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LP (4) | Special files and drivers | Unix Manual Pages | :man▋
The interface MTU defaults to 1500, but may be set to any value. Both ends of the link must be configured with the same MTU.
Cables with this wiring are widely available as ’Laplink’ cables, and are often coloured yellow. The connections are symmetric, and provide 5 lines in each direction (four data plus one handshake). The two modes use the same wiring, but make a different choice of which line to use as handshake.
When idle, all data lines are at zero. Each byte is signalled in four steps: sender writes the 4 most significant bits and raises the handshake line; receiver reads the 4 bits and raises its handshake to acknowledge; sender places the 4 least significant bits on the data lines and lowers the handshake; receiver reads the data and lowers its handshake. The packet format has a two-byte header, comprising the fixed values 0x08, 0x00, immediately followed by the IP header and data. The start of a packet is indicated by simply signalling the first byte of the header. The end of the packet is indicated by inverting the data lines (i.e., writing the ones-complement of the previous nibble to be transmitted) without changing the state of the handshake. Note that the end-of-packet marker assumes that the handshake signal and the data-out bits can be written in a single instruction - otherwise certain byte values in the packet data would falsely be interpreted as end-of-packet. This is not a problem for the PC printer port, but requires care when implementing this protocol on other equipment.
When idle, all data lines are at zero. Each byte is signalled in four steps: sender writes the 4 least significant bits and raises the handshake line; receiver reads the 4 bits and raises its handshake to acknowledge; sender places the 4 most significant bits on the data lines and lowers the handshake; receiver reads the data and lowers its handshake. [Note that this is the opposite nibble order to LPIP mode]. Packet format is: The length includes the 14 header bytes, but not the length bytes themselves nor the checksum byte. The checksum is a simple arithmetic sum of all the bytes (again, including the header but not checksum or length bytes). The start of packet has to be signalled specially, since the line chosen for handshake-in cannot be used to generate an interrupt. The sender writes the value 0x08 to the data lines, and waits for the receiver to respond by writing 0x01 to its data lines. The sender then starts signalling the first byte of the packet (the length byte). End of packet is deduced from the packet length and is not signalled specially (although the data lines are restored to the zero, idle state to avoid spuriously indicating the start of the next packet).
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