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what is ii and I2C?

ii is a flexible communication protocol which allows modules to send commands to each other digitally, which opens up possibilities that patch cables can’t facilitate. This digital networking is described as a ‘bus’.

Various monome eurorack modules can communicate with each other by sending digital messages over an I2C serial bus, using any type of cable to connect between pin headers on the rear of each module. Using the I2C bus as a transport, many different modules now support a similar set of control messages, which together constitute the ii protocol. This was originally conceived as a way for Teletype to interact with other devices, but now other devices also support initiating ii transmissions, including Crow.

An I2C bus consists of 3 lines - ground (GND), data (SDA) and clock (SCL). The data and clock lines are “pulled high” via pull-up resistors to 5V and data is transferred when these lines are “low”, thus power is also needed for the bus to operate.

Both monome’s Teletype and crow provide power over the i2c bus – however, Ansible does not. If you are in need of additional power or are planning to add more than three or four i2c-capable modules to your bus, we suggest invest in a powered bus board like the Txb.

While ii setups are usually straightforward, requiring the connection of matching ii headers (GND <-> GND, SCL <-> SCL, SDA <-> SDA) via 2.54mm connectors, there are a few consideration to keep things working:

  • Best practice is to daisy chain modules together. Several modules provide dual headers, allowing you to connect one module to the next. If your module only has one set of ii pins, like the Disting Ex, place this at the end of the chain.

  • Make sure to align your ii connections correctly, with each of the corresponding pins. These are usually marked on the PCB – a white line is often used to refer to the ground pin. Note that the vertical order of the pins on each module may be reversed from another in your case – always check to see where the GND line is!

  • Keep your cables as short as possible to reduce the risk of dropped data.

For additional information, please check out the helpful i2c/ii guide available on lines.

ii tips

A few quick ii tips:

  • Pins on each module will be labeled GND (ground), SCL (clock), and SDA (clock). These should be connected GND <-> GND, SCL <-> SCL, SDA <-> SDA, and you can connect them with any sort of 2.54mm pitch jumper cable. On all monome and Whimsical Raps modules, and the TELEX expanders, the pins are in this order: gnd, scl, sda. The order is the same from ground whether all pins are labeled on the board or not.
    Take care as other manufacturers may have different pin orderings. You won’t damage anything by getting these wired wrong, but modules might crash.
  • Trilogy modules support ii, but may need a pin header soldered on. See here.
  • I2C needs power to operate. This is achieved by “pulling up” the clock and data lines to 5V, meaning connecting an appropriate “pullup resistor” between SCL and 5V and between SDA and 5V. Some devices (generally those meant to act as leader devices) have pullup resistors wired, others (generally followers) do not – some such as Crow make this controllable in software. The right choice of pullup value may depend on the number of devices attached, but in many cases you should probably only have one device pulling the lines up.
  • Each device on the bus has an address, which the leader uses to indicate which device it wants to talk to. Some devices support choosing between multiple addresses, others use a fixed address. Generally all followers sharing the same address will respond identically to messages sent to that address.


While ii setups with a few devices are usually straightforward (e.g., connect headers between Teletype and Just Friends - done!), as networks involve more modules there can be some important considerations to keep things working. Here are some common user questions about ii networks involving several modules, focused on questions related to Monome devices specifically. Lots of other great information and links are available on lines.

Do I need to use a Teletype “backpack”?

The newer Teletype PCBs (black) provide more power to the I2C bus and provide more ii headers, allowing for somewhat larger ii networks out of the box. Older (green) Teletypes can often only support 2-3 followers without needing a powered busboard (DIY). Minimizing the total length of wire on your bus can also help – daisy chain rather than using a star.

Can I use Crow and Teletype simultaneously on the same ii bus?

Yes and no. The I2C specification is designed to allow multiple leaders to attempt writing to the bus simultaneously. However, only a single device in the network can be sending data at any given time. If multiple devices transmit at once, there is a possibility of one module locking up, or other odd behavior. As a result this type of setup is not currently supported, though some users have found varying degrees of success using multiple leaders.

However, Crow’s pullup resistors can be enabled or disabled in software, allowing it to be easily used as a leader or a follower. Support for controlling some of Crow’s functionality from Teletype is planned.