Port-powered serial converters
Port powered converters take their power from the connected RS-232 device's output signals. As an example, a PC normally has outputs on the Transmit Data (TD), Request to Send (RTS), and Data Terminal Ready (DTR) pins.
By measuring the voltage across the input lines of an RS-422/485 port powered converter, you can determine if the port is supplying power to the device. There are bias resistors on the RS-422 and RS-485 differential inputs. These bias resistors tie one side of the differential input high and tie one side to ground. On a port powered converter measuring the voltage across the Receive Data A and B lines (B is positive) will indicate if the device is getting power from the port. The typical measurement of slightly more than 4 V dc will indicate the device is being powered from the RS-232 port.
Why won't my converter work with my new computer? Some new computers use low power RS-232 Com Ports. The type of RS-232 driver components used in the computer will determine the amount of power available for port powering. There are three basic categories of RS-232 driver components that determine the amount of power available. The drivers that require a (+) and (-) power supply to function, the components that use an internal charge pump, and those that use low power components often found in laptops or notebook computers. If your computer uses low power Com Ports it will be unable to port power a converter.
How far can I communicate with port power? The typical distance for RS-422 and RS-485 communication is 4000 feet. If the RS-232 port supplies sufficient power to port power the converter, this distance can be obtained. Other factors that absorb power from the RS-232 port can reduce the power available to port power much more than the RS-422/485 distance. If quality 24 AWG twisted pair cable is used on an unterminated connection the effect of distance is minimal.
Are there port powered converters that provide an option for external power? Yes, most of our port-powered converters come with an option to connect an external power supply.