RS232 to TTL converter

A self-powered RS232-TTL converter. Also called SF Interface.

The interface for connecting (not only Casio) pocket databases, pocket calculators and cell phones is very suitable for all known microcontrollers.

Moreover, connecting One-Wire devices is simple. There may be a baud rate limit at ca. 38400 Baud.

This is the Eagle version of 2006. There is an Ultimate-PCB version of 2003.


This device fits into a usual plastic 9-pin D-Sub shell. Of course, SMD devices are used heavily. The power supply comes from serial port too and is designed for maximum flexibility. It uses all three output lines in both polarities (6 diodes, looks like a three-phase bridge rectifier). Even when all status lines are inactive, a toggling TxD line will be sufficient as power supply. Moreover, control and status lines are connected for built-in “hardware-handshake” for RTS+CTS, DTR+DSR, or both usual interlock systems.
PCB design
This PCB is designed for professional manufactoring.

The Eagle source can also be Augebrowsed on server.


The device without cable, pretty small

Depending on application (microcontroller, Casio SF-Interface, or One-Wire bus), making a suitable cable is up to the user.

The populated PCB

When using for One-Wire applikation, wire RxD to TxD, and connect your IC or bus.

Connections seen on picture (right):

Solder point (see picture)Signal name (see schematic)Data directionDirection
upperTxDto Computerinput
lowerRxDfrom Computeroutput
Note: The signal names on schematic are weird but will match to the pin names of a microcontroller.