MLAB wiki

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====== I²C bus as an MLAB's component ======

[[http://en.wikipedia.org/wiki/I²C|I²C bus]], also called TWI is a popular electronic bus designed for low-speed communication between different peripherals. As an MLAB's component it is mostly used to ensure communication between microprocessor and sensors. I²C derivative [[http://en.wikipedia.org/wiki/System_Management_Bus|SMBus]] differs from it in that it contains timeouts' definitions and Packet Error Checking and therefor is, in sense, more robust than the original I²C. 
   
Thanks to an advancement in computer technologies [[en:arm|new possibilities]] arise allowing the use of the bus for direct communication between sensors and Linux systems. Furthermore, with the help of [[en:usbi2c|I²C adapter]] the bus can be connected to PC as well.

===== Linking of I²C devices =====

Although the I²C bus was originally designed to link devices on small scales (within the dimensions of PCB), using certain techniques it is possible to enlarge its range to tens of meters (( [[http://cladlab.com/electronics/circuit-design/communication-protocols/i2c-protocol#gallery|I2C Transmission Distances And Buffer’s]] )). However, it must be noted that such approach may not be safe because over long wires a high voltage can be electrostaticilly and electormagnetically induced and can cause damage to connected devices. The exact distance over which it is safe to use the I²C bus depends on the surrounding environment and the required reliability of the system.

Linking of I²C devices within MLAB is implemented on several levels according to extent of a network. Locally (on scale of several meters), the modules are connected by MLAB cables and separated by [[en:i2chub]]. This module also solves the conversion between logic 3.3V and +5V.

Over more extensive distances the bus is linked through shielded signal cables ending in female connectors and connected to [[cs:uniserial]] module. This module also contains a basic surge protection and most importantly acts as a mechanical bridge between 'heavy external wiring' and fine internal linking of devices.

In the case of extensive networks I²C is replaced by other physical layer (CAN, Ethernet) that interconnects individual I²C network segments.

==== Pymlab package ====

Pymlab represents a software package that enables a creation of network made of I²C devices and modules, which serve as routing points. It defines the structure of a network and addresses of individual nodes in a source file of a control program. Individual points in the network can than be accessed using simple python commands. 
 
The installation of the [[en:pymlab|pymlab]] packed can easily be done using following command:

sudo pip install pymlab

In case of having some older version of pymlab already installed, it is enough to run:

sudo pip install --upgrade pymlab

Both commands are equivalent and use the python package manager that contains [[https://pypi.python.org/pypi/pymlab/0.2|the pymlab]].
This procedure should also download other packages used by pymlab such as cython-hidapi (reffered only as hid module in python). However it does not install alelibusb, which must still be done manually using:

sudo apt-get install libudev-dev libusb-1.0-0-dev libhidapi-dev python-setuptools python-smbus cython

===== Convertors to connect I²C to PC =====

Although I²C is quite popular bus suitable to connect different sensors over short distances, it is not usually found and readily available on regular computers with exception of [[http://www.paintyourdragon.com/?p=43|DCC leading out of free display port.]]

==== USB interface ====

Proprietary designs:

* [[http://www.robot-electronics.co.uk/acatalog/USB_I2C.html|USB-I2C  USB to I2C Communications Module]]
  * [[http://www.harbaum.org/till/i2c_tiny_usb/index.shtml|i2c-tiny-usb]]
  * [[http://www.headfuzz.co.uk/?q=usbtoi2c|OpenServo interface]]

These share a common problem with a quality of drivers. Furthermore, i2c-tiny-usb does not fully meet the I²C specification, completely fails to comply with USB physical layer specification and only has maximal communication speed of 50Kbps. MLAB replaces these constructions with [[en:i2c_avr_usb|I2C AVR USB]] device that is equivalent to i2c-tiny-usb design, but made of MLAB modules.  [[en:i2c-pic-usb| I²C to USB with PIC connector]] represent another, yet unfinished, alternative.

==== USB HID ====

To connect without having to use drivers designed for specific hardware [[http://en.wikipedia.org/wiki/USB_human_interface_device_class|USB HID]] specification can be used. MLAB has only one module that complies with this particular specification - [[en:usbi2c|USBI2C01A]]. It can be operated using python or HIDAPI - see below.

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