Serial to USB converter
using Nokia CA-42 cable

I bought some Nokia CA-42 cables from the usual stores for mobile phone components. We found there are two different types of such cables. One have Nokia logo on it, so we presume is an OEM cable made for Nokia. It has also the inscription for origin: Made in Thailand. (figures 1, 2 and 3). The cable includes in the USB connector a system with one ATMEL AT76C712 microcontroller and all the required peripherals. Figure 4 shows the signals on serial connector. This cable has 6 wires. The hardware id is VID 0421, PID 0802 (0421 means Nokia Mobile Phones).

The cable is recognized by computer only if we provide from outside a voltage around 3.3 V on "power in" pin (see figure 4). In normal use (connected to a mobile phone), this voltage comes from the phone battery. For this cable, the driver needed is "Nokia connectivity cable driver". It must be installed on computer before plugging the cable on USB.

Figure 1
Figure 2
Figure 3
Figure 4

The second type is presented in figures 5, 6 and 7. The circuit is equipped with an IC labeled PL-2303HX. Prolific CheckChipVersion software identifies it as PL-2303XHA (rev A). The cable has only 3 wires: TXD, RXD and GND (figures 8 and 9). The hardware id of this device is VID 067B, PID 2303 (067B means Prolific Technology Inc.).

Prolific website says the drivers that you can download from their site no longer support PL-2303 rev A IC's (Yellow Mark Error Code 10 in Device Manager under WinXP, Vista, and 7). There was a workaround for that. We used the old PL-2303 Vista and XP drivers. Both are working well in Windows 7. For finding these drivers, use the internet search engines.

Figure 5
Figure 6
Figure 7

All these CA-42 cables are based on PL-2303HX (except, of course, the first model presented in our article). It provides only RS232-3.3V (LVTTL). If you have a board that contains PL-2303H, then your convertor can support both RS232-3.3V (LVTTL) and RS232-5V (TTL). Depending on the schematic design of the device, one variant is selected. There are also some boards with PL-2303H that have inside a switch to can choose between the two voltage levels (3.3-5V) for serial interface.

Figure 8
Figure 9
Figure 10
USB to RS232 (PDF)

Converters with PL-2303HX having a RS232-3V serial interface can be connected directly to systems that have the same serial type (e.g. home routers, Raspberry Pi boards, LCD Displays and others). The things change when we are dealing with systems that have RS232 interface. This RS232 serial communication can reach voltages in the range from -15V to +15V. We need an additional transceiver (called "voltage level shifter" in some articles) from 3 V serial to RS232. The complete schematic of the converter from USB to RS232 (using CA-42 cable) I made is on figure 10. The circuit for transceiver is the typical application recommended in MAX3232 datasheet. It is powered with 3.3 V from USB using a LM2936Z-3.3 voltage regulator. RS232 serial in this converter does not includes all signals. It uses only RXD and TXD, enough to make the communication.

The power supply for converter must be taken from USB port (or from an independent source). If the target device is used for this, problems will appear at cold restart. There is a delay on USB port startup caused by the computer operating system. The boot sequence of the device, which usually is faster, will not be monitored if the converter has the startup in the same time.

In modern personal computers the serial ports are not included anymore. However, RS232 interface is still used in applications such as industrial automation systems and also in some consumer products. Network equipment usually use serial console for configuration. A serial to USB converter can be very useful.

Warning: Usually, in home routers, the serial interface header is internal (serial is 3.3V type). To use the interface, you may need to open the router case. That will void the warranty for the device.

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