Toggle-Stuck and Cross-Stuck Cases

A FAULT-TOLERANT MULTICAST OUTPUT-BUFFERED ATM SWITCH 175

6.4.3.1 Toggle-Stuck and Cross-Stuck Cases

By performing fault de- tection for the TS, the location of the faulty column can be identified. In Ž . order to locate the faulty row, we define a location test called the TS test for a TS fault. Figure 6.26 shows an example of the TS test. Cells coming from MPMs are forced to have a different FA than those from the AB; thus, all SWEs in the SWE array are set to a cross state. As aresult, fault-free condition cells from the MPMs all appear at the east side of the SWE array, while those from the AB appear at the south side. If there is a TS at Ž . SWE i, j , cells from the ith MPM will be delivered to the jth output link, while cells from the jth column of the AB are routed to the ith discarding output. Ž . Once we have exactly located the TS fault, say at i, j , we can reconfigure Ž . the SWE array by setting SWE i, j to a cross state, as shown in Figure 6.27. Fig. 6.26 Fault location test for a toggle-stuck SWE by forcing all SWEs to a cross state. KNOCKOUT-BASED SWITCHES 176 Ž . Fig. 6.27 Reconfiguration by forcing a toggle-stuck SWE 4, 2 to a cross state. This reconfigured SWE array has the same cell-loss performance as the SWE Ž . array with a CS fault at i, j . As mentioned before, a CS fault cannot be detected online. However, since a CS situation can be considered a reconfiguration of TS faults, we do Ž not need to identify the location of the CS fault although it can be detected . and located with a few steps of offline tests as described in the following . If we add one more bit to the priority field and modify the setting of the priority fields of the test cells, we can offline detect and locate a CS fault in the SWE array. The method of locating a CS fault is to force all the SWEs in the shaded square block of SWEs in Figure 6.28 to a toggle state while the rest of the SWEs are set to a cross state. If there exists a CS SWE in the square block, we will be able to identify its location by monitoring the outputs. By moving the square block around in the SWE array and repeating the test procedure, we can determine if there is a CS fault and locate its Ž . position if any . A FAULT-TOLERANT MULTICAST OUTPUT-BUFFERED ATM SWITCH 177 Fig. 6.28 Fault location test for a cross-stuck SWE by moving the square block around in the SWE array. Figure 6.28 shows an example of the fault location test for a CS SWE Ž . CS test . This test can also be used to locate a HS SWE. To diagnose the SWEs in the three uppermost rows, the priorities of the test cells, X3, X2, X1, V1, V2, and V3, are arranged in descending order. These cells’ FAs are set to be identical, but the FAs of test cells X4, X5, and X6 are set to be Ž different. The CS test forces all SWEs in the square block 3 = 3 in this . example to a toggle state and all other SWEs to a cross state. If there exists a CS in this square block, the output pattern will be different from the u v expected one. For offline testing of a switch module, at least NrL M tests 1 u v are required in MGN1, and L MrL tests in MGN2. 1 2

6.4.3.2 Vertical-Stuck and Horizontal-Stuck Cases