The MIDI-Theremin kit uses a MAX232 chip to handle serial output, via a DB9 connector on the main board.
There is also standard MIDI out, and the old MIDI via gameport, which I am not bothering to install as I don’t have any receiving hardware with a gameport, although I have soldered the opto-isolator (IC3: 6N139) to the board, I’ll remove it and re-use when I find a need for one.
The Theremin’s PIC16F628A microcontroller outputs serial on pin 11, which goes to :
- pin 5 of the MIDI connector (via 220R)
- the gameport optocoupler
- pin 11 of MAX232 (TTL Driver 1 input)
The MAX232 outputs RS-232 on pin 14 (Driver 1 output), which goes directly to pin 2 of the DB9 connector.
The Silicon Chip article states that a serial to USB converter can be used to connect the Theremin to a PC, I am hoping to eventually build in direct USB output.
Using a serial to USB converter cable, or even disassembling one and mounting it in the case, means that the serial signal is converted from TTL (5v) to RS-232 (-10V / 10V) to USB (~+/- 3V/5V?)
Removing the RS-232 chip completely and going directly from TTL to USB is the direct method.
First steps: mounting the MAX232 on a breakout board to test the kit as intended (before I potentially damage it with a mod), this also separates the RS-232 components completely from the main board so that I can swap in and out other serial output circuitry easily.
Breaking out the MAX232:
Three wires are required from the main PC to the breakout board:
- Ground
- TTL serial output from the microcontroller pin 11
- +5V power for the MAX232.
When I first assembled everything, I only had only two wires coming from the main board, I had forgot the +5V supply line. There are a number of tests for the data output in the kit instructions. The voltages I measured without the supply were obviously much lower than they should have been.
The DB9 connector is attached to the breakout board by only two wires. This is a basic transmit only serial implementation. The MIDI-Theremin does not receive anything from the connected PC.
Here are the layout and connection points for the MAX232 breakout. This circuit may be useful for other RS-232 experiments/projects:
I have soldered the DB9 pins directly together on the underside of the DB9 PC-mount right angle connector using short lengths of shielded and unshielded wire, being careful to avoid unintended shorts. The whole thing is held together with a rubber band, and shielded with a square of cardboard. It’s a bit messy, but compact, and hopefully only temporary.
With everything assembled (including the +5v supply wire!) and powered on, I am now reading the correct test voltages on the MAX232.
- +5V at pin 16
- -8.53V at pin 6 (ideal = -9V, but seems to be acceptable?)
- 9V at pin 2 (up to 9.6V is acceptable)
MAX232 info and pinout:
http://en.wikibooks.org/wiki/Serial_Programming/MAX232_Driver_Receiver
http://www.datasheetcatalog.org/datasheet/texasinstruments/max232.pdf
RS-232 serial pinout:
http://www.lammertbies.nl/comm/cable/RS-232.html#pins
Next post: TTL to USB plans using the FT232RL chip.