Back in October 2000 we described a versatile I2C bus interface that plugged into the PC printer port. Nowadays the PC serial port is rarely used for keyboard or mouse connection so it’s sensible to make better use of it. This design provides an I2C bus connection to the PC’s serial port.From a hardware point of view this serial port version of an I2C interface card could hardly be simpler. Looking at the circuit diagram (Figure 1) it can be seen that the MAX232 chip (IC1) is the main component of the circuit. This IC translates the signal voltage levels on the I2C bus to RS232 voltage levels that can used by the serial COM port of a PC. Even Notebooks that are notoriously fussy about serial port signal levels will work happily with this interface.The 9-way RS232 connector K1 that connects to the serial port of the PC has its DCD wire (pin 1) connected to the DSR wire (pin 6). The port driver software will check this connection during initialisation to determine if the interface card is present.