What is a Port Powered Serial Converter

| 5/4/2015 3:16:20 PM

Brian FosterA “port powered” converter is a serial conversion device used to convert RS-232 into RS-422 or RS-485. They are often used by field service technicians or even in cable harness assemblies.  The advantage of these devices is their compact size and the ability to take power directly from the RS-232 port. 

Power is available on the RS-232 port but it is usually not obvious. There are some proprietary schemes that provide regulated DC Power via the control signal pins. Our PCI Express Serial Card DSLP-PCIE-100 is one such device.  But today we are talking about using the actual data signal to provide power to a serial protocol converter.
Most of the time, we only care about two RS-232 signals,  Receive Data and Transmit Data… pins two and three on the DB 9 connector. But, there are a number of other signals present; these include the flow control signals RTS (Request to Send) and CTS (Clear to Send).  The DC voltage levels that define the state of the signals are between -3 to -25 Volts for logic 1 and +3 to +25 Volts for logic 0. Today, more often than not, it is around +/- 5 VDC. In addition to indicating the logic state of the signal, this voltage can be used to power a converter.  TD can be used to power the converter, but we need to compensate for the changing state between logic 0 and logic 1. Also, we usually need to drive an RS-422 or RS-485 signal a long distance and there may be termination on the circuit which eats power. So TD is not always enough. Luckily we have pins 7 and 8 available along with the other control signals. In our early converters we needed to go in an “assert” CTS or RTS to use them to power the converter. On our newer port powered converters, we don’t need to worry about the logic level. We take care of that using a diode network. This simplified schematic shows how this is done. 

Serial Converter Schematic
Our port powered line of converters offer a quick and easy way to converter RS-232 to RS-422 and RS-485. 

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