They have ieee1284 printer-port protocol baked into them, so you do an OUT transaction of several bytes and the adapter does the per-byte handshaking. The old p

They have ieee1284 printer-port protocol baked into them, so you do an OUT transaction of several bytes and the adapter does the per-byte handshaking. The old parallel ports were just a collection of gpio lines (originally some input-only and some output-only, later some turned bidirectional) connected to some io addresses.