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 example is a high quality polyethylene cable recommended for RS-422 & RS-485 applications. The port powered converter is operating at the highest power consumption when it is transmitting data to the RS-422 or RS-485 line. All available power that is not used by the internal converter circuit is used to drive power to the transmission line.
The power dissipated by the termination resistors is significant (75 mW for a very short transmission line). Under certain conditions it is not necessary to terminate the RS-422 or RS-485 transmission line. If the system is operating at low speed or short cable lengths, it does not need to be terminated. Low speed is considered to be signaling rates below 200 Kbits/sec. or when the cable delay (the time required for an electrical signal to transverse the cable) is substantially shorter than the bit width or when the signal rise time is more than four times the one way propagation delay of the cable (i.e. not a transmission line). As a general rule, if the signal rise time is greater than four times the propagation delay of the cable, the cable is no longer considered a transmission line. For most cables, which are 4000 ft. (1200 m) or shorter and baud rates at 19.2k baud or lower, termination is not required.
The RS-232 Port As A Power Supply
An RS-232 port can supply only limited power to another device. The number of output lines, the type of interface driver IC, and the state of the output lines are important considerations. The types of driver ICs used in serial ports can be divided into three general categories:
- Drivers that require plus (+) and minus (-) voltage power supplies such as the 1488 series of interface integrated circuits. (Most desktop and tower PCs use this type of driver.)
- Low power drivers, which require one +5 volt power supply. This type of driver has an internal charge pump for voltage conversion. (Many industrial microprocessor controls use this type of driver.)
- Low voltage (3.3 v) and low power drivers which meet the EIA-562 Standard. (Used on notebooks and laptops.)
Table 1 is a comparison of several types of driver ICs. For driver ICs, their ability to deliver power is greater if the output of the driver is in the positive voltage state.