The 16 by 16 switch fabric is constructed from eight 4 by 4 crossbar switching elements arranged in a delta network as shown in figure 4. The delta network gives rise to internal blocking. (Consider when port 0 and 1 both wish to send to ports 5 and 6 respectively. The cells both wish to use the internal link from element A to F; only one of them can). A similar 8 by 8 switch can be constructed by using half (cutting it horizontally) of this switch.
Figure 4: A 16 Port Delta Network
Each 4 by 4 crossbar switching element is almost identical to a 4 by 4 switch fabric. The routeing tag (or fabric routeing byte) is slightly different (see table 3). The first stage acts in the same way as the 4 by 4 switch fabric, removing the active bit from the routeing tag. The second stage routes using the upper nybble and, therefore, bit 4 as its active bit.
Table 3: Fabric Route byte (16 x 16 Fabric)
Since the 16 by 16 switch fabric has to be constructed from 2 boards and (to keep the design simple) these two are identical, an extra signal (set to 0 on one board and 1 on the other) is needed to cope with the antisymmetry of the two halves. That is, since the lower board (with elements C, D, G and H) is identical to the upper board, output 0 from element C is connected to G and output 1 to H instead of output 2 to G and output 3 to H, and similarly for element D. The extra signal (called bd) is exclusive-ORed with the most significant routeing bit of the lower nybble of the fabric routeing byte to give the correct route from the input stage to the output stage.
The delay through the 16 by 16 fabric equals the sum of the delays through each stage, which is 8 cycles. Since one byte is stripped from the cell as it passes through, the total delay from the start of the cell going in and the start of the cell coming out is 9 clock cycles.
The 16 by 16 switch fabric fits onto two identical double height eurocard PCBs connected by a 96 way ribbon cable. One with elements A, B, E and F on and the other with C, D, G and H. The switching elements are built on a 6400 gate equivalent Xilinx programmable gate array. The only other components on the boards are Xilinx PROMs (to program the Xilinx chips) and SIL resister packs which pull-down all the inputs to each switch element for when a slot has no device attached.