Pulse or digital communications – Transceivers
Reexamination Certificate
1999-11-10
2003-10-07
Chin, Stephen (Department: 2634)
Pulse or digital communications
Transceivers
C375S224000
Reexamination Certificate
active
06631159
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to transceivers and, in particular, to detecting a cable disconnect to provide a hot connection capability.
2. Description of the Related Art
Transceivers (combination transmitter/receiver devices) are widely utilized for digital and analog communication of electrical signals. Transceivers are often connected to each other by external cables. In some transceiver systems, the cable may be connected and disconnected while the system is running. This is sometimes referred to as a “hot connection.” To implement a transceiver system with a hot connection capability, the transmitting transceiver must be able to detect if the cable is disconnected within a relatively short time.
One type of transceiver that is utilized and that can be configured to allow hot connections to be made is a USB transceiver, i.e. a transceiver designed in accordance with the Universal Serial Bus (USB) specification, e.g. USB Specification, rev. 1.1 (Apr. 20, 1998). USB is well-known to those skilled in the art; a technical specification on the bus can be found on the World Wide Web at <http://www.ti.com/sc/docs/msp/usb/spec/spec1.htm>. Additional information on USB may be found at <http://www.intel.com/design/usb/>; <http://www.usb.org>; <http://www.usb.org/developers/index.html>. The two “sides” of a communication using transceivers may sometimes be referred to as the “host” side and the “device” side of the system or communication.
Referring now to
FIG. 1
, there is shown a high-level block diagram of a prior art communication system
100
having two transceivers X
1
, X
2
, connected by a cable
101
. Transceiver X
1
, consists of transmitter T
1
and receiver R
1
. Transmitter T
1
may be put into a high impedance state by means of enable signal EN
1
when X
1
is used in the receive mode. Transceiver X
2
is similar to X
1
and consists of transmitter T
2
and receiver R
2
. Cable
101
, consisting of one or more wires, connects X
1
and X
2
.
Differential transceiver connectivity may also be employed. Referring now to
FIG. 2
, there is shown a block diagram of a prior art transceiver communication system
200
with differential transceiver connectivity. Differential transceiver system
200
has two special, differential transceivers X
1
, X
2
, connected by a cable
201
. Transceiver X consists of
1
differential transmitter T
1
and differential receiver R
1
. Transceiver X
2
is configured similarly.
Referring now to FIGS.
3
A,B, there are shown circuit diagrams illustrating transceiver T
1
and receiver R
2
of transceiver system
200
of
FIG. 2
in further detail. (Transceiver T
2
and receiver R
1
of system
200
are configured similarly.) Transmitter T
1
steers a current I
1
through a termination resistor RES
2
of receiver R
2
, whose value matches the impedance Z
O
of the cable connecting these two components (e.g., a cable such as cable
201
). Typical values of Z
O
vary from 20&OHgr; to 150&OHgr;, depending on the type of cable employed. Receiver R
2
is merely a differential comparator. When input A is low and its inverse AN is high, transistors M
1
and M
4
are on, and transistors M
2
and M
3
are off. If switch SW
2
coupled to resistor RES
2
of receiving transceiver X
2
is closed, the current I
1
is steered from node N, through resistor RES
2
, back into node P, and then to ground. Thus, node P is at a lower voltage than node N, so the output OUT
2
of the receiver R
2
is low. When A is high and AN is low, the current I
1
is steered in the opposite directions, so that node N is at a lower voltage than node P, so the receiver's output is high.
Referring once more to
FIG. 2
, the termination resistors RES
1
, RES
2
may be placed at both sides of cable
201
, or at only one side. In order to minimize reflections from impedance mismatch, it is preferable to have a termination resistor on the side that is receiving. Placing another termination resistor on the side that is transmitting may help signal integrity further, but it comes at the price of doubling the power that is needed to produce and transmit a given signal.
This may be seen in the following example. Assume a 50&OHgr; cable impedance and a single 50&OHgr; termination resistor, placed at the receiver side. Typical differential signals are 200 mV to 800 mV. If a 400 mV signal is desired, this is achieved by having a current of 8mA flow through the 50&OHgr; termination resistor. With a 3.3V power supply, this means a DC power consumption of 26.4 mW. However, if there is a 50&OHgr; termination resistor at each side of the cable, a 16 mA current is needed to yield the same 400 mV signal. This results in a DC power consumption of 52.8 mW. For this reason, it is often a good idea to have the switch (SW
1
or SW
2
) open at the side that is transmitting and closed at the side that is receiving.
In order to detect cable disconnect, present USB transceivers use two external 15 K&OHgr; pull-down resistors at the host side of the cable, and a 1.5 K&OHgr; pull-up resistor at the device side of the cable. As long as the cable is connected, the host senses that one connection of the (differential) gain signal is high and the other is low. If the cable is disconnected, however, both connections are pulled low by the 15 K&OHgr; pull-down resistors. In this manner, the host side may detect whenever the cable has been disconnected.
This disconnect-detection technique has several disadvantages. First, it only works for the host side of the cable, not the device side. Second, this technique is relatively slow, and may be unacceptably slow for some applications. The RC time constant is typically 4.5 &mgr;s, since an approximately 300 pF load must be pulled down through the 15 K&OHgr; pull-down resistor. (Lower value resistors could be used to reduce the time constant and to speed up the detection, but this would utilize more DC power, which is also undesirable.) Third, this technique requires the use and presence of the external pull-down resistors at the host side.
SUMMARY
A transceiver comprises a transmitter for transmitting a transmitted signal to a second transceiver via a cable; a first receiver for receiving a received signal via the cable from the receiving device; and a second receiver for detecting a disconnect condition of the cable when the transmitter is transmitting the transmitted signal.
REFERENCES:
patent: 5461661 (1995-10-01), Buttle
patent: 6009527 (1999-12-01), Traw et al.
patent: 6279060 (2001-08-01), Luke et al.
Agere Systems Inc.
Chin Stephen
Duane Morris LLP
Lugo David B.
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