IBM 6400-D Generation II Series, Setup Manual

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Operating Modes
127
IEEE 1284 Parallel Interface
The IEEE 1284 is a parallel interface with bidirectional capabilities. Features
include the following:
•
Faster data transmission. Timing of the signals has been reduced.
•
Bidirectional communication. Both the host and the printer can send data.
•
Versatility. If a device cannot send data along particular lines, the 1284
can work around this and send data via other operating modes, such as
Nibble Mode which is discussed later.
•
Less user interaction. The host can ask the printer about printing status
and supported features, such as fonts and internal errors. For example,
instead of having to physically check if the printer has run out of paper,
you can create a program to query this from the host. The printer will
respond and a message will display on the host.
Operating Modes
The 1284 supports three operating modes, which are determined by
negotiation between the printer and the host.
Compatibility Mode
This mode provides compatibility with a Centronics or PC Parallel interface.
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 are sent over the
lines.
Data is transferred from printer to host in 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.
IEEE 1284 Parallel Interface