USB to RS232 Cable

Antoine Authier

This project lets you conveniently connect any computer with USB ports directly to a simple, traditional connector — the 9-pin RS-232 (anyone remembers it?). It converts the electrical signals from a USB<–>serial TLL convertor to the RS-232 standard. So in a nutshell, it converts a USB port into a standard but basic serial port: only the four basic signals are available. The USB<–>serial convertor chosen here is the TTL-232R USB to TTL UART cable from FTDI, available as part number 080213-71 and described in the June 2008 issue (see the Elektor website). The TTL logic signals available on the cable output are converted into RS-232 signals on the tiny board described here. The voltage level adaptor is a MAX3232 from Maxim. This industry-standard part comprises two transmitters and two receivers, ideal for our USB<–>serial convertor, which itself offers the four fundamental signals of an RS-232 standard port, namely TXD (Transmit Data), RTS (Request To Send), RXD (Receive Data) and DTR (Data Terminal Ready). Charge pumps built into the IC provide the 12 V levels required by the RS-232 standard.

This circuit works equally well from 3.3 V as from 5 V supply rails and supports both these levels on its logic input and outputs. In theory, it also ought to work correctly with the 3.3 V version of the cable mentioned above, the TTL-232R-3V3 – however we haven’t checked this experimentally in the lab. Instead of the complete cable, you can use just the TTL-232RPCB module (or TTL-232R-PCB- 3V3 for 3.3 V), currently only the former is available from the Elektor Shop. The 1206 cases size SMD (surface mount device) components used here make it possible to achieve a compact board, while still being easy enough to handle by constructors who may not be very used to this type of component, and who, in building this useful little project, will be able to practice and get an idea of how nimble-fingered they are before attacking more complex circuits using SMDs. No surprises in the construction of the project. Start by soldering the IC and the capacitors, then the connectors. Use a right-angle 0.1-inch SIL pin header to reduce the pull on the cable. With a straight header, the cable and board would form a cumbersome and inelegant right angle. The sub-D connector may be cannibalized from an old cable, as long as it’s a male one (i.e. a plug, not a socket). Slide the board between the two rows of pins on the connector and solder these directly onto the PCB copper islands provided. To finish off, you can protect the whole thing by slipping it into a piece of heat shrink  sleeving of suitable diameter.