1. Technical Field
The present invention relates in general to differential mode links, and in particular to a system and method for neutralizing signal echoes that reflect from physical discontinuities in a differential mode link interface to a cable.
2. Description of the Related Art
Serial data transfer is a form of data output that utilizes a digital communication interface for sending and receiving data in digital format. Serial data transfer has been a common method for transferring and collecting data because it provides a reliable and fully standardized link between a transmitter and receiver.
Any two devices equipped with a serial data interface can communicate with one another. A serial data interface also provides bi-directional communication so that operating parameters of a connected device can be remotely programmed by a host device. Another significant advantage of serial data transfer is that it is highly resistant to electrical noise.
Serial data interfaces are very versatile. Serial data that is sent and received between a sensor and host can be configured in a variety of ways. Differential signaling is a common serial transmission technique whereby differential voltages are utilized for transmitting data to allow the use of longer cabling and faster data transfer rates.
Differential signaling is typically implemented utilizing two wires (in addition to a ground wire) that maintain differentially opposed voltages of +V volts and xe2x88x92V volts. Due its superior noise suppression, differential signal transmission cables commonly have lengths of up to several hundred meters, and data rates exceeding 1 Mbps.
An further enhancement to differential signal transmission is a developing technology known as simultaneous bi-directional signaling. Simultaneous bi-directional signaling allows each differential wire pair to carry two data streams, one in each direction. The channel bandwidth is effectively doubled without raising the baud rate thus allowing the use of lower cost package options for a given data transmission rate.
Within a simultaneous bi-directional system, current source drivers are utilized to interface host devices on both ends of the cable. An exemplary differential driver suitable for such applications is discussed in related patent application ROC9-1999-0212, Ser. No. 09/506,754 the subject matter of which is incorporated herein by reference. A characteristic of such drivers is that an inverted replica of the driver output voltage is generated by a replica circuit. The purpose of this inverted replica signal is to permit a bi-directional receiver to properly receive and understand incoming traffic during periods in which the local driver is transmitting from the node.
To accomplish this masking effect, the inverted replica output is subtracted from the outgoing driver signal in the bidirectional receiver in order to extract the incoming signal that is superimposed on the driver output voltage.
A problem arises, however, due to physical discontinuities and imperfections on the interface between the line driver and the network cable utilized to carry differential network signals. Sources of such discontinuities include imperfections in packaging in addition to capacitance associated with substrate-to-card contacts.
Signals from the link driver switch at frequencies that result in portions of driver signals being reflected from the discontinuities. The result is that the discontinuities reflect the outgoing driver signals back across the driver substrate causing an echo that distorts the desired output transition. As previously explained the simultaneous bi-directional receiver receives the output driver signal and subtracts this from a replicated driver signal in order to extract an incoming network signal. Since the driver output signal is applied to the receiver from a common physical interface at which incoming bus signals are received from the network, the echo caused by interface discontinuities negatively impacts the receiver""s ability to receive and translate the incoming data reliably.
From the foregoing, it can be appreciated that a need exists for a system and method for canceling noise reflections generated in a differential link interface.
An echo cancellation circuit for neutralizing signal reflections in a differential link interface, and a method for achieving the same are disclosed herein. The differential link interface includes a line driver that generates a line drive signal and a replica driver for generating a replica signal that mirrors the line drive signal within the differential link. The echo cancellation circuit includes a slope adjustment device within the replica driver for temporarily altering the slope of the replica line drive signal during a signal reflection at the output of the line driver, such that the amplitude of the replica signal corresponds to the amplitude of the line drive signal during the signal reflection.
All objects, features, and advantages of the present invention will become apparent in the following detailed written description.