that triggered the operating system to read or write more data.

Unlike a cassette interface, where only a single device would normally be used, an external expansion port would need to be able to support more than one device. To support this, a simple protocol was developed and several new pins added to the original simple cassette port. Most important among these was the COMMAND pin, which triggered the devices to listen for a 5-byte message that activated one of the devices on the bus and asked