Interface-powered converters

Interface-powered converters plug directly into RS-232 devices, eliminating the need for external power supplies. These converters draw their power from the connected RS-232 device’s signals, normally the Transmit Data (TD), Request to Send (RTS), and Data Terminal Ready (DTR) pins.

When using an interface-powered converter, there are a few things to keep in mind, such as how much power is available from the RS-232 port—and how much power is dissipated in the system.

• The quality and length of the cable used on both the RS-232 and the RS-422/485 side can affect how power is dissipated. Because lower-quality and higher-gauge cable can cause power loss, a good-quality 24 AWG cable is recommended. Also RS-232 runs longer than 6 feet (1.8 m) and RS-422 runs longer than the 4000-foot (1219.2-m) standard can drain cable. You might even want to consider shorter runs for the RS-422/485 cable side. The tests shown in the chart (right) were all performed with cable lengths of 1000 feet (304.8 m).
• In addition, the data rate can also affect the dissipation of power along the line. The most common data rate for this type of equipment is 19.2 kbps.
• Termination resistors are not recommended with interface-powered converters. They add heavy DC loading to the system and may overload the converters.
• Also, it’s best not to make loopback connections because they dissipate a lot of power.
• Plus, because the RS-232 port can only supply limited power, you must consider the types of interface-driver integrated circuits used and the state of the output lines. The ability to deliver power is greater if the output is in the positive voltage state.

*NOTE: The voltage inverter in the interface-powered converter dissipates power when converting negative voltage.