You often need an RS-232-to-TTL adapter for debugging or testing UARTs using a computer. But most of these adapters require an external power-supply adapter to power up the RS-232 transceiver. This external adapter increases the number of cables on your desk and uses no flow-control signals. This Design Idea describes how you can use these signals as power sources. It uses the RTS (request-to-send) and DTR (data-terminal-ready) signals, which provide a positive voltage when you open the PC’s COM port (Figure 1). The voltage on those pins can differ from one computer to another but is generally higher than 6V, which is sufficient to power the adapter.

A standard RS-232 MAX3232 line driver from Maxim performs the TTL-to-RS-232 conversion. The MAX3232 accepts a 5 or 3.3V supply voltage, which is switch-selectable using S₁. D₁ and D₂ block the negative voltage that occurs when the COM port is closed. Q₁, R₃, S₁, and zener diodes D₃ and D₄ form a simple voltage regulator. LED₁ signals that the COM port is open. R₁, R₅, and R₆ protect the circuit under test and the line driver. The use of a pullup resistor for R₂ avoids the need for an open input. This circuit has successfully undergone testing with a laptop computer, which provides a 6V power supply. The circuit works well at speeds as high as 115,200 bps.