This is the connector between the master and slave devices.
| Pin | Description |
|---|---|
| 1 | Ground |
| 2 | +5V_SB |
| 3 | I2C SCL |
| 4 | I2C SDA |
| 5 | Address Latch |
| 6 | PS_EN |
+5V_SB is 5V standby power from the PSU. PS_EN is the power supply enable line. Address latch is used to auto-assign I2C addresses on startup (future use).
If I did this right, no magic smoke should escape if (when) someone plugs this connector in backwards. Off-by-one... not so much.
I am using a 160k / 10k resistor divider, with a 1 uF capacitor in parallel with the 10k resistor. Using the Teensy3's internal 1.2V precision reference, I can read a transistor voltage of 0 to 20.4 volts. The capacitor filters out the PWM frequency, letting me be extremely lazy with my firmware when checking how things are working.
This is the pinout of the connector on the high voltage power supply.
Note how the power supply datasheet counts terminal numbers! It might not be what you expect. For consistency, I use the same numbering as the datasheet.
| Pin | Description |
|---|---|
| 1 | RC+ |
| 2 | RC- |
| 3 | AUX (5V_SB) |
| 4 | AUX_GND |
| 5 | +S |
| 6 | -S |
The RC+ and RC- pins are for Remote Control of the power supply. Shorting RC+ to RC- will turn off the power supply. It also appears that RC- is internally tied to AUX_GND, but I probably should explicitly do that on the PCB to be safe.
This is a 5V standby power supply, limited to 300 mA. I think we come in under that, but it bears testing. If not, I might have to get creative...
This is a sense line to compensate for voltage droop at the load. We don't care, so these should be left unconnected. Probably best not to put wires in the header, so stray EMI doesn't get picked up.
I need external pullup resistors on the I2C bus, as the internal Teensy ones don't work all that well. For high speed, I probably want 1.5k or so.
I am going to support the legacy DIN MIDI jacks. MIDI uses a current loop and opto-isolator (optocoupler) to avoid ground loops. I will be using the 6N138.
I think I can use the MAB5SH part in Eagle as the footprint for the MIDI conenctor. Yeah, I think we're good. But just in case: http://www.newark.com/hirschmann/mab-5-sh/din-audio-video-connector-socket/dp/61T4646
I need to use a protection diode on the MIDI In port to protect the optocoupler from exceeding its breakdown voltage (5V max). I could use the diode I use to absorb flyback from the coils. Massive overkill, but simplifies the BoM. A small surface mount part might be easier to lay out, though.
I need some sort of output transistor for the MIDI out/through circuit. Worst case, a shorted cable could cause a load of up to 23 mA (5 V / 220 ohms) when pulling MIDI_OUT to ground, but the Teensy can only handle 9 mA per pin.
Plus, a faulty/malicious device could actually put more than 5V on Pin 5. A transistor with sufficient ratings solves all this.
Because this is a chance for me to have too much fun with design and learn some things, I could isolate the MIDI out as well. I would need an opto-isolator, and an isolated power supply to provide the 5V for the current loop and to run the optocoupler.
I will order a REE-0505S from Mouser and see if it works well enough. If not, I will try a MEV1S0505SC, but this would need a footprint designed for Eagle. (Actually, I need to verify the generic DC-DC footprint works on the REE...)
If that all works, I could route out an isolation gap in the PCB, and do all the high-voltage design. Of which I currently know nearly nothing. But once I learn, I would have a MIDI input/output stage with > 1000V isolation! heh
http://www.mouser.com/ProductDetail/Molex/15-91-2140/?qs=sGAEpiMZZMttdt6ek4ZYDGcRuHgvKenX This is for the pads on the back of the Teensy.