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
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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