Pulse or digital communications – Transceivers
Reexamination Certificate
1998-11-25
2003-09-23
Le, Amanda T. (Department: 2634)
Pulse or digital communications
Transceivers
C375S257000
Reexamination Certificate
active
06625206
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to digital communication systems, and more particularly to bidirectional digital data transmission via a single transmission medium.
2. Description of the Relevant Art
Digital electronic devices typically communicate via electrical signals (e.g., voltage and/or current) driven upon electrical conductors (e.g., metal wires). As the operating frequencies (i.e., “speeds”) of digital electronic devices increase, electrical conductors used to route signals between components (i.e., signal lines) begin to behave like transmission lines. Transmission lines have characteristic impedances. If the input impedance of a receiving device connected to a transmission line does not match the characteristic impedance of the transmission line, a portion of an incoming signal is reflected back toward a sending device. Such reflections cause the received signal to be distorted. If the distortion is great enough, the receiving device may erroneously interpret the logical value of the incoming signal.
Binary digital signals typically have a low voltage level associated with a logic low (i.e., a logic “0”), a high voltage level associated with a logic high (i.e., a logic “1”), “rise times” associated with transitions from the low voltage level to the high voltage level, and “fall times” associated with transitions from the high voltage level to the low voltage level. A signal line behaves like a transmission line when the signal rise time (or signal fall time) is short with respect to the amount of time required for the signal to travel the length of the signal line (i.e., the propagation delay time of the signal line). As a general rule, a signal line begins to behave like a transmission line when the propagation delay time of the signal line is greater than about one-quarter of the signal rise time (or signal fall time).
Resistive “termination” techniques are often applied to transmission lines, and signal lines long enough to behave like transmission lines, in order to reduce reflections and the resultant signal distortion. One or more electrically resistive elements may be inserted between each sending device and the signal line (i.e., transmission line) in order to cause the effective output impedances of the sending devices to more closely match the characteristic impedance of the transmission line. Similarly, one or more electrically resistive elements may be inserted between each receiving device and the transmission line in order to cause the effective input impedances of the receiving devices to more closely match the characteristic impedance of the transmission line.
Various techniques exist which allow signals to travel in opposite directions along a single electrical path (i.e., bidirectional data transmission). Such bidirectional data transmission techniques may be employed to reduce the total number of electrical conductors required in digital communication systems.
FIG. 1
is a diagram of an exemplary digital communication system
10
employing a bidirectional data transmission technique. Digital communication system
10
includes a first communication device
12
a
and a second communication device
12
b
connected to opposite ends of a transmission line
14
. Communication devices
12
a
and
12
b
are synchronized to drive data signals upon transmission line
14
during alternate periods of the clock signal. Transmission line
14
includes at least two electrical conductors, and may be, for example, a single wire routed above an electrically conductive ground plane, a coaxial cable, or a pair of wires twisted together (i.e., a twisted pair of wires). Communication device
12
a
includes an input/output (I/O) driver
16
a
and an I/O terminal
18
a
connected to one end of transmission line
14
. Communication device
12
b
includes an I/O driver
16
b
and an I/O terminal
18
b
connected to the other end of transmission line
14
. I/O drivers
16
include circuitry for driving electrical signals upon the respective I/O terminals
18
, and for receiving input signals from I/O terminals
18
. I/O drivers
16
a
and
16
b
operate synchronously in response to a periodic clock signal. Communication devices
12
a
and
12
b
may be coupled to receive the clock signal via a clock signal line, or may include circuitry to generate and synchronize two separate clock signals.
FIG. 2
is a diagram illustrating the cyclic nature of the bidirectional data transmission technique employed by digital communication system
10
. Each period of the clock signal begins with a transition from a first voltage level “V
1
” to a second voltage level “V
2
”, where V
2
>V
1
(i.e., a rising edge of the clock signal). During a first period of the clock signal (i.e., a first clock cycle)
22
, communication device
12
a
drives a data signal upon transmission line
14
via I/O driver
16
a
and I/O terminal
18
a
, and communication device
12
b
receives the data signal via I/O terminal
18
b
and I/O driver
16
b
. During a second clock cycle
24
immediately following first clock cycle
22
, communication device
12
b
drives a data signal upon transmission line
14
via I/O driver
16
b
and I/O terminal
18
b
, and communication device
12
a
receives the data signal via I/O terminal
18
a
and I/O driver
16
a
. The data transmission cycle repeats itself as shown in
FIG. 2
with communication devices
12
a
and
12
b
alternately driving and receiving data.
Transmission line
14
has a characteristic impedance “Z
O
”. In order to reduce signal reflections within transmission line
14
, I/O drivers
16
a-b
drive respective I/O terminals
18
a-b
with an output resistance equal to Z
O
, and electrically couple I/O terminals
18
a-b
to the second voltage level through an electrical resistance equal to Z
O
while in a receive mode.
FIG. 3
is a timing diagram illustrating exemplary voltage levels within digital communication system
10
during employment of the bidirectional data transmission technique. At a time “t
1
” in
FIG. 3
, the clock signal transitions from the first voltage level “V
1
” to the second voltage level “V
2
”, beginning a first clock signal period in which communication device
12
a
drives data upon transmission line
14
and communication device
12
b
receives the data. During the first clock signal period, communication device
12
a
is to drive the first voltage level “V
1
” (e.g., a logic ‘0’) upon transmission line
14
via I/O driver
16
a
and I/O terminal
18
a.
I/O drivers
16
a-b
cannot drive respective I/O terminals
18
a-b
immediately, and an output delay time “t
OUT
” results. At a time “t
2
”, delayed from time “t
1
” by “t
OUT
”, I/O driver
16
b
electrically couples I/O terminal
18
b
to the second voltage level, and I/O driver
16
a
drives I/O terminal
18
a
. As the output resistance of driver
16
a
is equal to the characteristic impedance “Z
O
” of transmission line
14
, the signal launched upon transmission line
14
by communication device
12
a
via I/O terminal
18
a
at time “t
2
” has a voltage level midway between “V
1
” and “V
2
”.
A propagation delay time “t
PROP
” is required for a signal to travel from one end of transmission line
14
to the other. At time “t
3
”, delayed from time “t
2
” by “t
PROP
”, the signal launched upon transmission line
14
by communication device
12
a
at time “t
2
t” arrives at I/O terminal
18
b
, and I/O terminal
18
b
assumes the voltage level midway between “V
1
” and “V
2
”.
I/O driver
16
b
compares the voltage level present upon I/O terminal
18
b
to a reference voltage having a value greater than midway between “V
1
” and “V
2
” (e.g., two-thirds the difference between “V
1
” and “V
2
”). At a time “t
4
” following “t
3
”, the clock signal transitions from “V
1
” to “V
2
”, beginning a second clock signal period. At time “t
4
” the voltage level present upon I/O terminal
18
b
is less than the reference voltage, and I/O driver
16
b
produces and provides voltage level “V
1
” (e.g., a logic ‘0’) to communication device
Kivlin B. Noäl
Le Amanda T.
Meyertons Hood Kivlin Kowert & Goetzel P.C.
Sun Microsystems Inc.
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