Methods and circuits for generating a preemphasis waveform

Details Methods and circuits for generating a preemphasis waveform Methods and circuits for generating a preemphasis waveform

1999-03-09

2002-05-21

Pham, Chi (Department: 2631)

Pulse or digital communications

Pulse code modulation

Differential

C375S295000

Reexamination Certificate

active

06393062

ABSTRACT:

TECHNICAL FIELD
The present invention is related to the design and operation of serial communication devices. More specifically, the present invention teaches a variety of transmitter devices that operate in such a manner as to improve performance by compensating for high frequency line losses during transmission. For example, the preemphasis circuitry of the present invention extends the distance and increases the data rate of reliable communication by reducing intersymbol interference caused by long cables.
BACKGROUND ART
Serial communication is perhaps the simplest possible communication available, transferring only one bit at a time. Extraordinarily common, multiple serial communication standards (such as RS-232, RS-422, and RS-485) have developed over time. These standards specify the interface (connectors, pin functions, voltages, logic states, etc.) between two or more devices (e.g., a modem and a computer) so they may exchange data. RS-485 is a differential communications standard similar to RS-422, but with additional specifications so that multiple transmitters and receivers may share a single line. For example, the published RS-485 standard provides that RS-485:
1) is a differential mode serial data standard;
2) is capable of 32 transmitters and 32 receivers, maximum;
3) can operate over a maximum cable length of 1200 meters;
4) can handle a maximum bit rate of 10 Megabits/second;
5) has transmit levels of +/−1.5V minimum; and
6) must withstand voltage levels up to 12V.
The standard is, not surprisingly, deviated from in practice. For example, manufacturers design and sell RS-485 transmitters and receivers that are compatible with the standard, but have better performance.
FIG. 1
illustrates an RS-485 network
100
of the prior art. The RS-485 network
100
includes a plurality of transmitters
102
(often referred to as “drivers”) and receivers
104
coupled in parallel across a wire pair
106
. A repeater circuit
108
couples the network
100
to a second RS-485 network
200
. The network
100
further includes termination devices
110
which present a known load to the transmitters
102
and serves to minimize signal reflection across the network
100
. Only one transmitter
102
at a time may drive a differential signal (Y−Z) across the wire pair
106
. (Note that the labels Y and Z are used herein to indicate both the differential signal pair and the channel carrying this signal. The appropriate meaning will be apparent from the context.)
FIGS. 2-4
illustrate one possible waveform for the differential signal pair present on channels Y and Z.
FIG. 2
shows a Y
ideal
signal,
FIG. 3
shows a Z
ideal
signal, and
FIG. 4
shows their differential Y
ideal
−Z
ideal
. As will be appreciated, the waveforms of
FIGS. 2-4
are merely ideal representations. In practice, limitations in circuit design and circuit components, as well as losses due to line impedance result in the actual waveforms being mere approximations of square waves.
The distributed series resistance and the parallel capacitance present on the wire pair
106
cause it to have a low pass frequency transfer characteristic. These low-pass filter characteristics distort and dampen signals transmitted upon the wire pair
106
. The low pass filter characteristic causes “Intersymbol Interference” or “ISI,” which is a variation in the propagation delay of a bit pattern down the cable depending upon the particular sequence of the data that preceded it. Because of ISI, a direct trade-off must be made between cable length and bit rate on the one hand, and an acceptable error rate on the other. Specifically, increasing the cable length or speeding up the bit rate will, due to the ISI, result in an increase in the bit error rate.
In order to overcome these limitations, network designers take actions like coupling two different networks together with a device such as the repeater circuit
108
. The repeater circuit
108
mirrors and amplifies received signals in order to further propagate them, thereby increasing the possible cable length and/or transmission speed for the RS-485 network without increasing the bit error rate. The repeater circuit
108
is typically a bi-directional device. While solutions like adding the repeater circuit
108
are effective in their own way, they increase cost and complexity of the RS-485 network and it would be preferable to enhance performance of communications networks without the use of expensive, complex repeaters.
DISCLOSURE OF THE INVENTION
The present invention overcomes ISI by precompensating for the anticipated high-frequency energy losses in the transmission media. This precompensation is accomplished using a preemphasis waveform, i.e., driving the differential signal to a value larger than normal on a signal edge. Preemphasis increases the slew rate of edges inside data with a lot of transitions relative to data with fewer transitions, thereby compensating for the low-pass effects of the transmission cable. The present invention further contemplates various adaptive control schemes for ensuring that varying operating conditions do not effect preemphasis waveform generation. The first approach involves tracking the operation of a replicate driver for each main driver and adaptively controlling each main driver depending upon feedback from the replicate driver. A second approach, that is both more complicated and more effective, involves measuring the signal and the line impedance at the output of the main driver and adaptively controlling the main driver in response.


REFERENCES:
patent: 4857906 (1989-08-01), Conner
patent: 5198818 (1993-03-01), Samueli et al.
patent: 5459749 (1995-10-01), Park
patent: 5528166 (1996-06-01), Iikbahar
patent: 5559441 (1996-09-01), Desroches
patent: 5600274 (1997-02-01), Houston
patent: 5642067 (1997-06-01), Grace
patent: 5798668 (1998-08-01), George
patent: 5818269 (1998-10-01), Brown et al.
patent: 5901179 (1999-05-01), Urabe et al.
patent: 5917340 (1999-06-01), Manohar et al.
patent: 5936450 (1999-08-01), Unger
patent: 6087847 (2000-07-01), Mooney et al.
patent: 6157215 (2000-12-01), Gabara et al.

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