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Current mode logic , or source-coupled logic , is a digital design style used both for logic gates and for board-level digital signalling of digital data.
The basic principle of CML is that current from a constant current generator is steered between two alternate paths depending on whether a logic zero or logic one is being represented. Typically, the generator is connected to the two sources of a pair of differential FETs with the two paths being their two drains. Bipolar equivalents operate in the same way, with the output being taken from the collectors of the BJT transistors.
As a differential PCB-level interconnect, it is intended to transmit data at speeds between 312.5 Mbit/s and 3.125 Gbit/s across standard printed circuit boards.
The transmission is point-to-point, unidirectional, and is usually terminated at the destination with 50 Ω resistors to Vcc on both differential lines. CML is frequently used in interfaces to fiber optic components. The principle difference between CML and ECL as a link technology is the output impedance of the driver stage: the emitter follower of ECL has a low resistance of around 5 ohms whereas CML connects to the drains of the driving transistors, that have a high impedance, and so the impedance of the pull up/down network is the effective output impedance. Having a drive impedance that is closer to the characteristic impedance of the driven line greatly reduces undesirable ringing.