The only bits that are worth mentioning are 0 and 1. These lower two bits are input/output for the link port wires. Bit 0 is the red wire, and bit 1 is the white wire.

If no calculator is connected, reading the port will return 00000011b. The 1 indicates that the line is high. A 0 is low. If two calculators are connected, and either brings the line low, the line will be low on both. The line cannot be set high if the other calc is holding it low. Think of it as an OR operation, if either calc sets a line low, it will be low. Otherwise high.

Anyway:

Outputting:

Bits 2-7 of your output to the port should be zero. For bits 0 and 1, set that bit if you want to bring the line low. If the bit is reset, then the line will not be held low. Example: The link port reads 00000011b. You output 00000010b. The white line goes low. A subsequent read of the port returns 00000001b.