iMac/PowerBook & IIC bus

Steven Bytnar sbytnar at
Wed Jul 31 09:03:46 EST 2002

Does anyone know anything about the IIC bus in the original iMac?
It looks like there's a 74FST3125 4-bit bus switch that allows
access to different IIC buses. One bus, the default bus(?), leads
to the J3/BOTTOM slot. The other bus leads to the J1/TOP. How do
you tell the Cuda/PMU/mem controller chip to switch between the
IIC buses? It seems like there's two to four different
packet/command mechanisms of reading from IIC devices on older
machines using the Cuda chip:
char type; /* 1 = ADB_PACKET_PSEUDO, 5 = ADB_PACKET_MACIIC */
char command; /* 0x22 = ADB_PSEUDOCMD_GET_SET_IIC,
                 0x25 = PSEUDOCMD_COMBINED_FORMAT_IIC */
char i2caddress;

packet format, but I haven't figured out any of the other combinations.

I think this IIC bus switching mechanism is used on PowerBooks also.

Any help greatly appreciated.


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