(1) Data transmission whereby all bits are sent one after the other, as opposed to using a parallel interface where bits are sent in groups. Parallel protocols are more complex than serial and the and the connectors are more complicated and , hence, more expensive, all of which makes it. It’s also harder easier to get the specification implemented correctly wrong among vendors., For this reason,hence, MIDI uses a serial transmission protocol and specifies a serial interface, even though the transmission time required is substantially higher than for a parallel protocol. As MIDI files are quite small, compared with files which contain actual audio or video content, the relative lack of transmission speed was felt to be a good trade-off for the simplicity of its implementation..
(2) Network connections where devices in the network are hooked up in a line, i.e., one to the next. This is called a daisy-chain, and is the opposite of a parallel topology.