Abstract:
Signals sent between a first device and a second device are split. A first user adjustable input signal equalization and gain compensates for losses to signals from the first device before the compensated signals are forwarded to the second device, a first analytic instrument and a second analytic instrument. A second user adjustable input signal equalization and gain compensates for losses to signals from the second device before the first compensated signals are forwarded to the first device, the first analytic instrument and the second analytic instrument.

Description:
BACKGROUND 
     Protocol Analyzers are used to capture, examine, and debug the complex protocols used by storage and networking busses. Oscilloscopes are used to view waveforms and verify signal integrity. Modern point-to-point busses, such as those operating in accordance with the Serial Attached SCSI (SAS) protocol, transfer data at a rate of gigabytes per second on each of multiple differential transmission lines, and employ complex protocols for equalizing or training the transmitters in order to compensate for the frequency dependent characteristics of the cable or transmission line. 
     Tapping into or splitting a transmission line degrades transmitted signals because splitting redirects a portion of the signal resulting in power loss, and tapping changes the impedance of the transmission line, creating signal reflections. As an alternative to tapping into a transmission line, protocol analyzers often use a technique of terminating the received signal and then retransmitting it again as a clean new point-to-point signal with full strength. Such a technique will usually change the signal waveform somewhat, so performing this technique multiple times on the same signal in order to attach multiple instruments is undesirable. 
     One aspect of modern GHz busses which adds to the difficulty of troubleshooting while maintaining high signal integrity is the large number of signals involved. For example, SAS hosts and expanders are typically four lanes wide, with each lane consisting of two differential pairs; one pair in each direction. Thus a typical SAS bus involves sixteen signals all operating at GHz speeds. The large number of signals makes it more difficult to tap or split without introducing stubs or discontinuities into the transmission line. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a simplified and idealized block diagram showing a Multi-Lane three-way splitter being used to make copies of original signals for two analytic instruments in accordance with an embodiment. 
         FIG. 2  is a block diagram of a four-lane, bi-directional, three-way differential splitter using typical SAS connectors and including gain and input signal equalization (ISE) adjustment functions in accordance with an embodiment. 
         FIG. 3  is a block diagram that provides information about controls for user adjustable gain and ISE in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     For a bus that operates point-to-point at gigahertz (GHz) speeds and employs transmitter training protocols, it is desirable to connect two analytic instruments, such as a protocol analyzer and an oscilloscope, in order to develop and debug the transmitter equalization protocol. For example, it is desirable to connect both a protocol analyzer and an oscilloscope to a GHz bus without changing the signal so that the equalization protocol sequence does not change significantly. Significant changes in the equalization protocol sequence makes it difficult to reproduce and debug a protocol problem with transmitter training. 
     In order to support simultaneous protocol analysis and waveform analysis using separate instruments, a device is needed which can create multiple true copies of the original signals, and with minimal change to the original signals. A desirable feature of the device is the ability to simplify the interconnections between the system under test and the protocol analyzer and oscilloscope so the user doesn&#39;t need to spend significant time organizing and labeling cables and connectors. For SAS, such a system under test necessarily involves sixteen signals to and from the Initiator, sixteen signals to and from a target, sixteen signals to the protocol analyzer, and sixteen signals to the oscilloscope. Thus, the ideal device is a four-lane, three-way differential splitter, with two bi-directional connections for the devices under test, and unidirectional connections to the protocol analyzer and oscilloscope. 
     Another desirable feature of a device used for splitting a GHz signal into multiple true copies, is a way to undo the frequency dependent losses incurred by the cabling and printed circuit board (PCB) traces required to implement and connect the device. By implementing adjustable input signal equalization (ISE) and gain in the device, a user can undo the frequency dependent losses and overall loss. 
       FIG. 1  shows a device  100  connected to a device  110  through a multi-lane three-way splitter  140 . For example device  100  is an initiator and device  110  is a target connected to each other via a bus that operates in accordance with the SAS protocol. Alternatively, for example, device  100  is a host and device  110  is a card connected to each other via a bus that operates in accordance with the Peripheral Component Interconnect Express (PCIe) protocol. Alternatively, for example, device  100  and device  110  connected to each other in accordance with some other bus protocol. 
     Splitter  140  duplicates the original signals and provides the copies to a protocol analyzer  120  and an oscilloscope  130 . A cable from device  100  is bidirectional, and includes transmit lines  101  and receive lines  102  each consisting of four sets or lanes of differential pair signaling. Likewise, a cable from device  110  is bidirectional, and includes a transmit direction  112  and a receive direction  111  each consisting of four sets or lanes of differential pair signaling. The implementation of a transmit signal splitter element  141  and a transmit signal splitter element  142  is shown in more detail in  FIG. 2 . Splitter  140  forwards copies of all device  100  and device  110  transmitted signals to protocol analyzer  120  using two separate cables: a cable  121  providing device  100  transmit signals, and a cable  122  providing device  110  transmit signals. Splitter  140  also forwards copies of all device  100  and device  110  transmitted signals to oscilloscope  130  using two separate cables: a cable  131  providing device  100  transmit signals, and a cable  132  providing device  110  transmit signals. While in  FIG. 1  the copies of signals are forwarded to oscilloscope  130  and protocol analyzer  120 , a person of ordinary skill in the art would recognize that these copies of signals could be sent to many other types of analytic instrument, such as a network analyzer, or any analytic instrument equipped to receive and analyze the copies of signals. 
       FIG. 2  shows a more detailed block diagram of four-lane bi-directional three-way differential splitter  140 , transmit signal splitter element  141  and transmit signal splitter element  142 . To support four-lanes of bi-directional differential pairs, an HD miniSAS Connector  201  is used to transmit to, and receive data from device  100  through receive lines  102  and transmit lines  101 . HD miniSAS connector  211  performs the same function for device  110 . Once the four lanes of differential signals transmitted by device  100  have entered splitter  140 , the positive (+) side of each differential pair goes into gain &amp; ISE block  242 , and the negative (−) side of each differential pair goes into gain &amp; ISE block  243 . ISE &amp; gain blocks  244  and  245  provide the same function for device  110  transmit signals as  242  and  243  provide for device  100  transmit signals. 
     The gain and ISE blocks provide the user with a way to select gain settings of half, unity, or double, and they provide equalization settings which can perform frequency dependent boost in two different frequency ranges, including a long bit-time boost used to boost non-return-to-zero (NRZ) signals that do not change for many successive bit times, and a short bit-time boost used to boost the rise/fall transition speed.  FIG. 3  provides more detail on the functions of the ISE &amp; gain blocks. 
     Each of the gain &amp; ISE blocks  242 ,  243 ,  244 , and  245  drives 3 output drivers. One driver forwards the signal on to the original destination of device  100  or device  110  through HD miniSAS connector  201  or  211 , another driver forwards the signal to the protocol analyzer  120  through HD miniSAS connector  221  or  222 , and the third driver forwards the signal to the oscilloscope  130  through SubMiniature version A (SMA) connectors  231  or  232 . 
     Because the four-lane Bi-Directional three-way Differential Splitter never interprets a differential pair as asserted or de-asserted, but treats each signal as a separate linear signal, it is able to correctly forward the common-mode, or out-of-band signaling used by modern GHz busses, such as PCIe, SAS, SATA, and USB 3.0 busses. These busses use the same transmission lines at low speed to communicate low-frequency information, such as RESET or WAKE-UP, by driving both the positive and negative side of a differential pair to the same voltage level, thus creating a common-mode voltage. To support common-mode signaling protocols, splitter  140  must not compare the positive and negative inputs and generate its output based on whether the positive or negative is higher, as a traditional differential receiver does. 
     Splitters  141  and  142  are each implemented with a Quad 1:2, 2:1 multiplexor (Mux), such as a TI SN65LVCP114 multiplexor available from Texas Instruments. A third output from the TI SN65LVCP114 multiplexor is realized by configuring a C input for loopback diagnostics mode, and by enabling loopback on the C port. The C port then becomes the third output copy of the input signal. 
       FIG. 3  provides additional details about the gain &amp; ISE controls. Gain &amp; ISE block  242  has two input control signals labeled VOD and gain in  FIG. 3 . The VOD control signal, when high, doubles the differential output voltages. The gain control signal, when high, selects unity gain, and when low selects half gain. Between the VOD and gain control signals, total gain selections of half, unity, and double can be made. A three-position switch  301  controls the VOD and gain control signals such that when switch  301  is in the down position, VOD and gain control signals are both low selecting half gain. When switch  301  is in the middle or no-contact position, the gain control signal is low and the VOD control signal is high, thus selecting unity gain. When switch  301  is in the up position, resistor  303  with a much lower resistance overrides resistor  302  with a much higher resistance causing both the VOD and the gain control signals to be high, thus selecting a gain of double. 
     Gain &amp; ISE block  242  has two tri-state inputs which select the equalizer (EQ) settings. Three-position switch  305  selects between the three ISE Long settings, and three-position switch  306  selects between the three ISE Short settings. 
     Gain &amp; ISE blocks  243 ,  244 , and  245  in  FIG. 2  function the same as gain &amp; ISE block  242  as described in additional detail above and as shown in additional detail in  FIG. 3 . 
     At first it may seem counter-productive to add ISE to the signals considering that the purpose of the three-way splitter is to ensure that the original signals are as unchanged as possible, but adding ISE counteracts the frequency dependent changes caused to the signals by the addition of the cables and PCB traces associated with splitter  140 . By adding ISE, the signal waveforms are brought back to their original wave shape, as if the cables and splitter were not in the path. 
     The foregoing discussion discloses and describes merely exemplary methods and implementations. As will be understood by those familiar with the art, the disclosed subject matter may be embodied in other specific forms without departing from the spirit or characteristics thereof. Accordingly, the present disclosure is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.