Electrical computers and digital data processing systems: input/ – Input/output data processing – Peripheral adapting
Reexamination Certificate
1998-12-04
2001-08-21
Lee, Thomas (Department: 2182)
Electrical computers and digital data processing systems: input/
Input/output data processing
Peripheral adapting
C710S010000, C710S120000
Reexamination Certificate
active
06279060
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains generally to parallel/serial bus bridges, and more particularly to Universal Serial Bus-to-peripheral generic bridges.
BACKGROUND OF THE INVENTION
The Universal Serial Bus (USB, described in its current release in the
Universal Serial Bus Specification Revision
1.1) is envisioned as an industry standard for the connection of all peripherals to a personal computer or workstation. USB seeks to replace the various specialized connectors and communication formats utilized with keyboards, mice, joysticks, multimedia inputs and outputs, printers, external disk drives, external CD-ROM drives, modems, parallel ports, serial ports, network connections, and virtually any other type of peripheral, with a single universal cable and connector type and a common communication format. Some of the other features of USB are: a true plug-and-play architecture requiring no user configuration; support for up to 127 peripherals on one bus; flexibility; and low cost.
As shown in
FIG. 1
, a USB physical interconnect has a tiered star topology
20
. A bus has a single host, and may have up to five tiers of hubs. A function provides a capability to the host, and may be a data source and/or a data sink. A hub is at the center of each star. A USB cable connects an upstream hub to a downstream USB device, i.e. another hub or a function. Hubs and functions may be self-powered, or bus-powered; a bus-powered device receives power from its upstream hub through the USB cable; a self-powered device receives power from somewhere else.
When a USB device is plugged in to a USB bus, the host detects the new device and “enumerates” it. The enumeration process includes assigning a unique USB address to the device, querying the device to determine what type of device it is, determining the device's communication requirements and whether a bandwidth allocation is available to meet these requirements, and allocating any necessary host software resources such as device drivers.
USB is a complete break from the past—it provides no backwards compatability with previous communication interfaces. But many new users of USB-equipped computers have significant investments in non-USB peripherals such as parallel cable-interfaced printers. Rather than junking these legacy peripherals, many users would prefer to somehow utilize non-USB peripherals with USB-equipped computers. Several vendors have addressed this need by offering USB bridges, which convert USB signals to a legacy format and vice-versa.
If a USB bridge is used to patch a non-USB peripheral into a USB bus, the enumeration process described above is, in effect, divided between the bridge and the peripheral. The host first enumerates the USB bridge itself. This consists of detecting and identifying the bridge device and allocating required resources to support communications between the host and the bridge device via the USB protocol. The host then queries the bridge to determine the type of peripheral attached and allocate resources appropriate to that device.
SUMMARY OF THE INVENTION
The present invention solves a problem present in prior art USB bridges that are not specifically designed for a specific peripheral device. These bridges are either self-powered or bus-powered, and thus they can be enumerated by a host even if their attached peripheral is powered off, not responding, or has even become physically detached from the bridge. In such instances, a generic bridge may be unable to complete enumeration, i.e., by describing a peripheral and its requirements. This situation may leave some hosts in a confused state; in other cases, a half-enumerated connection serves no useful purpose.
The present invention solves this problem by preventing a powered and USB-connected bridge from being recognized as such by the host unless its peripheral is ready to communicate. Instead of relying on the host to recognize that an attached function is a USB bridge to an uncooperative peripheral and respond appropriately, the bridge simply makes itself invisible to the host until such time as it can complete enumeration successfully. Besides avoiding a potentially troublesome host task, this invention creates several additional advantages: bus resources are not wasted on the bridge unless the peripheral is also present; the bridge may remain physically connected to the host regardless of the state of the peripheral, even if the bridge is powered from the host; the bridge need not have any peripheral-specific knowledge; large portions of the bridge itself may be idled when the peripheral is not ready; and the invention adds little complexity or cost to a bridge.
A USB host recognizes when a high-speed downstream device is connected because the downstream device is required to pull D+, one of a differential pair of USB data lines D+ and D−, high when the bus is idle. The present invention allows a connected and powered bridge to avoid recognition by a host by simulating a USB disconnect condition. This may be accomplished in a bridge by disconnecting the pull-up circuitry attached to D+, drastically increasing the resistance of the pull-up circuitry, or decreasing the pull-up voltage. Any of these acts will cause the upstream hub or host to create a low condition—one that looks like a disconnect—on the connected data line. A disconnect may also be simulated by the bridge actively pulling the D+ data line low and leaving it there.
A USB bridge using the present invention simulates a USB disconnect when no peripheral is attached and/or when an attached peripheral has not signaled that it is ready to communicate. In the case of a parallel port-interfaced peripheral adhering to the IEEE-1284-1994 standard, the peripheral asserts a Peripheral Logic High (PLH) signal when it is ready to communicate. The USB bridge of the present invention preferably uses this signal to control simulated disconnects—any PLH condition other than asserted results in a bridge-simulated USB disconnect, while a PLH assertion by the peripheral causes the bridge to show itself to the host as connected.
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Gilbert David C.
Luke David D.
Luttmann Eric J.
Richter Ronald J.
Du Thuan
In-System Design, Inc.
Lee Thomas
Marger & Johnson & McCollom, P.C.
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