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Asked — Edited

Using Multiple Ez-B

I see a number of people here using multiple EZ-B on their robots, if the reason is just running out of port lines and number of servos etc, then there is a really good alternative.

Many moons ago, long before the dawning of the EZ-B, the first AIMEC:3 used the SD20 (I2C 20 channel servo driver) chip


Using the SD20 requires only 2 port lines of the EZ-B (I2C bus) to control up to 20 servos!

Another big advantage is that the SD20 handles all the servo control overhead, so not only does it release a lot of EZ-B port lines for sensors etc, it also releases the EZ-B from having to generate all the necessary PWM pulses. With the SD20 the EZ-B only has to send one servo position command across the I2C bus and then the SD20 does the rest.

At around 6.50 per chip (and resonator), its quite a low cost alternative, also the same company do ready built servo control module boards.

Hope this is of help.

Dave, I know you have been using multiple EZ-B on your B9, if you want to try out the SD20 then I have a spare that I can send over in your Bosh servo package, let me know if you want me to add it?



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I've looked at their servo boards (literally just looked before I read this post actually) or more specifically the SD84 which I believe is USB controlled. Looking at the SD21 it's I2C which is better news (so thanks for that, I'd have assumed it was also USB controlled if I hadn't read this post).

I presume (possibly wrongly?) that these expander boards are not fit for sensors such as the ultra sonic ping sensors that require 2 ports though? But for servo positioning and controlling of LEDs, TIP transistor circuits etc. though these look to be ideal, you may have just sparked my interest in progressing on Jarvis again - thank you for that!

I guess now I need to revisit the other topic where the SD boards were mentioned and correct myself a little :)

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Hi Rich, The SD20 chipset and SD21 boards are for external EZ-B servo control only, but using them frees up all the port lines that the EZ-B may have been using for the robots attached servos. These freed up lines gives you more port lines for things like sensors or H bridge connections etc. Also there is quite a processing overhead for the EZ-B (microcontroller) to keep all the multiple servo's timing pulses running, and the SD20 and SD21 eliminates this and does all the hard work itself, so the EZ-B (I would imagine) has more resources for other things during runtime.

So I guess what I am trying to say is that if you have multiple servos hogging your EZ-B resources, then this I2C option is really neat!

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I have the Adafruit I2C controller - I've yet to get it to work... I haven't spent too long on it since shortly after getting it I got caught up in other things but it certainly wasn't as simple as it should be. It's next on my to-do list but other things have been preventing me from doing anything with the EZ-B or robots recently.

Devantech SD21 is the pre-made servo board Tony is talking about, it's I2C so should work and should be straight forward to script for.


Was the documentation and tutorial on the website helpful?

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Not really but I may have missed something. I need to look over the Arduino code for it and see if I can alter it for ARC, that'll probably be the easier method.


Hay Tony, Thanks for thinking of me.

Yes , I have 3 EZB's running in my B9 but it's mostly because it's a level thing. Each level of B9 twits, bobs or bends so I wanted to keep the wires running through these areas to a minimum. With an EZB on each level (Legs, Torso and Bubble) all I need is a power cable.

Furthermore, The EZB in the torso where most of the electronics are, I'm down to two digital ports and no ADC ports. I still have to build my arms with servos and sensors that will be controlled and powered from the torso section. I'll need more ports for sure. My plan was to buy another EZB (if they ever get released). With your little gizmo I may not have to buy another one and also free up a lot of processing power.

With that said, Sure! Please through it into the box with the motors. You're a kind and generous man. ;)

Thanks again! Dave Schulpius