Abstract:
A method of transmitting a signal on a bi-directional universal serial bus (“USB”) circuit for boosting a signal on a USB bus disclosed. The circuit includes a first stage inverting buffer coupled to a second stage inverting buffer to form a non-inverting buffer circuit. A high pass filter is coupled in series with the non-inverting buffer circuit to provide AC coupling to the USB bus and to allow fast signal edges through the circuit. The booster circuit is arranged to improve signal quality over a USB bus to allow additional USB devices and longer USB busses to be utilized.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is a continuation of U.S. patent application Ser. No. 12/023,659 entitled Bi-directional Universal Serial Bus Booster Circuit filed on Jan. 31, 2008 which is incorporated herein by reference in its entirety. 

   TRADEMARKS 
   IBM ® is a registered trademark of International Business Machines Corporation, Armonk, N.Y., U.S.A. Other names used herein may be registered trademarks, trademarks or product names of International Business Machines Corporation or other companies. 
   BACKGROUND OF THE INVENTION 
   This invention relates generally to a booster circuit for a universal serial bus (“USB”) and in particular to a circuit having a two stage buffer circuit coupled to the USB bus through a high pass resistor-capacitor filter. 
   Peripherals and external devices are commonly connected to a computer system using some type of bus arrangement. Traditionally, the connection was made using either a serial connection using a protocol such as RS-485 or via a parallel port using a protocol such as IEEE 1284. As the number of devices that needed to be connected increased, a new serial bus standard, known as universal serial bus (“USB”), was developed to aid in the interconnectability of computers and external devices. One aspect of the USB standard is that multiple devices could be connected to a single port on the computer using a device known as hub. 
   The USB standard provided a number of advantages over the previously used protocols. USB was designed to allow multiple external devices to be connected using a single standardized hardware interface. The standard also provided for power to be transferred to the devices allowing a number of devices to eliminate secondary power sources. Further, the devices could be connected and disconnected by the computer without rebooting or restarting the host computer. As a result of these features, the USB standard was widely adopted and the number of devices that coupled via USB grew rapidly. 
   To accommodate the increasing number of devices that connected via USB, peripherals known as USB “hubs” were developed that allowed multiple devices to be connected to a single USB port. Also, the USB standard increased the bit-rate that data could be transferred over the bus allowing devices such as external hard drives to become more practical. However, as the number of connected devices increases, the length of the bus also increases which in turn adversely impacts signal quality. Further, as the allowable bit-rate increased, the required level of signal integrity also increased. 
   Signal integrity is analyzed using an eye diagram that measures rise time, fall time, undershoot, overshoot and jitter. If a signal on the bus falls outside of the eye diagram specified by the USB standard, the signal will fail. A device that is normally USB compliant, meaning it transmits signals that comply with the standard, may start to fail if too many devices are coupled to the same bus, or if the physical length of the bus becomes too long. Thus, the number of devices and the speed of data transfer are limited by the quality ofthe signal. 
   While current USB bus topologies are sufficient for their intended purposes, it is desired to have computer systems incorporating longer USB bus lengths with more devices attached. In particular, it is desirable to have a system that is capable of boosting USB signs that have a slow edge rate to prevent rejection or signal delay. 
   SUMMARY OF THE INVENTION 
   A method of transmitting signal on a bi-directional universal serial bus booster circuit is provided. An input terminal is connected to a universal serial bus. Then a first inverting buffer is coupled in series with the input terminal. A second inverting buffer is then directly coupled in series with the first inverting buffer to form a non-inverting buffer with the first inverting buffer and the second inverting buffer. A resistor-capacitor (“RC”) passive filter is coupled in series with the second buffer. An output terminal is coupled to the RC passive filter wherein the output terminal is configured to connect with the universal serial bus. A first resistor is directly coupled in series with the input terminal and the first inverting buffer. A second resistor directly coupled in series with the second inverting buffer circuit and the RC pass filter. The first resistor, the second resistor and the RC passive filter are arranged to create positive feedback with respect to the input terminal. Further, the RC passive filter limits modification of signals on the universal serial bus to signals with high-speed edge rates without affecting slow speed signaling, full speed signaling and special bus events on the universal serial bus. 
   Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with advantages and features, refer to the description and to the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
       FIG. 1  illustrates a schematic representation of a universal serial bus circuit having a bidirectional booster circuit; 
       FIG. 2  illustrates a schematic representation of the bidirectional booster circuit of  FIG. 1 ; 
       FIG. 3  illustrates an exemplary embodiment of the bidirectional booster circuit of FIG.  2 .; and, 
       FIG. 4  illustrates signal waveforms at the input and output terminals of the bi-directional booster circuit of  FIG. 3 . 
   

   The detailed description explains the preferred embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
   DETAILED DESCRIPTION OF THE INVENTION 
   A system incorporating a universal serial bus (“USB”) compliant bus is illustrated in  FIG. 1 . The USB root hub  10  may be root hub coupled to a computer  12  such as a BladeCenter ® manufactured by International Business Machines Corporation. A root hub  10  is typically built into a host controller  14  that couples and translates the signals received over the USB bus with other circuitry in the computer  12 . Alternatively, the USB device  10  may be a USB hub that couples downstream USB Hubs  16  to an upstream bus (not shown). The USB device  10  is coupled to the USB bus  18  by a bi-directional booster circuit  20 . It should be appreciated that the USB standard allows for multiple tiers of USB devices, commonly called branches, to be coupled together and that the bidirectional booster circuit  20  described herein may be used with any or all of the tiers. 
   As it will be described in more detail herein, the bidirectional booster circuit  20  does not electrically isolate the components connected by the bus  18  and therefore keeps the signal delay to a minimum. An exemplary embodiment of the booster circuit  20  is illustrated in  FIG. 2 . In this embodiment, the booster circuit  20  is coupled in parallel to the USB bus  18 . A first buffer circuit  22  is coupled to receive signals from the USB bus  18 . A second buffer circuit  24  is electrically coupled in series to the first buffer circuit  22 . In the exemplary embodiment, each of the buffer circuits  22 ,  24  is an inverting type of buffer. The circuits  22 ,  24  are then arranged in a cascading manner to cooperate and form a non-inverting buffer. Finally, a resistor-capacitor (“RC”) passive filter  26  is coupled in series to the first and second circuits  22 ,  24  and couples to the output terminal  30  thus completing the connection of the bi-directional booster circuit  20  back to the USB bus  18 . 
   As best seen in  FIG. 4 , during operation, a signal  31  is transmitted over the USB bus  18  and received at the booster circuit input terminal  28 . The signal  31  may contain number of peaks  32  and valleys  34 . These peaks and valleys, commonly referred to as ringback, increase as the number USB Hubs  16  increases and the length of the USB bus  18  increases. The presence of ringback is indicative of signal degradation and may eventually lead to the signal being rejected by the USB devices. When the bidirectional booster circuit  20  detects a slow rising edge  36  on a signal being transmitted over the bus  18 , the first buffer circuit  22  drives the second buffer circuit  24  with the RC pass filter  26  reinforcing the direction of the signal change. Once the signal reaches a steady state  38 , the RC pass filter  26  de-couples the buffer circuits  22 ,  24  from the USB bus  18 . The resulting signal  40  at the output terminal  30  on the USB bus  18  has improved signal quality by smoothing out the peaks and valleys and a shorter time delay. By improving signal on the USB bus  18 , more devices  16  can be accommodated on a given bus. 
   The RC pass filter  26  also acts as a high pass filter that performs two functions. First, the RC pass filter  26  alternating current (“AC”) couples the bi-directional booster circuit  20  to the USB bus  18 . This prevents the direct-current (“DC”) level at the output terminal  30  from affecting the steady state voltage on the USB bus  18 . Second, the RC pass filter  26  only allows signal edges that are very fast through, this limits the signal modification to high-speed edge rates without affecting slow speed signaling, full speed signaling and special bus events. 
   For purposes of clarity, only one bi-directional buffer has been described herein as being coupled to the USB bus  18 , but the embodiments are not so limited. Communication between the external devices  16  and the USB device  10  is based on pipes (logical channels). Pipes are connections from the host controller to a logical entity on the device named an endpoint. A USB device  10  may have up to  32  active pipes,  16  into the host controller and  16  out of the controller. Each endpoint can transfer data in one direction only, either into or out of the device, so each pipe is uni-directional. In one embodiment, there is a bi-directional buffer circuit for each pipe. 
   Another embodiment of the bi-directional booster circuit  20  is illustrated in  FIG. 3 . In this embodiment, the bi-directional booster circuit  20  is coupled to the USB bus  18  by an input terminal  28  and an output terminal  30 . A first inverting buffer stage X 9  is cascaded with second inverting buffer stage X 10  to form a non-inverting buffer. The first and second inverting buffer stage X 9 , X 10  are connected in series with resistor R 60 , C 45  and R 59  to create a positive feedback arrangement with respect to the input terminal  28 . 
   A resistor R 58  is connected to ground between the resistor R 60  and the capacitor C 45 . The resistor R 58  and capacitor C 45  form a RC high pass filter  42 . As discussed above, the high pass filter AC couples the output of buffer circuits X 9 , X 10  to the USB bus  18 . This keeps the DC level of the buffer circuits X 9 , X 10  output from affecting the steady stage voltage on the USB bus  18 . 
   In an embodiment, the resistors R 59  and R 60  have a value of 100 Ω, while the resistor R 58  has a value of 1000 Ω. Further, the capacitor C 45  has a value of 120 pico-Farads. When this embodiment is coupled with a USB bus using a reference voltage of 1.85V, the resulting signal  40  produced by the bi-directional booster circuit  20  are shown in  FIG. 4 . Below 1.6V, the result indicates the natural response of the booster circuit and above 1.6V indicates improved slew rate. If the buffer circuit had not been coupled to the USB bus, there would have been no transitional boost to the USB signal and the delay time of the signal would have been longer. 
   While the preferred embodiment to the invention has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.