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  • Driving Twisted-Pair Cables with I2C Bus Extender

  • Created: Mar 11, 2014

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Summary

The P82B715 is an I2C-bus extender system for long cables with a net capacitance of about 3000pF. It causes few changes to bus logic signal levels without compromising the important features of the I2C-bus and it doesn’t allow any additional requirements on the levels of the signals that it handles. It is compatible with all I2C-bus devices and other related devices that use TTL switching levels instead of I2C levels. It permits inclusion of all other classes of I2C-bus buffers even those that impose additional logic LOW level requirements on the bus signals or that use static level or dynamic level offsets.


The slew control minimizes ringing and overshoot when driving I2C signals over twisted pair communication cables that have a low characteristic impedance (e.g., Category 5e and similar 4-pair cables, as typically used for Ethernet communications, are around 100Ω). A quick design method is presented for cable design applications to 100kHz and 30m. Application examples include driving 20m cables to 400kHz in Fast-mode, a very large system for driving architectural LED displays that could include more than 240m of cabling, and an ATCA radial IPMB distribution bus example driving 2800pF.


The system uses the P82B715 I2C-bus extender, a bipolar IC intended for application in IC-bus and derivative bus systems. While retaining all the operating modes and features of the IC-bus, it permits extension of the practical separation distance between components on the I2C-bus by buffering both the data (SDA) and the clock (SCL) lines. The IC-bus capacitance limit of 400pF restricts practical communication distances to a few meters. Using one P82B715 at each end of a long cable (connecting Lx/Ly to Lx/Ly) reduces the cable loading on the linked IC-buses by a factor of 10 and allows the total system capacitance load (all devices, cable, connectors, and traces or wires connected to the IC-bus) to be around 3000pF while the loading on each IC-bus on the Sx/Sy sides remains below 400pF. Longer cables or low-cost, general-purpose wiring may be used to link IC-bus based modules without degrading noise margins.

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