The ZIP Parallel Interface: Status

Once the zip has gained control of the parallel port. It presents status on the following signals

*(the interface signal nBUSYi is inverted at the PC). lo is relative to the programmer, not the interface)

Want = write out, relative to the programmer.

thus

1100 want data
1101 send data
1101 want command
1101 send status (or idle)

Note that the nACK signal has been replaced in the imm interface by nFault to be more in accord with IEEE.

Note the signals nBUSYi and nACK, are permanently hi when status is available and connected. nBUSYi drops low while zip interface is busy.

The natural state of the interface is 0xF0, all others are status presentations during transfers.

Note also, that read states (send data, send status) are bus reversals. For an imm chip in spp mode, this requires a negotiate, and acknowledge sequence