MIDI Hardware Interface (rev 1)

[ndf-zz]

On the Rev 1 DDRM, the MIDI input uses a 6n138 5V optocoupler, but powers it using 3V3. Incoming MIDI messages are sometimes garbled and often lead to stuck notes when DDRM is sequenced over MIDI. My fix for the problem is to replace the optocoupler with a substitute part, as follows:

• Install RV1S9160A on a SO-8 to DIP-8 adapter pins 2,4,5,6 & 7
• Short adapter board pins 3-4
• Short adapter pins 7-8
• Remove resistors R7 (1k) and R8 (330R)
• Connect 100nF ceramic capacitor from R7-GND to R8-3V3 (optional)
• Install SO-8 adapter in place of IC1

The MIDI Thru also suffers from errors in some cases because the current loop has too much resistance for use with 3V3. The fix I used was:

• Replace R11 with 33R/0.5W
• Replace R12 with 10R/0.25W

Straight out of the MIDI manual: (CA-033) MIDI 1.0 Electrical Specification Update

[DanLeuca]

I thought that the 6N139 solves this problem...