I wouldn't necessarily recommend building that circuit as-is, because it has a few shortcomings. The three changes I'd make would be to provide some buffering or clamping for the MIDI output and RS-232 pins (connecting a CMOS pin from the MAX232 to a connector is a bad idea). Pin 5 of the MIDI output always needs to have a 220 ohm resistor in series - I see this error a lot in hobby designs and it's completely wrong. I'd also provide a resistor on the base of the 6N138 to ground (about 4.7k) to turn it off faster. Otherwise, it may not reach 31.25k baud reliably. Alternatively, you could replace the 6N138 with a PC900 or H11L1 or H11L4 optocoupler (use a 280 ohm pull-up resistor - although Atari used a 4.7k pull-up and it seemed to work reliably).
You can drastically simplify the circuit though. Here's a design for an Amiga MIDI interface I did a while ago. NOTE: I haven't tested it yet!
The newer version is the better version for using as an Amiga MIDI interface device. The old version is better for interfacing to general RS-232 devices - because it can swing its output to -12V.
To use it stand-alone, you will need to provide +12 volts on pin 9, and -5 to -12 volts on pin 10. It should provide an acceptable degree of ESD protection. You can probably cheap out and connect pin 10 to ground instead of -12 volts, but that isn't strictly RS-232 compliant. It will work with most RS-232 devices, but not all. You can also use a regulated 5 volt supply on pin 9 by shorting resistor R3, replacing zener diode D2 with a 1N4148 and lowering R8 to 680 ohms.
The +/- 12 volt supply must not exceed 15 volts off-load.
Baud rate is mainly limited by the optocoupler. It will probably work at 38.4k. You might be able to go faster, depending on the characteristics of the particular optocoupler you use. Faster optocouplers are available like the TLP2361, but you'll need to limit the positive supply to exactly 5 volts if using a TLP2361. Don't just substitute any optocoupler because most of them aren't fast enough and/or have a too high threshold current.
You could remove the optocoupler entirely for faster speeds. In this case, instead of connecting the emitter of the 2N3906 to pin 9 of the modem port you connect it to pin 4 of the MIDI input jack. Pin 5 of the MIDI jack connects to where pin 4 of the optocoupler was. R6 needs to be changed to 3.3k. Pin 2 of the MIDI input jack connects to ground (not MIDI compliant, but should work with the MIST). You can remove D3 and R5, but you'll want a 1N4148 diode from pin 4 of the MIDI jack to ground - cathode to pin 4.
If you want a cheap and straightforward way of generating +/- 12 volts from a 5 volt regulated supply, you might use something like this: https://uk.rs-online.com/web/p/isolated ... s/1669112/
Also to keep some RS-232 devices happy, you might want to loop back the following pins on the RS-232 port:
RTS to CTS
DTR to DSR
Alternatively you can just connect RTS and DTR to a +12 volt supply.
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