108Chapter 5 IEEE 1284 Parallel InterfaceIEEE 1284 Parallel InterfaceThe 1284 supports three operating modes, which are determined bynegotiation between the printer and the host.Compatibility ModeThis mode provides compatibility with Centronics-like host I/O (see Table 5).Data is transferred from the host to the printer in 8-bit bytes over the datalines.Compatibility Mode can be combined with Nibble and Byte Modes to providebidirectional communication.Nibble ModeEight bits equals one byte. When a byte of data is sent to the printer, the eightbits 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 abyte equals one nibble.) Two sequential four-bit nibbles are sent over thelines.Data is transferred from printer to host in four-bit nibbles over the status lines,and the host controls the transmission.Byte ModeThe printer and host send data to each other along eight data lines (one bitper line).If bidirectional communication is supported by the printer and the host, thehost will take control of the data transfer.
