Ii bus board, v2, with pullups

edit - also see Ii bus board, v2, with pullups

for those not following the TT expander thread, we’ve found that putting a large number of devices on the i2c bus will add sufficient capacitance to break some things. timeouts are long and TT crashes, etc.

unfortunately the TT circuit has built-in pullups of 10k. these are not easy to change short of taking the unit apart and SMD soldering on new resistors. (see below for how to do this)

so, @sam proposed a bus board with pullups. but it needs 3v3, so it’ll need power.

2.2k pullups in parallel with the 10k on TT would give about 1.8k, which seems right. http://www.digikey.com/product-detail/en/yageo/RC0603JR-072K2L/311-2.2KGRCT-ND/729676

here’s a cheap, widely available, easy to solder 3v3 reg: http://www.digikey.com/product-detail/en/diodes-incorporated/AZ1117CH-3.3TRG1/AZ1117CH-3.3TRG1DICT-ND/4505206

just threw this together in 15 minutes… board and schem attached.

tt-i2c-v2.zip (16.4 KB)

here’s an osh order link. note, this is totally untested, but it’s so simple i can’t imagine there being a problem. if somebody wants to test this right away, be sure to volt-meter the levels before plugging in i2c, to ensure the 3v3 is correct. i’m saying this because kelli and i will be away for a bit soon, so those of you in a hurry wanting to take the torch on this, please do.

$9 for three boards!

Order from OSH Park

that said, if you’d rather just swap the resistors on your TT, here they are:

replace with 0603 package resistors. 2.2k or around there will be fine.


sorry, total electronics noob. does this just mean it will need a ribbon cable to the eurorack power too?

will this be fixed on future teletype units? or is this a do it if you need it only mod?

Two questions, firstly is there anyone in Europe who can do this sort of modification (for payment obvs)? SMD-level soldering is beyond my ability.
Secondly, how many headers on the busboard? I was thinking that the previous board was a little light, and would welcome having one or two spare. FWIW, I have WW, ES, AS, and hopefully in the near future a TXi/o. If there’s any possibility of I2C connection to an ER-301 at some point, it would be good to have some room. Is a larger busboard a commercially viable product?

yes it needs 10 pin eurorack power. 3v3 regulated off the 12v rail. almost no current used.

re future tt fix, all current stock has 10k. again this is only an issue with more than 4 modules on the i2c bus.


hard to call the busboard a “product” as there’s no potential for profit. i’d be making them at a loss. it will stay DIY, or if @bpcmusic wants to produce and sell them he is welcome.

there are 8 headers on the board i just designed. it is trivial to add more. image of the board is on the oshpark link.


Ok, thanks for the image and for all the effort in investigating a resolution to this.

Thanks a lot - I appreciate undertaking the efforts for this new bus board design proposal!

I understood that it has not been tested yet and is more of a trial that should work.

Is there supposed to be a functional difference between modding the teletype and using the powered bus board and has the resistor mod of the teletype been tested then?

What I am experiencing in my system is that i2c does not work reliably with 4 modules (two Ansibles, Earthsea and Meadowphysics) on the bus. Is this supposed to get solved by the tt mod or should I go for the bus board?

I would greatly appreciate a link or part number for the 2.2k pullups.

If any regular in europe wants a board, hit me up… could also be combined with the other parts, as I plan to fill a basket with other stuff too for free shipping. (this is low on my priority list though)


Count me as interested in that, particularly if you’re putting in an order for all the parts.

Great! I ordered 3 PCBs from OSHPark last night, and they were added to a manufacturing panel within ten minutes. Performing surgery on my Teletype would be a bit too anxiety inducing, so I appreciate this alternative.

I’ll report back after I build it. I currently have 2x Ansible connected. I had previously experienced a lot of JF crashes in remote mode, so I had disconnected that. I’m finishing the builds on 2x TXo and 2x TXi, so I’m about to have a lot of modules connected to one TT.

Are there any specific tests I should run and report after the build?

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any 0603 2.2k will do


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thanks tehn!! your support on this is very much appreciated.

If TT can’t support an ecosystem of attached modules it kind of defeats the purpose of offering an ecosystem. My plan was to attach a Just Friends, one Txo and one Txi so that sounds like it would work, but adding an Ansible or two would push it over.

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have ordered via oshpark for the board. Am planning to share the power with teletype using this diy mini flying bus. Probably not much strain on the power and able to save a spot on the power busboard for other module.


well, in short, it can. there’s no reason to overreact to an edge case. without modding your TT or getting this cheap-as-nothing busboard, you can do everything that is promised with these modules.

what the TT mod or bus board enables:

  • prevents crash on user-error addressing of non-existent modules
  • allow read/write speeds to rates never imagined when this feature was added

i think it’s important to remember that this is an open source endeavor. we’re collectively figuring out a way to wedge in more functionality than was intended-- which to me is far more interesting that having a locked protocol and a very private company never talking about their plans. the fact that @bpcmusic made expanders in the first place is simply wonderful-- so let’s embrace the ride we’re all on.


I apologize for what I realize, in retrospect, was an overly-harsh tone. As a product person myself, primarily in the corporate world but now in the non-profit world, I’ve suffered through many ecosystem debates, and among them none were as painful as those brought about by success.

So what you say is not just true but good: working through a problem as a community in an open way is an excellent way to address issues. Ensuring that conversation includes some clear guidelines about edge cases is important, as those are the places explorers go first, followed by everyone else if they are successful.



Hi sakul, I’m in Germany and would be more than happy if I can buy a complete board from you. Please let me know if this would be possible.

@tehn I try again, hoping not to be too annoying with my requests:

Is the bus board supposed to be the more efficient solution for the I2c stability bugs compared to the teletype resistor mod?


@Leverkusen The result on stability is identical between the two solutions. The difference is in whether you want the bus board, and whether you’re comfortable modifying your teletype.