View Full Version : How many WCK servos in one chain?

09-13-2010, 03:30 AM
How many wck servos would I be able to run off one serial port? I had read somewhere that it was limited to the number of IDs that could be set which is technically 255.

This seems logical to me and I should be able to run a good 10 on one line right? I assumed the limitation would be the cables not being ably to handle enough current.

I am having weird issues. All servos are set to a baud rate of 57600.

I can control all servos individual (only having one plugged in) but sometime as soon as i plug in another servo it stops working. This doesnt happen with all servos though. ie I can control 4 servos fine but when i add a 5th i cant control any of them. If i take out servo 4 and put in servo 5 everything works sweet again. This is sort of implying you can only have 4 servos on one line.

The thing thats confusing though is sometime a cant even control 2 servos.

What could be causing this problem? Is there a limit to the number i can run on one line?

Cheers for your help.

09-13-2010, 05:34 AM
In the normal robobuilder configurations 16 servos work off one serial port. I have had 22 working reliably at 921K.
While the protocol allows 255, you might have some logic level problems before that without buffers.

The normal problems are with a bad cable or a bad servo. Did you check all servos are programmed correctly ?

Try programming a different servo to ID 4 and using that, and replace the cables to servo 4

What are you using to drive the servos ? Is it the RCB ?

09-13-2010, 10:40 AM
How big is your powersupply/battery ?

09-13-2010, 04:17 PM
I have checked each servo and am pretty sure they are all the same. So are they known to have issues with dodgy cables? I have made up some extra long cables which work fine with a single servo but no more than that.

Ill give resetting them a go. Is there a specific baud rate that works best when connecting up a bunch?

At the moment i am running off a pretty low power supply, can only supply 1amp before limiting but i am only driving one servo at a time at the moment so it only every draws about 100odd mA.

At the moment i am using my laptop to run them but i will be using the RB110

09-14-2010, 03:37 AM
Ok so using the Robobuilder controller i can control about 18 on one line and it doesnt seem to be a problem.
The problem i am having is i am using an ST232 chip, bringing it to TTL, running from the same comport as what i am controlling the RBC and i can only control 4servos. Once i add the 5th everything stops.

Ibot: What were you using to run 22servos? Off a computer or RBC?

09-14-2010, 10:11 AM
Does the robobuilder controller have huge caps on it between the power and the servos? If so then this still looks like a powersupply brown out problem to me.

09-14-2010, 11:34 PM
The RBC does have a 470microF capacitor over power and ground so i have added that in. What i have found is that when 4 servos are plug in i get a nice signal on the oscilloscope all the way down to the last servo. When the 5th is plugged in the signal completely stops. ie i have a signal on the input side of the st 232 chip (from the computer, 12v) but nothing on the output side.
So something is happening in the chip, saturating or cant supply enough dump current?
I have the st232 set up exactly like this:
http://sodoityourself.com/max232-serial-level-converter (http://sodoityourself.com/max232-serial-level-converter)
Is there a difference between the ST and the MAX? All i can see is the maximum speeds are different.
Do i need to do something to the output signal? Add a buffer or up the current somehow?
The RBC has inductors on all pins. Assume that is for high buad rates. Do i need these?

Cheers for your help

09-15-2010, 05:46 AM
What voltage are you supplying the servos with?

When you say you use that circuit, where is the 5V supply sourced from for the SP232 ?

Ther are some differences in SP232 and MAX, circuit should work for both. Check capacitor polarity, especially the one on pin 6.

The MAX and SP do seem low on current source capability compared to the ATMega in the RBC. I did not look to see the input circuit on the servo, to check if pull-up or pull down is present, or any input resistor. Yet!

Let me know above, so I can analyse full circuit.

I usually drive directly from an FTDI chip or TTL serial cable.

09-15-2010, 03:11 PM
I am using a power supply for both at the moment. 11V for the servos, the st chip draws 8mA and the servos draw about 100ma with 5plugged in.