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Unread 29-01-2015, 01:20
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Network Switch

I was looking through the QA and rules and I can't find many rules regarding the use of network switches. Does anyone know about whether is has been permissible in the past to use switches? I feel like this must have been brought up before, but if it hasn't, I will QA it.

The only relevant rule I have found is
Quote:
R47 The DAP-1522 Wireless Bridge must be connected to the roboRIO Ethernet port (either directly or via a CAT5 Ethernet pigtail).
As long as the roboRIO is attached directly to the bridge, is it ok to use a netgear (or any other brand) network switch provided it complies with all other rules regarding power?
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Unread 29-01-2015, 06:55
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Re: Network Switch

In prior years, the Wifi Adapter had one port, and the cRio has 2 ports. The cRio connected directly to the Wifi Adapter, and you connected other stuff through the other cRio port. If you needed to connect more than one other thing, you had to connect the cRio to a hub or switch.

As long as your Network Switch is a non-broadcasting switch (does not have wifi), you should be fine. I don't see why the Network Switch is not considered a COTS Computing Device.
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Unread 29-01-2015, 07:19
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Re: Network Switch

I'm curious why you need this with the bridge already acting as a switch for the ports on it. Can you tell us what you are trying to do?
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Unread 29-01-2015, 07:25
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Re: Network Switch

Sam,
I have to ask what you are trying to accomplish. The DAP has four ports only one of which will be in use on the robot. I believe that a second switch can be used provided that there is no "collisions" with data on the robot and that the RoboRio be connected to the DAP and not routed through another switch. Please be sure that the list in R49 is satisfied. A question to the Q&A will get you a response from FIRST Engineering as to their thoughts on the subject and that is my recommendation.
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Unread 29-01-2015, 07:32
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Re: Network Switch

I can't think of any rules that would prohibit connecting an Ethernet switch to one of the D-Link's ports.

I'm curious why you would need extra connections, though. Do you have multiple cameras or off-board processors?
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Unread 29-01-2015, 11:12
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Re: Network Switch

Quote:
Originally Posted by Al Skierkiewicz View Post
Sam,
I have to ask what you are trying to accomplish.
The reason why this has come up is this year our robot has 5 co-processors. We considered using the SPI or I2C bus; however, ultimately we decided on using IP based communication. We only have 4 ports on the access point and we need 5 ports for processors, 1 for programming, 1 for roboRIO, 1 for camera, and potentially 1 for a driver station at comps. This adds up pretty quickly and we are looking at 9 ports.

An alternative is to switch to a different communication method; however, UDP is working pretty well and we have the most experience with it.

I will ask whether it is legal to use a switch in the QA. If they conclude that it isn't, does anyone have any experience with SPI or I2C and/or have any suggestions on what to use (or another communication protocol/bus I am missing).

Last edited by baumgartensam : 29-01-2015 at 11:52.
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Unread 29-01-2015, 11:27
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Re: Network Switch

Quote:
Originally Posted by baumgartensam View Post
The reason why this has come up is this year our robot has 5 co-processors


Quote:
I will ask whether it is legal to use a switch in the QA. If they conclude that it isn't, does anyone have any experience with SPI or I2C and/or have any suggestions on which to use (or another communication protocol/bus I am missing).
You didn't mention straight serial UART, or CAN.
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Unread 29-01-2015, 11:48
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Re: Network Switch

Quote:
Originally Posted by Alan Anderson View Post
You didn't mention straight serial UART, or CAN.
UART is something to consider.

I was hesitant about using CAN because 2 of the processors are on moving components and I don't want to kill our entire bus because of a wire snag. Is this a legitimate concern or am I imposing a flaw on CAN that doesn't really exist?
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Unread 29-01-2015, 11:56
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Re: Network Switch

Quote:
Originally Posted by baumgartensam View Post
I was hesitant about using CAN because 2 of the processors are on moving components and I don't want to kill our entire bus because of a wire snag. Is this a legitimate concern or am I imposing a flaw on CAN that doesn't really exist?
It's generally never a good idea to place your electronics on moving parts--do you mind going into a little more detail about that?

In any case, you're correct about the CAN bus: since everything is daisy-chained, a disconnect mid-way through the match is something you definitely don't want. I'm not sure if you would lose control just over all components past the disconnect, or if you would lose control of everything because you will have lost your endpoint. Either way, it's not worth the risk.
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Unread 29-01-2015, 12:02
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Re: Network Switch

Find a layer 2->7 managed switch and traffic shape on the robot

At the baseband non-packetized:
Differential RS422/485 would allow a bus where a device on the bus could disconnect or 'vampire tap' without killing the whole bus.
http://en.wikipedia.org/wiki/Vampire_tap
http://en.wikipedia.org/wiki/RS-422
http://en.wikipedia.org/wiki/EIA-485

However if you intend to run star configuration instead of a bus you may as well just use Ethernet and a switch.
The effort involved is really pretty similar and the conversion of the signals to differential built into most of the Ethernet devices.

Generally I think working around the broken wire issue is not entirely possible because sooner or later the FIRST control system has a few single points of failure.

Last edited by techhelpbb : 29-01-2015 at 12:12.
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Unread 29-01-2015, 12:07
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Re: Network Switch

Quote:
Originally Posted by aryker View Post
It's generally never a good idea to place your electronics on moving parts--do you mind going into a little more detail about that?
As a general rule, I definitely agree and we avoid it when we can.

Their are a few reasons why we chose to put processors on moving parts. The moving ones are on our elevator carriages. We are using them to get the absolute position of the elevator. The frame of our elevator has an overlay on it that has a gray code binary system encoded on it. We are then using reflective photo interrupters to read the encoded binary. We are encoding 10 bit numbers meaning we need 10 photo interrupters, each with 4 wires (2 for LED, 2 for photo diode). Rather than running 40 wires to our belly pan and doing the decoding there, we are using an arduino to convert the data into a decimal number on the carriage then sending that to our main controller.

We decided it was better to put the processor on the carriage and have a communication cable running through our cable carrier than to run 40 cables and put the processor on the belly pan.

Last edited by baumgartensam : 29-01-2015 at 12:11.
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Unread 29-01-2015, 12:11
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Re: Network Switch

I would use whatever you are familiar with. If devices talk to each other (vs. the roboRio), a switch would give them an independent pathway.

Quote:
Originally Posted by baumgartensam View Post
I was hesitant about using CAN because 2 of the processors are on moving components and I don't want to kill our entire bus because of a wire snag. Is this a legitimate concern or am I imposing a flaw on CAN that doesn't really exist?
I don't think it would kill the entire bus. It would just isolate that portion of the network.

CAN Bus tends to be Daisy Chained, rather than connected to a common bus. So anything after that failure point would be isolated from anything before that failure point. As a multi-master serial bus, you don't have to worry about being disconnected from the Bus Master.
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Unread 29-01-2015, 12:14
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Re: Network Switch

Quote:
Originally Posted by rich2202 View Post
CAN Bus tends to be Daisy Chained, rather than connected to a common bus. So anything after that failure point would be isolated from anything before that failure point. As a multi-master serial bus, you don't have to worry about being disconnected from the Bus Master.
Yes; however, my fear is that there would no longer be a terminator on the bus which to my understanding would bring the whole thing down. That being said, I don't have a thorough understanding of the inner-workings of the CAN bus and would love to be told I'm wrong.
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Unread 29-01-2015, 12:17
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Re: Network Switch

Quote:
Originally Posted by baumgartensam View Post
As a general rule, I definitely agree and we avoid it when we can.

Their are a few reasons why we chose to put processors on moving parts. The moving ones are on our elevator carriages. We are using them to get the absolute position of the elevator. The frame of our elevator has an overlay on it that has a gray code binary system encoded on it. We are then using reflective photo interrupters to read the encoded binary. We are encoding 10 bit numbers meaning we need 10 photo interrupters, each with 4 wires (2 for LED, 2 for photo diode). Rather than running 40 wires to our belly pan and doing the decoding there, we are using an arduino to convert the data into a decimal number on the carriage then sending that to our main controller.

We decided it was better to put the processor on the carriage and have a communication cable running through our cable carrier than to run 40 cables and put the processor on the belly pan.
Wow, that certainly sounds like a complex feedback setup!. Have you considered other options that may be simpler? For example, if your elevator is powered on a winch, you could hook up a potentiometer to the winch and use it to measure the location of the elevator. We did something similar a few years back with a winch and a 10-turn pot, and it worked perfectly the entire season.
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Unread 29-01-2015, 12:22
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Re: Network Switch

Quote:
Originally Posted by Jon Stratis View Post
Wow, that certainly sounds like a complex feedback setup!. Have you considered other options that may be simpler? For example, if your elevator is powered on a winch, you could hook up a potentiometer to the winch and use it to measure the location of the elevator. We did something similar a few years back with a winch and a 10-turn pot, and it worked perfectly the entire season.
It definitely is very complicated. We didn't want to gear down a shaft on our winch to only rotate 10 turns over 6'. We ran the numbers and we were aiming for more precision than we would be getting with a pot. We also have a large electronics and software team this year so we have an entire subteam working on this one sensor. They have made lots of progress and it actually is working really nicely.

There are a number of benefits most notably precision (1/8" over the whole travel). After this year we may package up the sensor so it's easy to reuse.

Last edited by baumgartensam : 29-01-2015 at 12:27.
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