Personal computer systems in general, and IBM personal computers in particular, have attained widespread use for providing computer power to many segments of today's society. A personal computer system can usually be defined as a desktop, floor standing, or portable computer that includes a system unit having a system processor, a display monitor, a keyboard, one or more diskette drives, a fixed disk storage, an optional pointing device such as a "mouse," and an optional printer. These systems are designed primarily to give independent computing power to a single user or small group of users and are inexpensively priced for purchase by individuals or businesses. Examples of such personal computer systems are sold under trademarks and include IBM's PERSONAL COMPUTER, PERSONAL COMPUTER XT, PERSONAL COMPUTER AT and IBM's PERSONAL SYSTEM/2 Models 25, 30, 50, 55, 56, 57, 60, 65, 70, 80, 90 and 95 (hereinafter referred to as the IBM products PC, XT, AT, and PS/2, respectively).
These systems can be classified into two general families. The first family, usually referred to as Family 1 Models, uses a bus architecture exemplified by the AT computer and other so called IBM compatible machines. The second family, referred to as Family 2 Models, uses IBM's MICRO CHANNEL bus architecture exemplified by IBM's PS/2 models 50 through 95. The bus architectures used in Family 1 and Family 2 Models are well known in the art.
Beginning with the earliest personal computer system of the Family 1 Models, the IBM PC, and through the current Family 2 Models, the system processor of the personal computer is from the Intel 86 family of microprocessors. The Intel 86 family of processors includes the 8088, 8086, 80286, 80386, and 80486 processors commercially available from Intel Corporation. The architecture of the Intel 86 family of processors provides an upwardly compatible instruction set which assists in preserving software investments from previous processors in the 86 family of processors. This upward compatibility preserves the software application base of the personal computers which use this family of processors. A variety of commonly available and well known software operating systems, such as a DOS or an OS/2 operating system, operate on various members of the Intel 86 family of processors.
Due to the ever increasing functional requirements in today's electronic systems brought about by the myriad of processors and their capabilities, the number of circuits being integrated into an integrated circuit (IC), as well as the number of IC's being integrated on a circuit board and the number of circuit boards being included in a system, such as a personal computer system, are being increased dramatically. Testing of such a densely packed system becomes more and more difficult. A testing standard presented by IEEE (P1149.1), which is identical to the testing standard adopted by Joint Test Action Group (JTAG) of Europe and North America was developed to address this issue. In these standards, a boundary scan technique or test protocol is presented to serially scan all the devices under test with a test data input (TDI) signal from a JTAG control unit, and the returned scan test data output (TDO) signal is looped back to JTAG control unit. By comparing the predefined serial signal pattern sent out from the JTAG control unit with the returned serial signal pattern, after it has been scanned through the identified devices, the operational readiness of all or certain selected system devices is determined.
In a system configured for options such as multiple plug-in cards, concern arises when the JTAG boundary test is considered to be implemented. In such an instance, a serial scan signal is scanned through from card to card via card connectors at card slots or at stations where electronic components or devices, including component boards, are plugged in or inserted and from which, as is usually desirous, such components and devices are readily removable. When the system is not fully populated with such components, devices or cards, a need exists to conduct a serial scan without interruption, regardless of which of the card slots or stations is occupied.
In solving this problem, there are many potential different approaches. Cost and reliability are two major deciding factors in selecting an implementation. A reliable and cost effective design, which is not overly complex, is essential. When making a test algorithm feasible, in an otherwise already complex system, it is important not to add more reliability problems to the existing system. To do so would defeat the whole purpose of the initial testing. Also it is desirable to accomplish the testing without the need to include additional signal pins.