Vintage Synth Repair

Thanks for the reply. I would be curious to know why a MIDI signal would need to be converted to RS232 levels at 31250 baud. MIDI is a 'current loop' in that the same electrons come from the MIDI transmitter through the MIDI receiver and then back to the original source. RS232 uses +9 volts for active signal and -9 (to -15) volts for inactive signal.

I have used an AVR microcontroller type Mega164 to make a MIDI-to-RS232 converter. This was easy to do because the Mega164 has two serial port UARTs in it that both have input and output signals. Set one serial port to 38400 baud and the other to 31250 baud. Then whatever byte comes into one port goes out the other. A PC-to-MIDI convertor for less than $10. But my difficulties aren't with hardware. It's being unable to use the PC MIDI port at all with straightforward code. If I could get the sysex dumps into the PC then I could put the parameters on the screen and make a general purpose editor program for all the tone modules out there.

With the old MPU401, it was simple. A bit in a byte located at a specific I/O address would go high to indicate that a new data byte had arrived. Another bit would go high indicating that the 401 was ready to receive a byte for transmission. Send the dump request string and then collect bytes from the 401 into a buffer until it received 0xF7. But it can't be done with modern Windows because there are several undocumented software layers between the MIDI port and the application program code. So MIDI dies a slow death and the tone modules sell on eBay for 1/20th of what they did 15 years ago.

Anyway, I will research Apple Mac MIDI interfaces. Or I will try too. Getting technical info about Apple computers can be difficult because everyone in AppleLand seems to believe that there is no need for anyone to know "stuff like that" and that you would be better off just giving them $500 for a program that "take care of stuff like that". After all, that is what makes Apple so superior to everything else; the mindset goes.

Thanks again. I don't even know if the Gmega that I bought works since it was sold 'untested'. Everything that I have bought 'untested' on ebay has worked and usually well. It's just that it's difficult to actually tell if a tone module works if you don't have a MIDI keyboard or the original power supply. I actually prefer that people not try to get MIDI gear working. It's easy to break the power supply section since so few MIDI manufacturers believe that putting a ten cent diode between the power jack and the voltage regulator circuitry is a worthwhile investment on an $800 tone module. People might not buy their $39 matched wall-wart and get a $8 Radio Shack instead when the original tone module power convertor gets misplaced.

Oh, the joys of electronic repair...

--- On Wed, 12/16/09, GB wrote: