The signal lines are used as follows:
Data0 (Pin 2)
Data out, bit 0.
Data1 (Pin 3)
Data out, bit 1.
Data2 (Pin 4)
Data out, bit 2.
Data3 (Pin 5)
Data out, bit 3.
Data4 (Pin 6)
Handshake out.
ERROR* (pin 15)
Data in, bit 0.
SLCT (pin 13)
Data in, bit 1.
PE (pin 12)
Data in, bit 2.
ACK* (pin 10)
Data in, bit 3.
BUSY (pin 11)
Handshake in.
When idle, all data lines are at zero. Each byte is signaled 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:
Length (least significant byte)
Length (most significant byte)
12 bytes of supposed MAC addresses (ignored by FreeBSD).
Fixed byte 0x08
Fixed byte 0x00
<IP datagram>
Checksum byte.
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).
FreeBSD calculates outgoing checksums, but does not validate incoming ones.
The start of packet has to be signaled 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 signaling the first byte of the packet (the length byte).
End of packet is deduced from the packet length and is not signaled specially (although the data lines are restored to the zero, idle state to avoid spuriously indicating the start of the next packet).