When using or installing any RS-232 port powered device, it is
important to keep a few things in mind. The two main
considerations are the power available from the RS-232 port and
the power dissipated by the RS-422 or RS-485 system. The power
dissipated when using a port-powered converter is consumed by: -
The cable on the RS-232 side of the converter. - Termination
resistors used on the RS-422 or RS-485 transmission line. - The
RS-422 or RS-485 cable. - The port powered converter. It is
assumed in this discussion that the RS-232 cable is short, 1.8
meters (6 feet), so that the RS-232 cable power is
insignificant. The power dissipated by our typical port powered
converter is 50 mW. Figure 1 shows the power dissipated by 2000
ft. (600 m) and 4000 ft. (1200 m) of transmission line
(unterminated). For the data shown in Figure 1, the RS-422 line
driver was not power limited by an RS-232 port. This table
clearly shows that the line power increases as the data rate
increases. The transmission line used in this exam