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B–11
Host I/O Interfaces
Prime Signal Enable
TOF Action Reset
Buffer Size In K 64
The 1284 supports three operating modes which are determined by negotiation
between the printer and the host.
Compatibility Mode
This mode provides compatibility with Centronics-like host I/O. Data is transferred
from the host to the printer in 8-bit bytes over the data lines.
Compatibility Mode can be combined with Nibble and Byte Modes to provide
bidirectional communication.
Nibble Mode
Eight bits equals one Byte. When a byte of data is sent to the printer, the eight
bits are sent over eight data lines.
Some devices cannot send data over their eight data lines. To bypass this, the
1284 permits data to be sent as half a byte over four status lines. (Half a Byte
equals one nibble.) Two sequential four-bit nibbles over the status lines, and the
host controls the transmission.
Byte Mode
The printer and host send data to each other along eight data lines (one bit per
line).
If bidirectional communication is supported by the printer and the host, the host
will take control of the data transfer.
Signals
Table B–6 lists which of the signals is associated with the corresponding pins on
the 1284 interface. Descriptions of the signals follow.