Introduction

The SCANSTA112 is the third device in a series that enable

multi-drop address and multiplexing of IEEE-1149.1 scan

chains. The SCANSTA112 is a superset of its predecessors

- the SCANPSC110 and the SCANSTA111. The STA112 has

all features and functionality of these two previous devices.

The STA112 is essentially a support device for the IEEE

1149.1 standard. It is primarily used to partition scan chains

into managable sizes, or to isolate specific devices onto a

seperate chain (Figure 1). The benefits of multiple scan

chains are improved fault isolation, faster test times, faster

programiing times, and smaller vector sets.

In addition to scan chain partitioning, the device is also

addressable for use in a multidrop backplane environment

(Figure 2). In this configuration, multiple IEEE-1149.1 acces-

sible cards with an STA112 on board can utilize the same

backplane test bus for system-level IEEE-1149.1 access.

This approach facilitates a system-wide commitment to

structural test and programming throughout the entire sys-

tem life sycle.

Architecture

Figure 3 shows the basic architecture of the ’STA112. The

device’s major functional blocks are illustrated here.

The TAP Controller, a 16-state state machine, is the central

control for the device. The instruction register and various

test data registers can be scanned to exercise the various

functions of the ’STA112 (these registers behave as defined

in IEEE Std. 1149.1).

The ’STA112 selection controller provides the functionality

that allows the 1149.1 protocol to be used in a multi-drop

environment. It primarily compares the address input to the

slot identification and enables the ’STA112 for subsequent

scan operations.

The Local Scan Port Network (LSPN) contains multiplexing

logic used to select different port configurations. The LSPN

control block contains the Local Scan Port Controllers

(LSPC) for each Local Scan Port (LSP

control block receives input from the ’STA112 instruction

register, mode registers, and the TAP controller. Each local

port contains all four boundary scan signals needed to inter-

face with the local TAPs plus the optional Test Reset signal

(TRST).

The TDI/TDO Crossover Master/Slave logic is used to define

the bidirectional B0 and B1 ports in a Master/Slave

configuration.

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FIGURE 2. Example of SCANSTA112 in a multidrop addressable backplane.

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