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Altered the iintroduction to include a proper definition of the principle and distingush between the two main uses of the term. |
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{{Short description|differential digital logic family}}
'''Current mode logic''' ('''CML'''), or '''source-coupled logic''' ('''SCL'''), is a
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.
[[File:CML line.svg|thumb|right|CML termination scheme]]The transmission is point-to-point, unidirectional, and is usually [[electrical termination|terminated]] at the destination with 50 [[ohm|Ω]] [[resistor]]s to [[IC power supply pin|V<sub>cc</sub>]] on both differential lines. CML is frequently used in interfaces to fiber optic components.▼
As a [[differential signaling|differential]] PCB-level interconnect, it is intended to transmit data at speeds between 312.5 [[bit/s|Mbit/s]] and 3.125 Gbit/s across standard [[printed circuit board]]s.<ref>Serial Interface for Data Converters, [[JEDEC]] standard JESD204, April 2006</ref>
▲[[File:CML line.svg|thumb|right|CML termination scheme]]The transmission is point-to-point, unidirectional, and is usually [[electrical termination|terminated]] at the destination with 50 [[ohm|Ω]] [[resistor]]s to [[IC power supply pin|V<sub>cc</sub>]] 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 (typically 50 ohm resistive) is the effective output impedance. Having a drive impedance that is closer to the characteristic impedance of the driven line greatly reduces undesirable ringing.
CML signals have also been found useful for connections between modules. CML is the [[physical layer]] used in [[Digital Visual Interface|DVI]] and [[HDMI]] video links, the interfaces between a [[display controller]] and a monitor.<ref>{{cite web|url=http://www.bicsi.org/pdf/Regions/charlotte_se_4_11/Understanding%20DVI-D,%20HDMI%20and%20Display%20Port%20Signals.pdf|title=Understanding DVI‐D, HDMI And DisplayPort Signals|accessdate=2013-10-30|archive-url=https://web.archive.org/web/20131102130352/http://www.bicsi.org/pdf/Regions/charlotte_se_4_11/Understanding%20DVI-D,%20HDMI%20and%20Display%20Port%20Signals.pdf|archive-date=2013-11-02|url-status=dead}}</ref>
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