[Dave Flowerday]

This will probably end up being a long post, and hopefully I can answer a lot of questions from this thread. But before I do that, I need to stress one thing: DO NOT connect a computer directly to the tether port of either the OI or the RC! The tether port carries standard serial data, but it also has +12V on one of the pins (think about it: the OI gets power from the RC when in tether mode, so this power has to come from somewhere). If you hook this up to a computer, there is a good chance you will ruin either the serial port on the PC or the RC itself or both.

OK, now that that's out of the way... Myself and some other members of team 111 experimented a lot with this last season. We successfully built a miniature operator interface out of a Motorola HC08 microprocessor that talks directly to the RC over the tether port. So it definitely can be done. However, we also have a lot of fancy equipment here at Motorola that allowed us to figure it all out before we destroyed anything. If you don't know exactly what's on each pin of the tether port, then you shouldn't be hooking the RC up to anything other than the OI.

The only reason why you still need the OI is that otherwise the master uP stops execution of the Stamp when it doesn't detect a signal from the OI. Strictly speaking, the OI just sits there. In fact, it doesn't even send any important data to the RC as the only things that are present in the Serin command are the sensors and switches.

The OI is sending several "important" pieces of data to the RC, even in this case. One of the bytes in the datastream from the OI to the RC is your team number. The RC will not function unless it is receiving this team number in the stream. There's also 2 checksum bytes, an "enabled" bit, etc.

Finally, do you have an OI and an RC and the necessary cables (one F-F and one M-F)? If so, I'll try to write a little program in the next few days to record everything that gets sent back-and-forth between the two.

We've obviously figured out the whole bi-directional protocol since we can control the RC with our own software. Right now I'm thinking it's not a good idea for me to share the protocol as it will almost certainly just lead to lots of dead RCs from people trying to hook up to them. I can tell you this though: the RC-->OI data stream is the exact stream you get out of the Dashboard port. So if you read the spec for that you'll get a very good idea of what's going on between the OI and the RC. The reverse direction is extremely similar.

If the main stamp is on the RC, then how does the OI respond to it? Are the screamers bi-directional? I hooked up the modems in backwards for kicks once, and the RC got short spurts of signal. IE. Pause .5sec, lurch, pause, lurch.

The screamers are indeed bidirectional. I'm a little surprised that hooking them up backwards didn't work. Functionally, the radios on each side of the link are identical, except for the Tx/Rx frequencies are opposite. If you want to learn more about the radio modems, visit www.electrowave.com and look for the SCREAMER422 technical manual.

The important thing to keep in mind is that the Stamp is a VERY small part of the OI/RC system. Basically, it is only there to allow us to program more easily. After all, a Stamp IIsx is only about $30-$45, but the whole RC/OI setup is $1145. There are SEVERAL other microprocessors that are far more expensive than the Stamp in both the RC and the OI. These are the ones that deal with all the communications between the two pieces.

It's been about 6 months since I looked at this, but if I recall correctly, there are 3 processors in the RC: Two PICs and the Stamp. The two PICs do the majority of the work, and run considerably faster than the Stamp. However, they are a lot cheaper. At $30-45, the BS2SX is easily one of the most expensive microcontrollers on the market. The PICs in the RC are probably between $5-10. The HC08 that we used in our Mini OI was $3. With the Stamp, you are paying a hefty price premium for the "ease of use" of the PBASIC language, though I'd argue that programming a regular microcontroller in assembly isn't really that much more difficult, and would be a lot more useful to students interested in computer science/engineering (but that's another topic altogether!).

So if the microcontrollers in the RC and OI are so cheap, why does it cost $1200, you ask? Well, I can't say for sure, but just guessing I'd say that it's because it's such a low volume product that they have to charge that much to recoup their costs in designing the hardware and writing the software to run it all.


Please let me know if you guys have questions about any of this. It's great to see so many people interested in the inner workings of the control system! This is exactly the kind of stuff that electrical/computer engineering is all about, and unfortunately I don't think either of these fields get enough exposure in the FIRST world.