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View Full Version : Stamp to AX-12 Servo



Bill Chennault
02-08-2007, 04:11 AM
All--

I would like to play with the two AX-12 (http://robosavvy.com/RoboSavvyPages/Support/Bioloid/AX-12(english).pdf)servos I have sitting in a parts drawer. Of course, I want to use one of my Stamps . . . probably a BS2p40, since it will communicate ALL THE WAY up to 9.6k baud. (I think. Or, is it 19.2k?) The AX-12 smart servo's INITIAL baud rate is 1 million bits per second, but is capable of being set all the way down to 9.6k baud.

I know I could buy a DiosPro and reset the AX-12's baud rate down to 9.6k baud and I don't exactly mind buying the DiosPro just for that purpose. However, my learning curve and path are with Parallax, so--at least at this time--I am not too interested in another microcontroller type, unless that is the only feasible way to set the AX-12's initial baud rate.

Another solution is the CM-5 controller, which I think is a UART. But, since I am a newbie, it scares me.

Do you have any suggestions for setting the AX-12's initial baud rate so that I may experiment with these servos within the Stamp environment?

Thanks.

--Bill

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You are what you write.

Chris Savage
02-08-2007, 06:50 AM
Bill, even a PC serial port doesn't typically go that fast...However you could use our Prop Plug or USB2SER Development tool to establish a connection to the device from the PC at the specified baud rate and change it that way. You may need a PC side application to do this for you. I don't think Hyper Terminal will work for it. Take care.

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Chris Savage
Parallax Tech Support

Bill Chennault
02-08-2007, 07:15 AM
Chris--

Of course, I have studied the USB2SER specs (since I have one or two) and do not see why it would not work. If anyone has a pin diagram that would allow connection to the AX-12, I would certainly appreciate it! Just remember that the AX-12 is an RS-485 device (TTL).

--Bill

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You are what you write.

Chris Savage
02-08-2007, 10:41 PM
Bill,

I didn’t realize the device was RS-485…That is a differential mode system and you would need a driver chip to translate the signals. Take care.

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Chris Savage
Parallax Tech Support

Bill Chennault
02-09-2007, 04:12 AM
Chris--

My fault! Even a newbie like me should realize that what I really needed was a "USB2RS485" adapter. Do you know if such is available? If not, can we get rich by making one? :)

--Bill

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You are what you write.

Chris Savage
02-09-2007, 07:19 AM
Bill, how about a MAX485?

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1111

Take care!

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Chris Savage
Parallax Tech Support

John Abshier
02-09-2007, 09:25 AM
It is not a RS-485.· It is a half duplex ttl serial.· There is only one data wire.··Attached is a diagram from the manual.·

·Perhaps whoever sold it to you will reprogram it to a lower baud rate.

Bill Chennault
02-09-2007, 10:21 AM
John--

Thank you for the clarification. But, isn't the AX-12 servo network·itself·an RS-485 net? I thought that the connection from the first AX-12 to the next and ensuing·AX-12s was RS-485. Is this incorrect? And, yes, the vendor has offered (several times) to set the baud rate low so the Stamp will talk to it. In fact, they have experimented and already have a Stamp talking to the AX-12 because the Stamp is so popular and I beat them unmercifully. (They were supposed to ship it to me pre-set to 9.6k baud.)

But, I hate to ship stuff BACK. I suppose I will in this case, though. Still, I would rather figure it out--with all the help everyone is willing to give me--and LEARN something in the process than send it back and have it return, ready to go.

Theoretically, there exists a USB2Dynamixel adapter that will allow my PC to program the RAM table in the AX-12. I wish I could find a place to buy one. So far, it appears to be vaporware.

By the way, I have seen that diagram many times. It is obvious that it is a single-wire data device. What I THOUGHT the documentation said was that two AX-12's formed an RS-485 network.

--Bill

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You are what you write.

James Long
02-09-2007, 12:21 PM
Bill,

As I recall ( and I don't recall well....so cut me a little slack). The other servos that Dynamixel sells are rs-485. I think the AX-12 is a different communication.

But like I said....I don't recall well.

James L

John Abshier
02-09-2007, 11:08 PM
From Wikipedia: EIA-485 (formerly RS-485 or RS485) is an OSI Model physical layer electrical specification of a two-wire, half-duplex, multipoint serial connection. The standard specifies a differential form of signalling. The difference between the wires’ voltages is what conveys the data.

The AX-12 look to be mostly 485 but does not use the 2 wire differential form of signalling.