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Chapter
4
IEEE 1284 Parallel Interface
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 IEEE 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.
Signals
Table 13 lists each of the signals associated with the corresponding pins on
the IEEE 1284 interface. Descriptions of the signals follow.
Pin
Source of Data
1
Host
2
Host/Printer
3
Host/Printer
4
Host/Printer
5
Host/Printer
6
Host/Printer
7
Host/Printer
8
Host/Printer
9
Host/Printer
10
Printer
11
Printer
300
Table 13. IEEE 1284 Signals
Compatible
nStrobe
Data 1 (LSB)
Data 2
Data 3
Data 4
Data 5
Data 6
Data 7
Data 8 (MSB)
nAck
Busy
Type of Mode
Nibble
Byte
HostClk
Host/Clk
PtrClk
PtrClk
PtrBusy
PtrBusy
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