Low voltage differential dual receiver

Details Low voltage differential dual receiver Low voltage differential dual receiver

2001-09-26

2002-11-12

Chang, Daniel D. (Department: 2819)

Electronic digital logic circuitry

Signal sensitivity or transmission integrity

Signal level or switching threshold stabilization

C326S033000, C326S031000, C326S083000

Reexamination Certificate

active

06480022

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to data communications within a computer system. More specifically, the present invention relates to a differential receiver of a bus line.
BACKGROUND OF THE INVENTION
Within a computer system, it is often desirable to connect a variety of peripherals to the system bus of the computer itself for communication with the central processing unit and other devices connected to the computer. A variety of bus types may be used, and for any bus it is important to have bus drivers and bus receivers that allow devices to communicate quickly, efficiently and accurately.
FIG. 1
illustrates an embodiment of a computer system
10
. Computer system
10
includes a computer
12
, a disk drive
14
, a tape drive
16
, and any number of other peripherals
18
such as card reader units, voice input/output, displays, video input/output, scanners, etc. The computer and peripherals in this example are connected via a Small Computer System Interface (SCSI) bus
20
, although a wide variety of other buses may also be used.
Any number of computers or hosts may be present in computer system
10
and be connected to SCSI bus
20
. Each computer may also contain a variety of hardware and software. By way of example, computer
12
includes a monitor
30
, a motherboard
32
, a wide variety of processing hardware and software
34
and a SCSI host adapter card (or interface card)
36
. Host adapter card
36
provides an interface between the microcomputer bus of computer
12
located on motherboard
32
and SCSI bus
20
.
SCSI is a universal parallel interface standard for connecting disks and other high performance peripherals to microcomputers. However, it should be appreciated that computer system
10
is an example of a system, and other interface standards having characteristics similar to SCSI may also be used in such a computer system. By way of example, an Intelligent Peripheral Interface (IPI) standard is one such other standard.
In one embodiment, SCSI bus
20
is an 8-bit parallel flat cable interface (18 total signals) with hand shakes and protocols for handling multiple hosts and multiple peripherals. It has both a synchronous and an asynchronous mode, and has defined software protocols. In the embodiment shown, the SCSI bus uses differential drivers, although SCSI is also available with single wire drivers. SCSI interface cards (such as card
36
) plug into most microcomputer buses including VME, Multibus I and II, PCI, ISA, VL, EISA and MCA. In another embodiment, SCSI bus
20
is a 16-bit parallel cable interface (27 total signals). In other embodiments motherboard
32
has SCSI adapter card
36
incorporated into the motherboard itself, and a separate, plug-in adapter card is not needed. SCSI bus
20
is a multi-drop bus typically produced as a flat cable that connects from a computer
12
to any number of peripherals. In this example, disk drive
14
, tape drive
16
, and other peripherals
18
connect to SCSI bus
20
by tapping into the bus. In other examples, it is possible for any number of peripherals to be inside computer
12
in which case SCSI bus
20
may also be present inside computer
12
also.
Disk drive
14
includes the physical disk drive unit
40
and SCSI controller card
42
and other internal cables and device level interfaces (not shown) for enabling the unit to communicate with computer
12
. After connecting to disk drive
14
, SCSI bus
20
continues on to connect to tape drive
16
. Tape drive
16
includes the physical tape drive
50
, SCSI controller card
52
, and other internal cables and interfaces (not shown) for communicating with computer
12
.
SCSI bus
20
also connects to any number of other peripherals
18
. In alternative embodiments of the invention, any of the peripherals shown may eliminate the SCSI controller card by using an “embedded SCSI” architecture in which the SCSI bus becomes also the device level interface. In these peripherals, a cable such as SCSI bus
20
may be connected directly from motherboard
32
of a computer to a peripheral without the need for connecting to an internal controller card.
FIG. 2
shows in greater detail
50
SCSI bus
20
and connections to it from computer
12
and a peripheral
18
. SCSI bus
20
may come in a variety of standards. Illustrated here by way of example, is a 16-bit SCSI bus with a variety of its control signals shown. Shown are the signals data[
0
]
52
through data[
15
]
54
, parity
56
, ACK (acknowledge)
58
, REQ (request)
60
, and a variety of other control signals
62
.
This example illustrates how one value from computer
12
may be transferred via SCSI bus
20
to peripheral
18
. It should be appreciated that any number of data or control signals may be transferred back and forth on the SCSI bus. For example, computer
12
has a value
70
that passes through a driver
72
and over an electrical connection
74
to the bus line data[
15
]. At the peripheral end, the signal on bus line data[
15
] is passed by way of an electrical connection
76
to a receiver
78
whereby value
70
is received in peripheral
18
. Techniques by which a value may be transmitted by a driver over a SCSI bus to be received by a receiver in another electronic device are well known to those of skill in the art.
FIG. 3
shows in greater detail a proposed apparatus
80
by which value
70
is transmitted from computer
12
to peripheral
18
.
FIG. 3
illustrates a proposed SCSI standard known as the ULTRA
2
Specification being proposed by the SPI-
2
working group. As in
FIG. 2
,
FIG. 3
shows a value
70
being transmitted by a driver
72
from computer
12
to a receiver
78
in peripheral
18
. Because SCSI bus
20
uses a voltage differential technique of transferring information, value
70
is transmitted using a signal line
82
from driver
72
and also using its complement, signal/
84
. In other words, signal lines
82
and
84
are used to transmit information for bus line data[
15
]
54
. In a similar fashion, information for other bus lines is transmitted using two signal lines.
The SCSI bus also uses a bias voltage in the termination at each end of the SCSI bus. The termination bias voltage is used during the arbitration phase of SCSI protocol in order to help determine which devices are asserting which bits on the 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. The need for a termination bias voltage on a SCSI bus and its ramifications are discussed in greater detail below. To achieve the termination bias voltage, computer
12
includes a voltage source V(A)
86
(for example, 1.5 volts) and a voltage source V(B)
88
(for example, 1.0 volt) which are connected in series using resistors
90
(for example, 270 ohms), resistor
92
(for example, 138 ohms), and resistor
94
(for example, 270 ohms). This termination bias voltage circuit is connected to signal lines
82
and
84
as shown. Thus, point
91
is typically at 1.3 volts due to the termination bias voltage, and point
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.
In a similar fashion, peripheral
18
also includes a termination bias voltage. As in computer
12
, resistors
95
,
96
, and
97
connect in series voltages V(A) and V(B). These voltages and resistances may have similar values as for 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
50
.
This proposed technique for transmitting data over a SCSI bus uses a low-voltage swing differential (LVD) driver and 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 st

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