Midi on the RS232 Port

Early 1995, Yamaha introduced a new range of synthesizers with interesting interface capabilities. For the first time, these machines were compatible with three standards: MIDI, Macintosh and PC. To support the PC interface, the MU5, MU10, MU80 synthesizers and the SU10 sampler came with an RS232 serial port, providing connectivity with the PC. This spurred the author into designing an RS232-to-MIDI converter that’s supported by a freeware driver from Yamaha.Initially, these sound generators were designed to work with what Yamaha called ‘HOST PORT’ supporting software. In fact, only a few sequencers were actually capable of directly supporting this functionality. Fortunately, Yamaha designed a driver that permits the use of a serial port as a standard MIDI interface to almost any sequencer running under Windows.An interesting spec of the Host Port mode of the MU5 is its ability to retransmit all messages arriving on the MIDI IN connector to the RS232 port. The same ability was available in the other direction, from RS232 to MIDI OUT. In industry-speak, this is called a ‘gateway’.

Resistors:
R1 = 470kOhm
R2 = 470Ohm
R3 = 1kOhm
R4 = 150Ohm
R5,R6,R8-R13 = 220Ohm
R7 = 10kOhm
Capacitors:
C1 = 220nF
C2,C3 = 33pF
C4-C7 = 10µF 10V radial
C8,C9 = 100nF
C10-C16 = 47nF
Semiconductors:
D1,D3,D5,D6 = LED
D2 = 1N4001
D4 = 1N4148
IC1 = 80C320-MCG (Dallas Semiconductor)
IC2 = 74F573
IC3 = EPROM 27C256 100ns (programmed, order code 000139-21)
IC4,IC6 = 74LS04
IC5 = MAX232 (Maxim Integrated)
IC7 = 7805
IC8 = 6N137 (Atmel-Temic)
Inductors:
L1 = 1µH5
Miscellaneous:
X1 = 24MHz quartz crystal (third overtone)
X2 = 1.843 2MHz quartz crystal
K1 = 9-way sub-D socket (female), PCB mount
K2,K3,K4 = 5-way DIN socket, pins at 180 degrees, PCB mount
PCB, order code 000139-1
Disk, order code 000139-11 (driver, source code, hex file)