I don't know much about this. I was hoping for an actually computer engineer to chime in, but I do know a little so I'll post about that.
The diagram is for a "level converter" to convert the "TTL level" signals that the teensy outputs on pins 6&7 (which are transmit and receive). If your serial device is fine with TTL level, just hook it up to pins 6&7, power and ground, call it a day. If you need RS232 compatability, then the use of the diagram is needed to wire up an additional level converter chip.
Now, looking at the
MAX232 Datasheet as well as the
wikipedia page on RS232 and especially
the pinout you can start to see what's going on.
So they are using pins 2, 3, and 5 which correspond to send, receive, and ground. Here is the full pinout table for the normal DB9 / RS232 connector / standard: Note no pin for VCC, power or +12V. I'm pretty sure this is why there is a need for a level converter.
Name | Abbr. | Pin |
| | |
Transmitted Data | TxD | 3 |
Received Data | RxD | 2 |
Data Terminal Ready | DTR | 4 |
Data Carrier Detect | DCD | 1 |
Data Set Ready | DSR | 6 |
Ring Indicator | RI | 9 |
Request To Send | RTS | 7 |
Clear To Send | CTS | 8 |
Signal Ground | G | 5 |
Finally, I found the following:
Limitations of the standard:
No method is specified for sending power to a device. While a small amount of current can be extracted from the DTR and RTS lines, this is only suitable for low power devices such as mice.
So the device would often need it's own power supply. This is the expectation.
---
To answer the question of it being incomprehensible:
The pins on the teensy output serial data directly. Unfortunately, the voltage isn't enough, so the voltage driver chip will convert it to a higher voltage required for serial communication. All the capacitors and jumpers and things are just what the chip needs to work properly.
The standard specifies a maximum open-circuit voltage of 25 volts: signal levels of ±5 V, ±10 V, ±12 V, and ±15 V are all commonly seen depending on the voltages available to the line driver circuit. Some RS-232 driver chips have inbuilt circuitry to produce the required voltages from a 3 or 5 volt supply.
'
I'd probably recommend opening the case and seeing what actual wires are connected to the serial plug. I'd guess you'd probably only have pins 2(data) pin 5(gnd) and the remain pins would be what was used to power it.
It's not great, but it's all I have:
I see 5 pins connected. Full album:
https://www.flickr.com/photos/dork_vader/with/14700843108/