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The ZIP Parallel Interface: Data Transfers.
Both types of zip drive will operate in any of the modes, ecp, byte or nibble. The actual mode used is dependent solely on the interface capabilities.
Note also, that If the interface has a fifo for epp, it will also be utilised.
In the specific case of nibble mode, this is input only. This means, that for output data (and commands), data is sent in the special byte mode, NOT, in spp mode which would confuse a printer.
The zip interface remains static. The driver, and the zip, communicate solely in the mode chosen for all data , command, and status retrieval. There is no special case, or special 'mode' for disinguishing zip commands and zip status returns.
However, prior to transferring any 'data', the zip interface is interrogated for status
In the specific case of incoming data or status from an IMM chip in byte/nibble mode, the interface must first be turned around. This is achieved with a negotiate sequence. At the end of data transfer, the interface is acknowledged. Regardless of the number of data packets sent, the negotiate / acknowledge sequence occurs only at the beginning and end of the entire transfer.
Data transfers occur in successive burst-packets of (up to) 512 bytes. There is nothing inherently in the zip that requires this. It is simply that the driver utilises scsi block data capabilities.
As will be noted in the byte mode for imm, the zip interface requires data to be sent in pairs. This means command and data to an imm drive need to be an even number of bytes.