Electronic digital logic circuitry – Interface – Current driving
Reexamination Certificate
2002-04-12
2003-12-16
Tran, Anh (Department: 2819)
Electronic digital logic circuitry
Interface
Current driving
C326S082000, C326S026000, C326S028000
Reexamination Certificate
active
06664811
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to data communications and more particularly to transmission of high frequency signal over transmission lines for high-speed data communication.
2. Description of the Related Art
Modem computer systems often utilize one or more buses to connect to peripheral devices to enhance its resources. For example, the resources of a computer system may be substantially increased by connecting the computer system to one or more peripheral devices such as disk drives, tape drives, printers, scanners, optical drives, and the like. These peripheral devices are attached to the computer system by means of a peripheral bus (e.g., cable).
One of the most widely used peripheral buses is the well known small computer systems interface (SCSI) bus, which is defined in conformity with well known SCSI protocols (e.g., SCSI-1, SCSI-2, SCSI-3, etc.). These SCSI protocols are universal parallel interface standards for connecting disks and other high performance peripherals to computers and are incorporated herein by reference. The SCSI protocols are designed to provide an efficient peer-to-peer I/O interface between a host computer and peripheral devices in a computer system.
FIG. 1
shows a block diagram of a conventional computer system
10
including a host computer
12
, one or more SCSI devices
14
,
16
, and
18
, and a SCSI bus
20
. The host computer
12
includes an SCSI host adapter
22
for communicating with the SCSI peripheral devices
14
,
16
, and
18
. The host adapter
22
in the computer system
10
controls communication between the host computer
12
and the SCSI devices
14
,
16
, and
18
. For example, the host adapter
22
provides a physical connection between the host computer
12
and the SCSI bus
20
. In addition, it is configured to receive data, address, and control signals from the host computer
12
and convert the signals into corresponding SCSI compatible data, address, and control signals. Conversely, the SCSI host adapter
22
is also configured to receive SCSI compatible data, address, and control signals from the SCSI devices
14
,
16
, and
18
through the SCSI bus
20
and convert them into corresponding host-bus compatible data, addressing, and control signals. The SCSI host adapter
16
is well known in the art and may be implemented, for example, by using AIC-7890A™ packaged semiconductor device, which is available from Adaptec Inc., of Milpitas, Calif. Although the computer system
10
is illustrated using a SCSI bus, it should be appreciated that computer system
10
may employ other interface standards having characteristics similar to SCSI such as Intelligent Peripheral Interface (IPI) standard.
In the computer system
10
, the host adapter
22
and SCSI devices
14
,
16
, and
18
typically use bus drivers and bus receivers to allow devices to communicate data and control signals.
FIG. 2
shows a more detailed schematic diagram of the host computer
12
and peripheral device
18
connected via the SCSI bus
20
for driving and receiving a signal. The SCSI bus
20
is a 16-bit SCSI bus with a plurality of control and data lines
52
,
54
,
56
,
58
,
60
, and
62
to transmit control and data signals. For example, data lines
52
to
54
are used to transmit data signals from data[
0
] through data[
15
] while parity line
56
is used to carry a parity signal. On the other hand, control lines
58
to
62
are used to transmit control signals. For example, the control line
58
is used to carry ACK (acknowledge) signal while the control line
60
transmits REQ (request) signal. Other well known SCSI control signals are also carried on control lines
62
.
Both the host computer
12
and peripheral device
18
typically include a driver and a receiver for each data and control line in the bus
20
. In the example, the host computer
12
is shown with a driver
72
and the peripheral device
78
is shown with a receiver
78
to illustrate transmission of a data signal over data line
54
from the driver
70
to the receiver
78
. It should be noted, however, that the signal transmission may proceed in either direction because the host adapter
22
and peripheral devices
14
,
16
, and
18
each typically includes a driver and receiver pair for bi-directional communication.
The output of the driver
72
is electrically coupled to the data line
54
while the input of the receiver
78
is electrically coupled to the data line
54
in the bus
20
. To illustrate transmission of a signal value
70
over bus line
54
, the driver
72
receives the signal value
70
from the host computer
12
(i.e., host adapter
22
) and drives the signal value
70
onto the data line
54
corresponding to data[
15
]. The receiver
78
then receives and outputs the signal value
70
from the data line
54
for use as data[
15
] by the peripheral device
18
.
FIG. 3
shows a more detailed schematic circuit diagram of the driver
72
and receiver
78
for transmitting signal value
70
over line
54
. Typically, SCSI bus
20
employs a voltage differential technique to transmit signals. Accordingly, the driver
72
transmits the signal value
70
using a signal line
82
and a complement signal line
84
to the receiver
78
. In this configuration, the signal lines
82
and
84
are used to transmit information for bus line
54
. Other bus lines typically employ a pair of signal lines to transmit information.
To determine which devices are asserting which bits on the bus during arbitration phase of SCSI protocol, the SCSI bus also implements a bias voltage in the termination at each end of the SCSI bus. Without a termination bias voltage, it would be difficult to determine which device is asserting a data bit because bits not being asserted would be floating. To provide the termination bias voltage, computer
12
includes a voltage source V(A)
86
(e.g., 1.5 volts) and a voltage source V(B)
88
(e.g., 1.0 volt) which are connected in series using a resistor
90
(e.g., 270 ohms), resistor
92
(e.g., 138 ohms), and resistor
94
(e.g., 270 ohms). This termination bias voltage circuit is connected to signal lines
82
and
84
as shown. Thus, a junction
91
is typically at 1.3 volts due to the termination bias voltage, and a junction
93
is typically at 1.2 volts due to the termination bias voltage. The termination bias voltage also results in an approximate termination resistance of 110 ohms.
Similarly, the peripheral device
18
also includes a termination bias voltage. As in the host computer
12
, resistors
95
,
96
, and
97
connect in series between voltage sources V(A) and V(B). These voltages and resistances may have similar values as for the host computer
12
and are connected to signal lines
82
and
84
as shown. Also shown in
FIG. 3
are multiple bus taps
98
symbolizing the variety of other devices, computers, and peripherals that may also tap onto SCSI bus
20
.
In the driver and receiver configuration, the driver
72
uses a low-voltage swing differential (LVD) driver and the receiver
78
is a low offset voltage, high-speed, differential input receiver. The driver for this type of SCSI bus uses an asymmetrical output, where one direction has more drive strength than the other. The reason for this asymmetrical output is because of the termination bias voltage discussed above.
The termination bias voltage and the asymmetrical driver output that it necessitates often cause undesirable effects. The reason for the need for a termination bias voltage has to do with the dual use of the data lines of a SCSI bus. A SCSI bus includes data lines that are used during a data phase of communication, i.e., high-speed transmission of data. A SCSI bus also includes various control lines that are used to transmit control signals at a lower speed during a protocol phase of communication on the bus. However, the data lines of a SCSI bus have a dual use. During the protocol phase of communication, the data lines are also used to transmit control
Adaptec, Inc.
Martine & Penilla LLP
Tran Anh
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