MIDIbox-to-COM Interface

Created in January 2002

The MIDIbox-to-COM interface enables you to connect the MIDIbox to a COM (RS232) port instead of a MIDI port. It saves one MIDI-IN and one MIDI-OUT for your MIDI equipment. The data is transfered about 20% faster than via MIDI.

Hardware Requirements

The MIDI interface of the MIDIbox must be replaced by a RS232 interface. The MAX232 chip does this simple job, it just shifts the TTL voltage levels of the PIC to the -12/+12 V levels of the RS232 port.

Driver Requirements

I tested the RS232 interface with the CBX serial port driver by Yamaha and noticed no problems. :)
The Roland Driver works also fine (search for serial MIDI driver)!
And here a third driver, provided by Kawai for the K5000 workstation. Nenad reported that the two driver above failed on his computer, but the (much simpler) Kawai driver works.

Firmware Requirements

PIC16F: The RS232 interface is supported by MIDIbox Plus since v1.908, MIDIbox64, MIDIbox16E, MIDIbox64SEQ and MIDIO128.

PIC18F (MIOS): MIDIbox-to-COM is part of the operating system. See also the functional description of MIOS_MIDI_InterfaceSet.

How to plug a MIDI device to a MIDIbox with MIDIbox-to-COM Interface

This could work with the MIDI Merger project. To use the Merger as a baudrate converter, just change following lines in the MIDI Merger source code:

;; code located in init.inc, function: Init_USART:
;;
        SWITCHBANK_0_1
        movlw   (1 << TXEN)     ; set baudrate: 31250
        movwf   TXSTA
        movlw   0x09
        movwf   SPBRG
        SWITCHBANK_1_0

must be changed to:

        SWITCHBANK_0_1
        movlw   (1 << TXEN) | (1 << BRGH) ; set baudrate: 38400
        movwf   TXSTA
        movlw   0x20
        movwf   SPBRG
        SWITCHBANK_1_0

On this way, the Hardware UART is clocked with 38400 baud, but the software UART runs with the MIDI baurate anymore. Following interconnections are necessary:

• MIDI In1 of the merger to MIDI-Out of the MIDI device (i.e. a MIDI Keyboard)
• MIDI In2 of the merger directly - without optocoupler - to the output of the MAX232 (Pin 12)
• MIDI Out of the merger directly - without optocoupler - to the Rx port of the MIDIbox-PIC.

Download

Schematic/PCB
File	Size	Description
midibox_com_schematic.gif	9k	RS232 Interface
MBHP LTC	LINK	This PCB provides a MIDIbox-to-COM port
Testing the RS232 interface
PIC16F MIDI Troubleshooting	LINK	HowToDebug the MIDI Interface

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