the best AP students sometimes.

Try an experiment just for fun, next time you are around high school students ask them what the decimal equivalent of 7/16" is and see how many can pull it off without a calculator or the internet.

[FrankJ]

Originally Posted by MrBasse

...
Try an experiment just for fun, next time you are around high school students ask them what the decimal equivalent of 7/16" is and see how many can pull it off without a calculator or the internet.

Or have them half 13/16. Not really a knock on the quality of students or teaching. There is a art or craft to all of this that takes time & practice to learn.

[SoftwareBug2.0]

Originally Posted by EricH

All. The. Time. It's really awkward when you forget to account for the thickness of a structural member in design and have to account for it in hardware with a sawzall. Been there, done that. Ever since, when working that sort of project, it's been "Did I account for ALL of the intermediate stuff when making this length?"

The other factor, of course, is that it's quite possible to mess up numbers with the mechanical drawings. (Trust me--even with a good drawing, sometimes the only thing saving your bacon is some really good measuring to interpret the drawing.) By saying that X needs Y distance of clearance, they are taking an aspect of the drawings that may or may not be clear to the average EE/programmer and saying "Pay attention to this. You need to take this into consideration. The drawings are less idiot-resistant than we'd like to be able to make them."

I guess I put things too strongly. Sure, people are fallible but maybe we would be better off letting them fail occasionally. Sometimes airports have moving walkways with a speakers at the end to announce that you should get off. Most of the time it's an annoyance. Too often I read through the documenation and feel like I'm listening to one of those speakers.

I think it's appropriate that a person would have to look ahead and see where they have to start walking again. And I think it's appropriate that people would have to determine clearances themselves. I say this as a person who has started daydreaming on one of those walkways and noticed the end by a foot coming out from under me. (It's actually kind of amusing; I recommend trying it sometime.)

[Greg McKaskle]

Sorry to be getting to this late. The MXP is a standard developed for myRIO and utilized for roboRIO. At least some of the drawings, tools, and documents are there because myRIO is used for college level design classes. If you don't need SPICE and don't want to expose the kids to it, I don't think anyone is going to twist your arm. If your comment is that some basic information is missing, please ask and I'm sure it can be provided.

As for the mechanical design. There are inserts for screwing the MXP card to the roboRIO, and the assumption was that this would be a common approach. The MXP docs also defines a planar variant that is intended to be used on myRIO side connectors. It is the team's design, and if you would rather have the board extend up into the robot, that is your choice, but unless the board is tiny, I suspect it will better serve as a lesson in how not to do mechanical design. Thus the recommendation is to use the ninety degree connection and screw the accessory to the roboRIO.

The primary goal with the MXP was to open up and more fully support the daughter-card or co-processors that a few teams implement. This can be limited to physical connector selection or can include integrated sensors and even processing. Even with the documentation, there is a chance of failure. There is also an opportunity for more open discovery and innovation. Please let us know how to improve it.

Greg McKaskle