Tips for Using Port Powered Converters
When using or installing any RS-232 port powered device, it is important to keep a few things in mind. The two main items 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:
a) The cable on
b) The termination resistors used on RS-422 or RS-485 transmission line
c) The RS-422 or RS-485 cable
d) The port powered converter
In this discussion, it is assumed that the RS-232 cable is short, 1.8 meters (6 ft
Figure 1 shows the power dissipated by 600 meters (2000 ft) and 1200 meters (4000 ft) of the 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. In this example, the transmission line used is a high-quality polyethylene cable recommended for RS-422 and 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
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 that are 1200 meters (4000 ft) or shorter and baud rates at 19.2k baud or lower, termination is not required.
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 that 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 (3.3V) and low power drivers that 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.
WHAT TYPE OF DRIVER DO I HAVE ON MY PORT?
To determine the type of driver used on a serial port without looking at the ICs, put a 3K Ohm load on the driver output line to signal ground and measure the voltage. If possible, measure the voltage under both (+) and (-) voltage conditions. These voltage measurements should give values as shown below.
Figure 2. PC Serial Port Used with Port-Powered Converter
LOOP BACK CONNECTIONS
Many port-powered converters have
SUMMARY OF TIPS
- Connect all port driver lines to the converter.
- Keep all unused port driver lines in positive voltage state.
- Determine if the port can drive the converter (what type of driver).
- Don’t terminate the transmission line, if not necessary.
- Don’t make
loop backconnections, if not necessary.