I soldered the included female 2x20 header onto the hat, facing down. I soldered a red jumper wire between the transceiver VCC pin and the +3.3v rail on the hat. I soldered a black jumper wire between the transceiver GND pin and the GND rail on the hat. I soldered all of the capacitors in place. I soldered the yellow wire to the transceiver R1IN pin. I soldered the blue wire to the transceiver T1OUT pin. I soldered the green wire to the GND rail on the hat. I soldered a green wire to pin 5, a blue wire to pin 3 and a yellow wire to pin 2. I scrounged around my junk boxes for a standard DB9 serial port. Then a yellow jumper between the transceiver R1OUT pin and the Raspberry Pi GPIO RXD pin.
I soldered a blue jumper wire between the transceiver T1IN pin and Raspberry Pi GPIO TXD pin. Then I angled the pins on the IC properly and inserted it into the socket. I started by soldering a 16 pin dip socket onto the hat. I'm going to use this to make building my circuit onto the Raspberry Pi quick and easy. Instructables sent us a Perma-Proto Pi Hat board along with a Raspberry Pi 2 as part of the Raspberry Pi 2 Build Night earlier this year. So there we have it, a simple circuit that will allow our new-fangled Raspberry Pi to talk to 40 year old telecommunications equipment. Connected to the RS-232 cable are transceiver pins T1OUT, R1IN, and GPIO GND (need that ground reference, buddy). Connected to the Raspberry Pi's GPIO UART are transceiver pins T1IN and R1OUT. While this part is capable of two serial channels, we only require one for this job. In other words, with a little help from its capacitive friends, the transceiver is capable of generating its own +/-15 rails. Where do these voltages come from? The transceiver chip uses a set of external capacitors as part of its charge pump circuits. The Raspberry Pi, however, does not have the ability to source +/-15v. This part gets its power from the Raspberry Pi. The part I chose for the circuit is SP2322E a 'true +3.0v to +5.5v RS-232 Transceiver'. This device only has one job, to translate back and forth between TTL and RS-232. To get around this, we are going to use a special integrated circuit called a transceiver. Vintage equipment (like the beautiful 300 baud acoustic coupler we are going to interface to) use +/-15v RS-232 for their hardware serial communications. Modern devices like a Raspberry Pi generally use a 3.3v TTL UART for hardware serial communications.
Let's dust it off and build our own dumb terminal using an old phone, the coupler, a Raspberry Pi, some wires, a few capacitors, a serial transceiver chip, and some solder! I recently acquired an early 300 baud acoustic coupler (how modems worked before you were allowed to connect modems directly to a phone jack). I may not be old enough to have lived through it, but the tech that fueled this paradigm shift to a new age is still around, collecting dust in surplus stores and buried in boxes in our grandfather's basements. This time period was so magical! The computer industry and telecommunications industry were colliding like two galaxies, and no one had any idea how it would all pan out. Ok I'm not actually that old, but how amazing it must have been to be using telephones to connect up to the ARPANET before it was legal to even connect a modem directly to your phone line (yeah for real, direct modems were illegal)! Back in my day the internet was a dumb terminal connected at 300 baud through an acoustic coupler to a timeshare system, and we liked it!