Sometimes you get very lucky when searching the Web!
A Japanese blogger (darekasan_net) posted a review of the Yamaha PSS-A50 and a mod adding 5-pin MIDI OUT. [Google translation of the review]
There are a set of test pads in the upper left corner of the A50 main board (DM) as shown in the picture below. [Click image to enlarge it.]
The two larger rectangular pads (orange) are digital ground (DGND). The four smaller pads (blue) from left to right are:
- MIDI_IN (RXD MIDI_IN)
- MIDI_OUT (TXD MIDI_OUT)
- 3.3V
- Digital ground
The blogger connected the MIDI_OUT signal to a 5-pin DIN connector, which is mounted nearby on the enclosure.
By the way, Yamaha conveniently mark test points with a circle (bullseye). You can see several test points for digital ground (DGND), the +3.3V digital rail, and the +5V digital rail. The USB interface chip is an NXP ARM microprocessor. The micro USB connector is in the upper right.
Direct connection is too trusting. The MIDI_IN and MIDI_OUT pads go directly to SWLL pin 55 (RXD) and SWLL pin 54 (TXD). I suggest adding a 220 ohm current limiting resistor in series with MIDI_OUT. Adding a signal buffer would be even better since you would rather blow up the buffer instead of the main processor (YMW830-V or SWLL) should someone radically misconnect the 5-pin MIDI port. A current limiting resistor on the +V MIDI pin wouldn’t hurt either.
If you get the urge to add 5-pin MIDI IN, you’ll need an optoisolator as shown in the schematic below.
Although the schematics indicate 5V, the circuits should work with 3.3V instead. Fortunately, the unpopulated test connector provides +3.3V as well as MIDI_IN and MIDI_OUT.
Here’s an idea. Instead of hacking in a 5-pin DIN connector alone, why not add a Bluetooth MIDI plug like the CME WIDI Master?
Our Japanese blogger considered adding a sustain input. The PSR-F50 dedicates SWLL pin 53 (PORTC0) to sustain. Unfortunately, the PSS-A50 has other ideas and SWLL pin 53 mutes the headphone output instead. You could put an external switch in parallel with the front panel sustain switch, but it toggles sustain and, thus, it doesn’t behave like a true piano sustain pedal.
As with any mods, make them at your own risk and kiss your warranty good-bye!
Copyright © 2021 Paul J. Drongowski