Here is the second version of my updated MIDI circuit, now able to control a 4x4 grid of a chessboard.

Pic Basic Pro Code

INCLUDE "modedefs.bas"

define OSC 20

define HSER_TXSTA 20h  ' enable the transmit register
define HSER_BAUD 31250 ' set the baud rate

DEFINE  ADC_BITS        10
DEFINE  ADC_CLOCK       3
DEFINE  ADC_SAMPLEUS    10

tempo VAR WORD

TRISA = %11111111
ADCON1 = %10000010
TRISB = %11111111

output PORTC.0
output PORTC.1
output PORTC.2
output PORTC.3

pause 500

main:

    high PORTC.0
        gosub playNotes
    low PORTC.0

    high PORTC.1
        gosub playNotes
    low PORTC.1

    high PORTC.2
        gosub playNotes
    low PORTC.2

    high PORTC.3
        gosub playNotes
    low PORTC.3

goto main

playNotes:

    if PORTB.7 = 1 then
        hserout [$99, $40, $40]
    else
        hserout [$89, $40, $000]
    endif

    if PORTB.6 = 1 then
        hserout [$99, $4C, $40]
    else
        hserout [$89, $4C, $000]
    endif

    if PORTB.5 = 1 then
        hserout [$99, $50, $40]
    else
        hserout [$89, $50, $000]
    endif

    if PORTB.4 = 1 then
        hserout [$99, $3C, $40]
    else
        hserout [$89, $3C, $000]
    endif

    pause 200

return