Trouble communicating at 1Mbps

[rikkov]

Hi Guys,

I am using a few dynamixels for one of my robots and I am seeing a problem with few of my RX-28 dynamixels.

Its like I was able operate my Rx-28 dynamixels at 1Mbps baud rate without any problem for a while(that is 10-15 days) and after that it now doesn't respond well at 1Mbps baud rate. Most of the reads/writes done at 1Mbps baud rate to these Rx-28 dynamixels fails. But if I operate these servos at 57Kbps(the default value) baud rate, it works fine without any problems.

I tried updating the firmware of the dynamixel and that didnt help.

I see this problem only in few of the Rx-28s.

Please let me know what I can do to get things working at 1Mbps again.

Thanks a lot,

Rikkov

[billyzelsnack]

Are your wires ok?

[rikkov]

Hi,

The wires are alright, the reason why I say this is that it works fine using same wire at 57Kbps.

Thanks a lot,
Rikkov

[MikeG]

I'm not sure about your RS-485 setup so I'm guessing here.

You might be experiencing signal reflection at the higher frequency. Add a terminating resistors at the far ends of the RS-485 cable. The resistor should be the same resistance as the impedance in the line. Probably about 120 ohms (standard). The resistor would go between the A and B signal lines at each end of the cable. Be careful, your master node might already have a terminating resistor, again I'm not sure about your setup. Adding a resistor to an already terminated node will only make the reflection worse.

Edit: I'm assuming that you have a single bus not multiple cables connected to a node. Do NOT connect parallel signal lines like you can do with an AX-12 network. It cause impedance mismatches and reflection on a RS-485 network.

[rikkov]

Hi,

I am actually using USB2Dynamixel for my communication.

I am also not using any nodes, all are connected one after the other in daisy chain fashion.

Thanks a lot for the reply,
Rikkov

[Bullit]

It sounds like you have damaged a max485 transceiver.
Remove power and use an ohmmeter to measure from Vdd to D+. The impedance should be high. If its 200 Ohms or lower then there is definitely a problem. What happens is that since D+ and Vdd are right next to each other on the bus if the two short it causes damage to max485 chips on the bus. Each servo and your usb2dynamixel has a max485 chip. Remove the servos from your bot one by one or measure each separately to find which has the problem. I've seen many of these fail. If you are skilled soldering its not too hard to replace these chips.

Hope this helps.

[rikkov]

Hi,

Thanks for the reply and I will check out what u have suggested.

I have a questions, if the transceiver of the servo is damaged, will it be able to respond at other baud rates?

I am kinda sure that the USB2Dynamixel I am using is quite ok since its able to talk to all the RX-64 and some RX-28 dynamixels I have at 1Mbps.But the RX-28 dynamixels that is showing this issue works fine at lower bauds.

-Rikkov

[Bullit]

What you have described matches exactly what I've seen when transceivers are damaged. They will typically operate at lower baud rates. You may even be able to add a 100 ohm resistor on the bus across D+ D- that may balance the pair a bit. In the end if the test shows transceiver(s) are damaged then the best thing to do is replace them. I've seen many on the same bus get damaged all at once. I've also observed the same happening to others. Some times when you take the servos apart and go to replace the chip you can see a small bump on the top of the chip. I think static can cause this type of damage too.

[MikeG]

@rikkov add a 120 ohm resistor between A and B (-D, +D) at the end of the line and see if that helps.