Components List:

Name	Q	Sample Picture	Cost	Comments
Phone Connector	1		$5	Can be hard to get locally. You can order it online from: -PC-Mobile (Hong Kong/UK) -Actioncode-GSM (Yugoslavia) -Handystecker (Germany) -GSM-Technology (Poland) -IT GSM (France)
Serial DB9 Connector	1		$3	Fairly easy to get. However, I recommend getting a cheap or old serial cable, cutting the other end and using the existing wires.
MAX3232 Integrated Circuit	1		$3	This is the core of the cable, so be careful about connecting it correctly. MAX3232 is the low power, wide power supply range version of the MAX232. It works perfectly with 3V power supply, but the 3.46V supplied in this scheme is ok for it as well.
100 nF Ceramic Capacitor	5		15¢	Small capacitors like this one do not usually have a negative leg and positive leg, so the direction doesn't matter. This capacitor is not essential as my cable worked without it, but it can make the cable more stable.

Circuit Diagram:

 

The whole process usually takes 2-6 hours, depending on your skill and equipment. The parts should cost about $11. A ready cable doesn't cost much more, considering the difference in quality & reliability and the time & effort you'll spend. Unless you are skilled enough and have time and patience, buy a ready cable.

This cable supports all baud rates up to 115kbit/sec, with hardware flow control (RTS/CTS). As opposed to when blinding RTS/CTS at both the PC and the phone side, you can now use the phone as modem, even at the default fixed data speed of 115kbit/sec.

You can build the circuit on a small piece of prototyping board (these are also available at resellers). You can also design a board if you're good at making PCBs. You can maybe omitting the board issue completely, and 'assemble' the circuit as a bunch of components with wires, then apply some epoxy resine to have it fixed. If you're really experienced, or just simply want the interface to be small, go for surface mounted components. The MAX 3232 is also available in SO16 packaging. Soldering SM components isn't hard - but if this is your first such work, then you probably shouldn't take this possibility.

Start with connecting wires to MAX3232 pins. Other components should be connected to the wires, not the pins directly. It's better to use colored wires from the beginning as this makes telling them apart easier.  Many people will prefer soldering, but if you miss the skill or equipment, you can try this trick. Get thin insulated wires and a sewing pin. Pass the pin through each wire 5 mm before the end to make a small hole. Now, flip the MAX232 upside down and insert the wires in its pins. Secure them together with a small piece of adhesive tape, and stick them to the base plate.

Pin 1 on MAX3232 -and other integrated circuits in general- is marked by a small hole. After connecting the wires to MAX232, connect the 100 nF capacitors to them. Small capacitors can be soldered to pins directly and placed over the chip, but you need skill to do this. After you finish with the capacitors, connect the direct wires.

Connecting wires to the serial port should be easy. As for the phone, you'll need to solder long wires to the connector and connect the other ends of them to the wires on the circuit. Be careful about the connector. Soldering is preferred there, because the wires can get shorted together wrongly by the slightest move. It's best to close the connecter's wires hole with glue after connecting them.

You can place a switch between data and service modes, which simply shorts 3 to 7 (data), or 3 to 8 (service). For flashing the phone, use service mode. For other purposes, use data mode.

Double-check that everything is connected correctly before trying the cable. When you plug a data cable into the phone -regadless of what's plugged into the other end-, the phone will beep after 5-10 seconds (service cables don't make it beep). This doesn't necessarily mean that the cable works, because shorting pin 3 to pin 7 on the phone connector makes it also beep. But if it doesn't beep at all, the cable doesn't work for sure. The fastest way to test the cable with is using All Phones Data Suite (APDS). Set it to connect at startup, and run it after each try. Mistakes are easy to make, but hard to discover. If your cable doesn't work, check...

-The type of wires you're using
-The direction of capacitors and diodes
-The phone connector's contact with the phone
-The type of MAX232 used
-Shorting of pins on the phone connector

Remember : The fault is always found in the last place you search for it in :)

Insight
The main part (MAX3232) provides standard RS-232 level interfacing between the phone and the PC. This is a true RS-232 line transceiver for low-voltage (3v) systems. Power supply voltage is drawn from the phone (therefore, unlike if it was powered from the PC serial port, the operation is independent from special RS-232 settings). The capacitors are needed for the internal voltage doubler / inverter of the chip.

You'll find that unlike other phones, the Sony Jx needs two pairs of wires to be interfaced -- Rxd / TxD and CTS / RTS. Received data and transmitted data are obvious. The reason for the other pair, "Clear to send" and "Request to send" is: the phone supports high serial speed and hardware data flow control. Without CTS/RTS, the PC can flood the phone with data, especially when using it as a modem. (No other purposes were found where CTS/RTS support would be important, especially not the service functions where it seems to be not needed at all -- still, since Sony intended to provide hardware flow control, it's best to support it to avoid later surprises. Theoretically, flooding the phone could be possible in any data mode operations).

The simpler part (the switch connected to the respective pins of the Sony data connector) is for making the phone recognize the interface as either a service or a data cable. When pin 3 is shorted with pin 7, the phone assumes that there is a data cable connected. Yet, when pin 3 is shorted to pin 8, it is assumed to be a service cable.

Pin-Outs

1- GND: Represents the digital interface and charger return courrent.
2- Charge: Represents the positive contact for the charging functionality.
3- Gen I/O : Connected to a general I/O pin on the baseband digital chip.
4- Power Out : Has a different functionality with different accessories.
5- RX (In) : Part of RS232 Interface.
6- TX (Out) : Part of RS232 Interface.
7- ID : Part of RS232 Interface. Shows the presence of an accessory and is also an analog ID for the accessory. Accessories which provide charger functionality only do not support this pin and their presence is detected with a valid charger voltage.
8- RTS : Part of RS232 Interface.
9- CTS : Part of RS232 Interface.
10- Audio Out : Single ended output delivers 0.8 Vpp into a 1kOhm load.
11- Audio In : Single ended input with a maximum sensitivity of 365 mVrms.
12- Audio GND.

Wires colors inside an original Sony cable

1- Black
2- Red (not connected)
3- Grey
5- Yellow
6- Orange
7- Green
8- Purple
9- Blue