# Patent

## Title
Physical layer loopback

## Abstract

In some embodiments, a chip comprises control circuitry to provide inband signals, inband output ports, and transmitters to transmit the inband signals to the inband output ports. The control circuitry selectively includes loopback initiating commands in the inband signals. Other embodiments are described and claimed.

## Background
The present patent application is a Continuation of application Ser. No. 10/394,363, filed Mar. 21, 2003 now U.S. Pat. No. 7,324,458.


BACKGROUND
1. Technical Field
The present inventions relate to loopback of signals from a first chip to a second chip and back to the first chip.
2. Background Art
Loopback refers to a process wherein a first chip transmits signals to a second chip and the second chip sends the signals back to the first chip. Loopback is used in analog and digital validation, test, and debug. “Inband” interconnects (conductors) carry inband signals that include data. “Side band” interconnects (also called “out of band” interconnects carry commands, but not the data. The inband signals on the inband interconnects may be packetized to include traditional data with other information such as commands, address (destination), and headers. The signals to be looped back are typically sent from the first chip to the second chip on inband interconnects. When the second chip is in a loop back mode, it transmits all or almost all of the signals back through inband interconnects to the first chip. The inband interconnects carrying the signals back to the first chip may be the same as or different than the inband interconnects that carrying the signals to the second chip.
In prior art techniques, the loopback mode is initiated through loopback initiating signals provided to the second chip through sideband interconnects. Further, the loopback initiating signals are provided to the second chip in response to software, such as an operating system, rather than in response to physical circuitry. Using an operating system to initiate loopback may mask a failure because of error correction. Product validation can be significantly impaired because of a lack of ability to isolate exact failing conditions.
Various techniques have been developed for chips to communicate with each other. The techniques include those that have been standardized and those that have not been standardized. An example of standardized techniques include versions of the Peripheral Chip Interconnect (PCI) Local Bus Specification, such as a PCI Local Bus Specification rev. 2.2, dated Dec. 18, 1998, by the PCI Special Interest Group (SIG). A PCI Express specification, formerly known as 3GIO (3rd generation input output), is in the process of being defined by the PCI SIG. A PCI Express Base Specification Revision 1.0, Jul. 22, 2002, has been released and is available with the payment of a fee.
Signals are often applied differentially on two interconnects. The term “common mode” refers to the average voltage on the interconnects. Systems have used capacitors in series with interconnects to pass the difference between signals on the two interconnects, but not the common mode under ordinary operation. The 8b/10b code is a commonly used code in serial links.



BRIEF DESCRIPTION OF THE DRAWINGS
The inventions will be understood more fully from the detailed description given below and from the accompanying drawings of embodiments of the inventions which, however, should not be taken to limit the inventions to the specific embodiments described, but are for explanation and understanding only.
FIG. 1 is a schematic block diagram representation of a system according to some embodiments of the inventions.
FIG. 2 is a schematic block diagram representation of some details of chip A of FIG. 1 according to some embodiments of the inventions.
FIG. 3 is a schematic block diagram representation of some details of chip A of FIG. 1 according to some embodiments of the inventions.
FIG. 4 is a schematic block diagram representation of a system according to some embodiments of the inventions.
FIG. 5 is a schematic block diagram representation of a system according to some embodiments of the inventions.
FIG. 6 is a schematic block diagram representation of a system according to some embodiments of the inventions.
FIG. 7 is a schematic block diagram representation of a system according to some embodiments of the inventions.
FIG. 8 is a schematic block diagram representation of a system according to some embodiments of the inventions.
FIG. 9 is a schematic block diagram representation of a system according to some embodiments of the inventions.
FIG. 10 is a schematic block diagram representation of a system according to some embodiments of the inventions.
FIG. 11 is a schematic block diagram representation of a system according to some embodiments of the inventions.



DETAILED DESCRIPTION
1. Overview of Examplary Systems
FIG. 1 illustrates a system 10 having a first chip (chip A) and a second chip (chip B) coupled through a link 14. In the example of FIG. 1, chips A and B are sometimes in ordinary operating modes in which chip A sends inband signals to chip B over inband interconnects 16 without the expectation that the signals be looped back by chip B and, in fact, chip B does not loopback the signals. However, when chip A is in a loopback initiating mode, it provides a loopback initiating command to chip B on at least one of inband interconnects 16 (which include inband interconnects 30-1, 32-1 . . . 30-N, 32-N). As an example, the loopback initiating command may be bit 2=1 (assert) in a PCI Express training sequence, but that is not required.
In response to receiving the loopback initiating command, control circuitry 34 of chip B goes into a loopback mode and loops back some or all of the inband signals it receives through inband interconnects 18 (which include inband interconnects 36-1, 38-1 . . . 36-M, 38-M, where M may or may not equal N). The loopback mode of chip B continues until chip B detects an end of loopback indication, discussed below. In the case of differential signaling as in FIG. 1, the signals are returned from chip B on at least two of the inband interconnects 18. In the case of single ended (non-differential) signaling, discussed below, the signals are return on at least one of the inband interconnects back to chip A.
When it is said that a signal is looped back, it is understood that voltage and timing characteristics of a signal from chip A that is being looped back may be to some extent modified by circuitry in chip B (e.g., by one or more of transmitters TXB1 . . . TXBM and perhaps other circuitry). Accordingly, the voltage and timing of the signals returning from chip B are not necessarily identical to the signals from chip A to chip B. Chip B may try to maintain the voltage and timing of the signals being loopback back, or it may try to “correct” the voltage and timing.
Although FIG. 1 is described in terms of chip B being in loopback mode in response to a loopback initiating command from chip A, in some embodiments chip B could send a loopback initiating command to chip A so that chip A is in loopback mode and loops back signals to chip B.
In operation, control circuitry 24 provides signals VoutA1+, VoutA1− . . . VoutAN+, VoutAN− to transmitters TXA1 . . . TXAN which transmit these as differential inband signals D+ and D− on inband interconnects 30-1, 32-1 . . . 30-N, 32-N. Receivers RXB1 . . . RXBN provide the received signals to control circuitry 34. As shown in FIG. 1, receives RXB1 . . . RXBN provide the received signals as differential signals VinB1+, VinB1− . . . VinBN+, VinBN− to control circuitry 34, although receivers RXB1 . . . RXBN could convert them to signal ended signals. Likewise, control circuitry 34 provides differential signals VoutB1+, VoutB1− . . . VoutBM+, VoutBM− to transmitters TXB1 . . . TXBM which transmit these as differential inband signals D+ and D− on interconnects 36-1, 38-1 . . . 36-M, 38-M. Receivers RXA1 . . . RXAM provide the received signals (or inverses of them) to control circuitry as differential signals VinA1+, VinA1− . . . VinAM+, VinAM−for control circuitry 34 or could convert them to signal ended signals. In some embodiments, the transmitters and receivers do not invert the signals they receive, but in other embodiments, they could invert them.
Chip A include ports 20-1, 22-1 . . . 20-N, 22-N. Examples of ports include die pads. Likewise, chip A includes ports 52-1, 54-1 . . . 52-M, 54-M and chip B includes ports 26-1, 28-1 . . . 26-N, 28-N and ports 48-1, 50-1 . . . 48-M, 50-M. These ports are referred to as inband ports because they are ports to interconnects that carry inband signals. In the example of FIG. 1, ports 20-1, 22-1 . . . 20-N, 22-N and 48-1, 50-1 . . . 48-M, 50-M are inband output ports and ports 26 28-1 . . . 26-N, 28-N and ports 52-1, 54-1 . . . 52-M, 54-M are inband input ports, but ports could be bi-directional. Chips A and B may be chips on a card(s), motherboard(s), or other substrate(s). The interconnects illustrated represent one or more of the following: slots, fingers, traces, and other conductive materials that may be present between the chips. There may be more than one chip on a card, motherboard, or other substrate. Multiple chips may be coupled to one or more than one chip.
FIGS. 2 and 3 illustrate details of some embodiments of chip A of FIG. 1, but other details may be used. The details of FIGS. 2 or 3 may be used in chip B of FIG. 1 and chips in FIGS. 4-11, but they are not required. Control circuitry 24 includes a mode controlling register 90 that controls the modes of control circuitry 24. In FIG. 2, at least part of the contents of register 90 can be controlled directly through an interconnect(s) 86 and physical layer port(s) 88. As used herein, a physical layer port is a port through which register 90 can be accessed without the aid of a layer higher than the physical layer, such as BIOS, an operating system, or software. Of course, there may intermediate receivers or other simple circuitry between port(s) 88 and register 90. The signals provided in interconnect(s) 88 may have one or more bits. As an example, port(s) 88 may be a JTAG port(s) or another similar port(s), but that is not required.
An advantage of controlling register 90 directly at the physical level is that the chip can be tested without higher levels such as operating systems, software programs or BIOS. This allows additional validation and compliance opportunities. For example, high volume testing can be applied to a PCI Express link without a fully functional personal computer (PC) platform. Progress can be made in system validation through debugging systems even if BIOS and/or other critical platform issues do not allow a successful operating system initialization. Loopback may be more controlled and repeatable when performed at the physical layer.
In FIG. 3, register 90 is not controlled directly through an interconnect. If it is controlled from a signal external to chip A, it is done so indirectly. Register 90 in FIG. 3 may be controlled through signals on inband interconnects and/or through other events. Note that it is not necessary to have a mode controlling register in control circuitry 24. The mode can be controlled through a state machine in which the result is not held in a register.
When a particular bit or bits in register 90 has a particular state, control circuitry 24 goes into loopback initiating mode either immediately or after another event(s) (e.g., configuration or self tests) and/or signals are received. While it is in loopback initiating mode, control circuitry 24 may control timing and voltage margins of one or more of signals VoutA1+, VoutA1− . . . VoutAN+, VoutAN− in control circuitry 24 or through transmitters TXA1 . . . TXAN through conductors 94 and 96.
Loopback mode may be used for testing, fault isolation, and validation purposes. Comparison circuitry 92 compares what was intended to be sent with what is looped back and received by receivers RXA1 . . . RXAM. The voltage and timing margins of signals D+ and D− may be altered until a miscompare is generated, although that practice is not required. In some embodiments, chip A may send a continuous loopback training sequence to chip B. Chip B may recognize the training sequence, retime the incoming inband signals, and resent them to chip A. Receiver scrambling and 8b/10b decoding/encoding are not required. Loopback initiating mode may end when a sequence is done, when certain miscompares are detected, or in response to some other event(s) or signal(s).
There are various ways in which chip A can enter loopback initiating mode. In FIG. 4, a testing controller 42 causes chip A to enter into loopback initiating mode through a signal provided through interconnect(s) 40. Interconnect(s) 40 may be an example of interconnect(s) 86 of FIG. 2 or they may be provide signals to other parts of the chip. After receiving the signals on interconnect(s) 40, chip A may go into loopback initiating mode immediately or only after some other event(s) occur and/or until chip A receives additional signals. Once it is in loopback initiating mode, chip A sends a loopback initiating command to chip B through at least one of inband interconnects 16.
In FIG. 5, chip B may cause chip A to be put into a loopback initiating mode through a signal on interconnect(s) 44. Again, interconnect(s) 44 may be an example of interconnect(s) 86 of FIG. 2 or they may be provided to other parts of chip A. As with FIG. 4, chip A may go into loopback initiating mode immediately or only after additional events occur and/or signals are received. Once it is in loopback initiating mode, chip A sends a loopback initiating command to chip B through at least one of inband interconnects 16. Chip A can also cause chip B to go into loopback initiating mode with a signal through interconnect(s) 46. In that case, chip B would send a loopback initiating command to chip A through at least one of inband interconnects 18.
In FIG. 6, in some embodiments, chip A may enter loopback initiating mode following power up or some other event independent of receiving a signal from another chip, or through a signal from another chip. In other embodiments, chip A may enter loopback initiating mode following receiving a signal from another chip that is not designed for that purpose but which nevertheless causes chip A to go into loopback initiating mode in combination whether another condition(s) and/or signal(s). In other embodiments, Chip A may go into loopback initiating mode in response to a signal over at least one of inband interconnects 18.
Chips including the loopback features described herein may be in computer systems. FIG. 7 is an example of such a computer system, although the inventions are not limited to use in such systems. An memory controller hub (MCH) 78 is coupled to a processor 84, memory 82, and an input/ouput hub (ICH) 70. ICH 78 is coupled to a hard drive 72, BIOS 74, and a chip 56. Chip 56 is also coupled to chip 58. Inband interconnects 16 and 18 are coupled between ICH 70 and chip 56. Inband interconnects 64 and 64 are coupled between chips 56 and 58. Interconnects 62 and 68 may include interconnects that carry signals like those of interconnects 40, 44, or 46 in FIGS. 4 and 5. In the case of FIG. 7, an operating system or program in hard drive 72 or memory 82, or firmware in BIOS 74 may cause a chip to be in loopback initiating mode either immediately or following some other event(s) and/or reception of other signal(s). It is also possible that testing controller 42 have an operating system, program, or BIOS but that is not required.
The transmitters, interconnects, and receivers act as serial links. In some embodiments, the link includes lanes, where a lane is formed of interconnects between two opposite pairs of transmitters and receivers. For example, a lane might include interconnects 30-1, 32-1, 36-1, and 38-1, where interconnects 30-1 and 32-1 carry signals from chip A to chip B and interconnects 36-1 and 38-1 carry signals from chip B to chip A. However, the use of such lanes is not required. Control circuitry 24 and 34 may be such that a loopback initiating command in any of the lanes causes all the lanes to loopback or some specified lanes in addition to the one containing the loopback initiating command. In some embodiments, loopback is initiated only for the lanes including the loopback initiating commands. In other embodiments, a command in one lane can initiate loopback for some or all lanes in the link. There may be different types of loopback initiating commands.
In FIG. 1, decoupling capacitors C1-1, C2-1 . . . C1-N, C2-N, and C3-1, C4-1 . . . C3-M and C4-M are in series in interconnects 30-1, 32-1 . . . 30-N, 32-N, and 36-1, 38-1 . . . 36-M, 38-M, respectively. The decoupling capacitors block direct current (DC) voltage but pass higher frequency signals. Decoupling capacitors C1-1, C2-1, C1-N, and C2-N are illustrated as being on the interconnect externally from the chip, whilst decoupling capacitors C3-1, C4-1, C3-M, and C4-M are illustrated as being in the chip. However, each of the decoupling capacitors could be external. Depending on the capacitance of the decoupling capacitors, it may be impractical to have a decoupling capacitor on die. For example, the decoupling capacitors may be about 75 nanofarads, although they may be more or less than that. The decoupling capacitors may be in the chip package, on a card supporting the chip, on the motherboard or elsewhere. The label 30-1 is intended to be the label for the interconnect on each side of decoupling capacitor CAC1-1. Decoupling capacitors may be useful where chips A and B have different voltages. Some embodiments do not include decoupling capacitors.
The inband signals are not restricted to any particular type of signaling. The inband signals may be, for example, be packetized or time multiplexed. In some embodiments, the signaling may be of the type in which the clock is recovered from the inband signals D+ and D− and used to provide a sampling clock in phase with data of the inband signals.
FIG. 8 illustrates that serial decoupling capacitors are not required in the interconnects. FIG. 8 further illustrates the receivers may convert from differential to single ended signals. The transmitters could convert a single ended signal to a differential signal.
FIG. 9 illustrates that single ended signals may be used for the interconnects rather than differential signals.
FIG. 10 illustrate bi-directional interconnects. The bi-directional signal may be sequential or simultaneous. In the case sequential bi-directional signals, signals to be looped back and signals being loopbacked could be interspersed or completely sequential. In the case of simultaneous bi-directional signals, the interconnects could simultaneously carry signals to be loopedback and other signals being loopbacked back.
FIG. 11 illustrates that there may be an intermediate chip 98 between chips A and B. Interconnects 102-1, 104-. . . 102-N, 104-N and 106-1, 108-1 . . . 106-M, 108-M carry differential signals, but they could be signal ended and/or bi-directional as discussed. The various features described in connection with the different figures may be mixed and matched in different embodiments.
There are different ways in which chip A can indicate the end of loopback. One way is to provide an end of loopback character through signals D+ and D−. A drawback of this approach is that using such a character may limit voltage or timing stressing in loopback. Under another approach, chip A indicates the end of loopback by causing signal D+ to equal signal D− (sometimes called squelch and electrical idle) for at least a particular amount of time. During the time D+=D− in electrical idle, the inputs to the corresponding transmitters of chip A may be in high impedance. Under yet another approach, chip A indicates the end of loopback by not changing D+or D− for a certain amount of time. In the case of single ended interconnects (not differential interconnects), loopback mode may be ended in response to the interconnect being held to a certain voltage or within a certain voltage range for a particular amount of time.
2. Additional Details of Some Implementations
Some embodiments of the invention are implemented so as to be compliant with a PCI Express specification, but that is not required. The invention may be implemented in a way that is not PCI Express compliant. Further, the relevant aspects of the PCI Express standard may change over time. Neither PCI Express nor the inventions are not restricted to these details.
Only the entry and exit behavior is specified, all other details are implementation specific. Loopback can operate on a lane or a link basis. When a Link is in loopback, the symbols received are “looped back” to the transmitter on the same Lane. The entry mechanism for chip A is device specific. The system designer is responsible to ensure that two connected devices do not initiate chip A requests at the same time. Chip B enters loopback whenever two or more consecutive training sequences TS1 or TS2 ordered-sets are received with the loopback bit set. Loopback is exited by the sending of an electrical idle ordered-set followed by electrical idle.
Once in the loopback state, chip A can send any pattern of symbols as long as the rules of 8b/10b encoding are followed. The mechanism used by chip A to enter the loopback state is implementation specific.
Chip A enters loopback when directed. In a Loopback. Entry state, linkup=0 (status is set false). Chip A transmits TS1 ordered sets with the loopback bit (bit 2) asserted to chip B until chip A receives TS1 ordered sets with the loopback bit asserted. This indicates chip B has successfully entered loopback. The next state of Loopback.Active. In the initiating device (chip A), timeout from loopback entry is implementation specific. The exit is to the Loopback.Exit state.
Chip B enters loopback when two or more consecutive TS1 ordered-sets are received with the loopback bit set. Under one proposal, loopback must be entered within the receipt of 1024 consecutive TS1 ordered sets with the loopback bit set. The next state is Loopback.Active.
In the Loopback.Active state, chip A is sending valid 8b/10b data. The next state of chip A is the Loopback.Exit state if directed.
In the Loopback.Active state, chip B is required to retransmit each 10b data and control symbol exactly as received, without applying scrambling/descrambling or disparity corrections, with three exceptions:
1. If a received 10b symbol is determined to be an invalid 10b code (i.e., no legal translation to a control or data value possible) then chip B must instead transmit the EDB (bad character) symbol in the corresponding time slot of the invalid symbol. Either a positive or negative disparity can be chosen for the EDB symbol.
2. If a SKP (skip) ordered set retransmission requires adding a SKP symbol to accommodate timing tolerance correction, the SKP symbol is inserted in the retransmitted symbol stream anywhere in the SKP ordered set following the COM symbol. Either a positive or negative disparity can be chosen for the inserted SKP symbol.
3. If a SKP ordered set retransmission requires dropping a SKP symbol to accommodate timing tolerance correction, the SKP symbol is simply not retransmitted and transmission continues with the next received symbol or an EDB, as defined above.
As a result of these rules, received valid 10b codes are retransmitted even if they fail to match expected disparity in the receiver and result in retransmission violating normal disparity rules.
The next state of chip B is the Loopback.Exit state when an electrical idle ordered-set is received, or if an electrical idle condition is detected. Note, chip B must be able to detect electrical idle was entered within 2 milliseconds (ms) in case the electrical idle ordered set is not properly detected.
In the Loopback.Exit state, chip A sends an electrical idle ordered set and goes to a high impedance electrical idle for a minimum of 2 ms. Chip B echoes the electrical idle ordered set and goes to a high impedance electrical idle for a minimum of 2 ms.
Training sequences (TS) are composed of ordered-sets used for bit alignment, symbol alignment and to exchange physical layer parameters. Training sequence ordered sets are not scrambled but are 8b/10b encoded. TS1 and TS2 are shown in table 1 below. SKP ordered-sets may be transmitted during training sequences but not interrupt a TS1 or TS2 ordered-set.







TABLE 1







TS1 and TS2 Ordered-Sets










Symbol
Allowed
Encoded



Number
Values
Values
Description





0

K28.5
COMMA code group for symbol





alignment


1
0-255
D0.0-D31.7,
Link number within component




K23.7



2
0-31
D0.0-D31.0,
Lane number within port




K23.7



3
0-255
D0.0-D31.7
N_FTS. This is the number of fast





training ordered-sets required by





the receiver to obtain reliable





bit and symbol lock.


4
2
D2.0
Data Rate Identifier





Bit 0 - Reserved, set to 0





Bit 1 = 1, generation 1





(2.5 Gb/s) data rate supported





Bit 2:7 - Reserved, set to 0


5
Bit 0 = 0, 1
D0.0, D1.0,
Training Control



Bit 1 = 0, 1
D2.0, D4.0
Bit 0 = 0, de-asserted reset



Bit 2 = 0, 1
D8.0
Bit 0 = 1, asserted reset



Bit 3 = 0, 1

(asserted)



Bit 4:7 = 0

Bit 1 = 0, enable link





Bit 1 = 1, disable link





(asserted)





Bit 2 = 0, no loopback





Bit 2 = 1, enable loopback





(asserted)





Bit 3 = 0, enable scrambling





Bit 3 = 1, disable scrambling





(asserted)





Bit 4:7, reserved (set to 0





unasserted)


6-15 for

D10.2
TS1 identifier


TS1





6-15 for

D5.2
TS2 identifier


TS2









Additional Information
An embodiment is an implementation or example of the inventions. Reference in the specification to “an embodiment,” “one embodiment,” “some embodiments,” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments, of the inventions. The various appearances “an embodiment,” “one embodiment,” or “some embodiments” are not necessarily all referring to the same embodiments.
If the specification states a component, feature, structure, or characteristic “may”, “might”, or “could” be included, that particular component, feature, structure, or characteristic is not required to be included. If the specification or claim refers to “a” or “an” element, that does not mean there is only one of the element. If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element.
The inventions are not restricted to the particular details listed herein. Indeed, those skilled in the art having the benefit of this disclosure will appreciate that many other variations from the foregoing description and drawings may be made within the scope of the present inventions. Accordingly, it is the following claims including any amendments thereto that define the scope of the inventions.

## Claims

1. A chip comprising:
control circuitry to provide inband signals;
an inband output port; and
a transmitter to transmit the inband signals to the inband output port, wherein the control circuitry selectively includes a loopback initiating command in the inband signals and selectively causes the transmitter to have a differential pair of signals equal each other to indicate an end of a loopback mode for a remote receiver.

2. The chip of claim 1, wherein the control circuitry includes a register to store a signal to control whether the chip is in a loopback initiating mode.

3. The chip of claim 2, further comprising at least one physical layer port to receive the signal stored in the register, wherein the signal is stored in the register through the at least one physical layer port without the aid of a layer higher than the physical layer.

4. The chip of claim 2, wherein if the signal in the register has a first value, the control circuitry places itself in the loopback initiating mode.

5. The chip of claim 2, wherein if the signal in the register has a first value, the control circuitry places itself in the loopback initiating mode but the control circuitry does not include a loopback initiating command in the inband signals until an additional event has occurred.

6. The chip of claim 2, wherein the signal is stored in the register through means other than a physical layer port with the assistance of a layer higher than the physical layer.

7. The chip of claim 1, wherein the transmitter vary voltage and timing margins of the inband signals under the control of the control circuitry.

8. The chip of claim 1, further comprising:
a receiver to receive signals; and
comparison circuitry to compare a state of some of the inband signals with some of the received signals.

9. The chip of claim 1, wherein the loopback initiating command include a bit asserted in a training sequence.

10. The chip of claim 1, wherein the control circuitry selectively provides an end of loopback mode command to indicate an end of loopback mode.

11. The chip of claim 1, wherein the inband signals provided from the control circuitry are differential signals and are transmitted differentially by the transmitter.

12. The chip of claim 1, wherein the inband output port is one or more die pads of the chip.

13. A chip comprising:
a receiver to receive inband signals at an inband input port;
an inband output port;
a transmitter; and
control circuitry to receive the received inband signals from the receiver, wherein the control circuitry places itself in a loopback mode if it detects a loopback initiating command in the inband signals, wherein in the loopback mode, the control circuit loops back at least some of the received inband signals to the transmitter to be transmitted to the inband output port, and wherein the control circuitry ends the loopback mode if it detects that a differential pair of signals at the inband input port equal each other for at least a particular amount time.

14. The chip of claim 13, wherein the control circuitry loops back all signals received if any differential pair of inband input ports provides the loopback initiating command.

15. The chip of claim 13, wherein the control circuitry only loops back signals received through a differential pair of an inband input port that also included a loopback initiating command.

16. The chip of claim 13, wherein one or more SKP symbols are at times added or removed by the control circuitry.

17. The chip of claim 13, wherein if received inband signals are not 8b/10b valid, the transmitter sends back an EDB control character in place of the invalid character.

18. The chip of claim 13, wherein when the control circuitry detects an electrical idle ordered set received by the receiver, the control circuitry causes the transmitter to echo the electrical idle ordered set and go into a high impedance electrical idle.

19. A system comprising:
a first chip and a second chip coupled through inband interconnects;
wherein the first chip includes first control circuitry to selectively provide a loopback initiating command in the inband signals provided to at least some of the inband interconnects, and a receiver coupled to at least some of the inband interconnects; and
wherein the second chip includes:
a receiver coupled to at least some of the interconnects to receive the inband signals from the first chip;
a transmitter coupled to the interconnect; and
second control circuitry to receive the inband signals from the receiver, wherein the second control circuitry places itself in a loopback mode if it detects a loopback initiating command in the inband signals, and wherein in the loopback mode, the second control circuit loops back at least some of the received inband signals to the transmitter to be transmitted through the at least some interconnects to the receiver of the first chip, wherein the second control circuitry ends the loopback mode if it detects that a differential pair of signals at the inband input port equal each other for at least a particular amount time.

20. The system of claim 19, further including an intermediate chip between the first and second chips.

21. The system of claim 19, wherein the control circuitry of the first chip includes a register to store a signal to control whether the chip is in a loopback initiating mode in which case the control circuitry includes a loopback initiating command in the inband signals and wherein the first chip further comprises at least one physical layer port to receive the signal stored in the register, wherein the signal can be stored in the register through the at least one physical layer port without the aid of a layer higher than the physical layer.

22. The system of claim 21, further comprising a testing controller chip coupled to the register through the at least one physical layer port.

23. The system of claim 19, wherein the loopback initiating mode is initiated from BIOS.

24. The system of claim 19, wherein the loopback initiating mode is initiated from an operating system.

25. The system of claim 19, wherein the loopback initiating mode is initiated from a software program.

26. The system of claim 19, wherein the at least some of the interconnects coupled to the transmitter of the first chip are different than the at least some of the interconnects coupled to the transmitter of the second chip.

27. The system of claim 19, wherein the at least some of the interconnects coupled to the transmitter of the first chip are the same as the at least some of the interconnects coupled to the transmitter of the second chip.

28. The chip of claim 2, wherein the control circuitry includes a loopback initiating command in the inband signals when the chip is in a loopback initiating mode.