Whenever two MIDI signals have to be combined to drive a receiver, you need a MIDI merger. In the design presented here, two coupled microcontrollers merge the data.Since the MIDI signal is unidirectional and sequential, the data are transmitted without any feedback from the receiver. This means that it is not easy to simultaneously combine the signals from two MIDI sources. However, it can be desirable to combine the data streams from two input devices in order to drive a single destination device (such as an expander). This function is performed by a device called a ‘MIDI merger’. It produces a meaningful signal from the combination of several data streams from different sources. This is not a simple task, since the data can range in length from a single byte for a control instruction up to several hundred bytes for SysEx instructions, and the tone information can vary considerably (it is frequently sent as ‘running status’, occasionally interrupted by SysEx instructions).