Each stage of a fabric routes using a nibble the meaning of which is shown in table 1.
For a zero stage fabric there is no fabric routeing and the nibble should be entirely zero including the active bit.
For a single stage fabric the low nibble of the first octet is used as the entire route.
For a two stage fabric both nibbles contain a route. The nibble in bits 0-3 is considered more significant than the nibble in bits 4-7 for the point of numbering the possible outputs.
All devices plugged into an output of the switch fabric route on the first byte received as described in table 2.
Table 2: Port Controller Route byte
Some devices ignore cells with the loop-back bit set and some ignore cells with the loop-back bit clear as appropriate.
