Pulse or digital communications – Cable systems and components – Transformer coupling
Reexamination Certificate
1998-12-18
2001-12-04
Pham, Chi (Department: 2631)
Pulse or digital communications
Cable systems and components
Transformer coupling
C333S131000, C375S257000
Reexamination Certificate
active
06327309
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of data communications. More specifically, it relates to the field of bidirectional data communications channels and to the field of bidirectional data communications channels for Fast Ethernet Local Area Network (LAN) communications and the IEEE standard for physical media connections referred to as 100BaseT4.
2. The Prior Art
For Fast Ethernet LAN communications (i.e. 100 MBit/sec), there are currently three approved IEEE standards for physical media connections. These three standards are commonly referred to as 100BaseTX (TX), 100BaseFX (FX), and 100BaseT4 (T4).
TX specifies that the transmission media is to be two pairs of unshielded twisted pair (UTP) cable of Category 5 or better. Of the two pairs, one pair is used as the transmission media for a dedicated transmit channel and the other pair is used as the transmission media for a dedicated receive channel. FX specifies that the transmission media is to be a fiber-optic cable. T4 specifies that the transmission media is to be four pairs of UTP cable of Category 3 or better.
As used throughout this discussion, a “T4 device” is any device that is designed to communicate using the 100baseT4 standard. Referring initially to
FIG. 1
, a first T4 device
10
and a second T4 device
12
are shown hooked together according to the 100BaseT4 communication standard. As noted above, the 100baseT4 standard specifies that the transmission media is to be four pairs of unshielded twisted pair cable. Each of these pairs are shown schematically in
FIG. 1
as lines
14
,
16
,
18
, and
20
. How each of the four pairs
14
,
16
,
18
, and
20
is utilized depends on the perspective of the T4 device.
From the point of view of the first T4 device
10
, the first pair
14
is the transmission media for a dedicated “transmit” channel and the second pair
16
is the transmission media for a dedicated “recieve” channel. From the point of view of the second T4 device
12
however, the first pair
14
is the transmission media for a dedicated “recieve” channel and the second pair
16
is the transmission media for a dedicated “transmit” channel. From either perspective, the third and fourth pairs
18
and
20
are each used as the transmission media for channels that both transmit and receive. The third and fourth channels are therefore “bidirectional” channels. Each of these bidirectional channels can be configured to be either a transmit channel or a receive channel, but neither can be configured for both transmit and receive simultaneously.
When the first T4 device
10
is transmitting, data signals are transmitted from the first T4 device
10
to the second T4 device
12
on the dedicated transmit channel of the first T4 device
10
and each of the two bidirectional channels while the dedicated receive channel of the first T4 device
10
is used by the first T4 device
10
for sensing a collision. A collision occurs when any data signals are received by the first T4 device
10
on its dedicated receive channel while data is being transmitted on any of the other three channels.
When the first T4 device
10
is receiving, data signals are received by the first T4 device
10
from the second T4 device
12
on the dedicated receive channel of the first T4 device
10
and each of the two bidirectional channels while the transmit channel of the first T4 device
10
is used to send a collision signal if required.
Even though a bidirectional channel can be configured as either a transmitter or a receiver, the impedance looking into the bidirectional channel of the T4 device must always be matched to the characteristic impedance of the transmission media that is attached to it. This is true whether the T4 device is transmitting or receiving.
There are two known configurations of prior-art bidirectional communications interfaces being used for T4 bidirectional channels. These are shown in
FIGS. 2 and 3
.
Referring now to
FIG. 2
, the first prior-art bidirectional current source type communications interface
30
is shown. The first interface
30
includes an integrated circuit
32
that contains a transmitter
34
and a receiver
36
. The output of transmitter
34
is connected to a pair of output pins
38
and
40
. The input of receiver
36
is connected to a pair of input pins
42
and
44
.
The first interface
30
has two paths from the four pins of the integrated circuit
32
identified above that are combined into a single path on two lines at connector
46
. A transmit path has a first end at output pins
38
and
40
and a receive path has a first end at input pins
42
and
44
. The transmit and receive paths are combined into a single bidirectional path such that both the transmit path and the receive path have a common second end at pins
48
and
50
of the connector
46
. Transmission media
52
is connected to pins
48
and
50
of the connector
46
.
According to the 100BaseT4 standard, the connector
46
is a RJ-45 connector and the transmission media
52
is a UTP cable that exhibits a characteristic impedance of 100 &OHgr;.
In operation, a transmit data signal is generated by the transmitter
34
and enters the transmit path at the integrated circuit
32
at output pins
38
and
40
. It then passes through a transformer,
54
. Finally, it exits the transmit path and passes on to the transmission media
52
through pins
48
and
50
of the connector
46
. The transformer
54
has a common first winding
54
a
that is coupled to both a second winding
54
b
and a third winding
54
c
. The second winding
54
b
has a center tap
56
which is connected to a fixed voltage potential VCC
58
. A first resistor
60
is connected between pin
38
of the integrated circuit
32
and VCC
58
. A second resistor
62
is connected between pin
40
of the integrated circuit
32
and VCC
58
.
A receive data signal is generated by a remote device (not shown) on the transmission media
52
and enters the receive path at pins
48
and
50
of the connector
46
. It then passes through transformer
54
. Finally, it exits the receive path and passes into the integrated circuit
32
to receiver
36
at input pins
42
and
44
.
In circuits which are integrated onto a semiconductor chip, it is desirable to provide the same functions using fewer pins or to provide more functions using the same number of pins. The advantage of using fewer pins is that it reduces the size of the chip and the expense of packaging the chip. For example, a design that uses only two pins to perform a function is superior to a design that uses four pins to perform the same function. Similarly, in circuits requiring magnetics, it is desirable to provide the same function using fewer or smaller magnetics. The advantage of using fewer or smaller magnetics is that it takes up less space and is less expensive. For example, a design that uses only one two winding transformer to perform a function is superior to a design that uses a pair of two winding transformers or a three winding transformer to perform the same function.
Also, in electrical circuits, it is desirable to provide the same function using fewer separate paths. The advantage of using fewer circuit paths is also that it takes up less space and is less expensive. For example, a design that uses only one path to perform a function is superior to a design that uses two paths to perform the same function.
The first bidirectional communications interface
30
is less than ideal by virtue of the fact that it uses two pairs of input/output (I/O) pins, output pins
38
and
40
and input pins
42
and
44
, at the integrated circuit
32
it uses a three winding transformer
54
, and it uses two paths for transmit and receive. These each add to the cost and the size of the T4 device.
Referring now to
FIG. 3
, a second prior-art bidirectional voltage source type communications interface
70
is shown. The second interface
70
includes an integrated circuit
72
that contains a transmitter
74
and a receiver
76
. The output of transmi
Dreyer Stephen F.
Jin Robert X.
Yiu Lee-Chung
Bayard Emmanuel
D Alessandro & Ritchie
LSI Logic Corporation
Pham Chi
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