HP QMS 1660 - Manual (Page 131)

Parallel
Interface Modes
5-31 Additional Technical Information
Byte Mode
Printer-host communication is done in bytes.The byte mode may be
used by the host device in a DMA (Direct Memory Access) mode for
more efficient operation.
When byte transfer is complete and there is no more data to transmit,
the host may do one of the following:
n
Terminate and return to the compatibility mode.
n
Stay in the Host Busy, Data Not Available phase.
n
Set Host Busy Low, putting the interface into the idle phase.
If there is additional data, the host may do one of the following:
n
Set Host Busy Low, indicating that the host can accept additional
data.
n
Stay in the Host Busy, Data Not Available phase.
n
Terminate and return to the compatibility mode.
Check your host documentation to see if the host is 1284 compatible.
Compatibility Mode
Printer-host communication is done in a manner that ensures com-
patibility.
ECP (Enhanced Compatibility Port) Mode
This is an advanced version of byte mode which allows transfer of
data in either direction without returning to the compatibility mode.
The communication is a half-duplex channel with either device, the
host or the printer, making a request for data transfer when there is
available data. In the case of simultaneous requests for transfer, the
printer always defers to the host.