Lynxmotion Smart Servo (LSS) and wiring

Hi everyone, and happy New Year to all!

I am new to lynxmotion and I ordered 8 “Lynxmotion Smart Servo (LSS) Motor - High Torque (HT1)” with 2 “Lynxmotion Smart Servo (LSS) - Adapter Board” for a project.

I see that the “Lynxmotion (LSS) - 300mm Serial Cable” is the longest available, but I need about 2 - 3 meters long.
So I will need to extend it a lot… I would like to be sure that it’s not a bad idea to do so.
(I remember on smaller arduino project that extending wires with servomotors could be tricky: trembling motors etc…).

1 - Any hints on the wire I need to order for 2-3meters long connections? maybe shielded cable? what thickness and what materials?

2 - I would like to know also if I could extended Y wire and control 8 servo from one board (I now it’s possible to add up to 6 servo on one hub but I am not sure if I can put a Y wire straight from the hub?

Thanks for your reply,

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You can certainly use longer cables. As you may already know, resistance in the cable increases proportionally with the length. The only way to reduce that impact is to increase the size of the cable. That being said, for 2-3 meters is still shouldn’t be all that bad. I have not tried something that long yet, but it should be easy enough to test.

This is an example of what happens if you use 3 meters of AWG-22 wires to run 12 V DC over it (to a 1 A load):

As you can see, it is not very significant.

Longer cables mean loss of voltage of the signal. The main different you’ll have with the LSS is that it is a digital communication, so either it gets through or it doesn’t. Longer cables can certainly mean more noise and signal issues.

You’ll notice the VCC & GND in the standard cables are in the middle between TX & RX. If you end up with custom longer cables, try to keep TX & RX away from each other to prevent cross talk.

The most I’ve used at one time was 36 LSS connected using many Y-cables and hubs! That was done for testing for electrical noise at 500k baud. It worked really good with pretty much no packet loss. I didn’t have any section that was more than 2 cables away though, so maybe 600 mm total at most?

If you do have noise at 2-3 meters, simply drop baud rate until you have no errors. The main difference between slower and faster baud rate is the duration of each “bit” of information. Slower baud rates (longer bits) have more tolerance to noise. The default baud rate of the LSS, 115220, should work at those cable lengths… most likely! :smiley:

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Great, thank you for your very well documented answer @ scharette

The voltage volt calculator is very useful also. I have found this 4pins AWG 18 cable which I think will be perfect to extend the wires.

I think it would be easier to extend existing wire rather than finding and buying every molex # 50-37-5043 connectors and corresponding 2.5mm pins inside…

The most I’ve used at one time was 36 LSS connected using many Y-cables and hubs!

Wow, so impressive! So it’s possible to simply add motors with an Y wire from hub, not only in chain from one motor to another… nice!

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Wow, that looks great! Back when I was a student we helped with some minor renovations at our engineering association’s building and we were donated large rolls of AWG-12 cable! Lets just say that we had plenty of cables for a long time! AWG-18 is plenty big enough for the amount of power you’ll be using so it should be fine.

If it does become an issue though (more than 4-5% drop, for example if using 5-6 servos), just run two more VCC & GND so that you can split the power demand in parallel on multiple power cables but keep all the data signals on the same pair (separated by the power cables to prevent cross-talk, of course!).

Yeah, the easiest if you don’t already have the molex connector is to simply cut the cable (I’d say smack in the middle of it, so you have extra length available in case you mess up :stuck_out_tongue: ) and then join your new wires there.

This is going to sound like it is very obvious (but from experience teaching labs, it happens all the time): make sure to keep all the wires connected to the same color on both side of your extension! :smiley: The last thing you want is shorting your 12 V into TX or RX and burning a bunch of servomotors.

I’d go a bit further: Once you have your extension setup, connect it to a power supply and test the pins with a multimeter to make sure you have 12 V & GND where you expect. Also, do continuity tests between both ends of the connectors to ensure there’s not short between two pairs (or more!).

At the time we figured if you built a large robot (like the old Lynxmotion Phoenix in 4 DoF version, with head & tail) or a full humanoid you’d need about 32-36 servos (at most). And you’d want fast communication, too! So we tested for it to make sure it was at least doable at an electrical and firmware level! :slight_smile:

Yes, it is. Basically, the TX and RX pins (and the VCC & GND, really) are all just parallel connections. Aside from the small details concerning voltage drop due to resistance (and other fun stuff relating to inductance/capacitance on the data lines), you can pretty consider all connections to be at the same point (from a data/interface perspective). Therefore, any signal on a TX pin is visible at all TX pins in the circuit (shared bus). Similar for the RX pin, too.

image
They are basically both the same: all the ports are connected in parallel.

Good luck with your project! If you feel like it, maybe you can show some pictures of your extension and/or document it in a tutorial/project page. You can create a project here and a tutorial here!

Sincerely,

Thanks a lot for all the informations.
I will report my experience here soon and why not, make a small documentation :wink:

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I’m certain others in the community (and outside of it!) will appreciate! You know what they say about a question, right? One person asks it and 10 others are thinking it but don’t ask… But they’ll certainly enjoy the answer! :smiley:

Here is a first step with 4meters wires connected :wink:

But I have a strange power supply problem to fix before to be able to report this test as you can see here!

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That’s pretty amazing! So, you’re saying it’s possible to add motors using a Y wire from the hub and make sure to get that insured, rather than just chaining them from one motor to another? That sounds like a cool innovation!