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.
Power Dissipation
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 which 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.
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.
| 1488 Driver |
+/- 9 volts |
< Vout |
<+/- 11 volts |
| Charge Pump Driver |
+/- 7.5 volts |
< Vout |
<+/- 8 volts |
| EIA-562 Driver |
+/- 3.7 volts |
< Vout |
<+/- 5 volts |
Loop Back Connections
Many port powered converters will have loop
back connections, such as RTS connected to CTS. These loop
back connections each dissipate about 8.3 mW per input line.
The only reason to make these connections is that some software
requires these connections. If you have control over the software,
change the software so that the loop back conditions are not
required. Also, don't complete the loop back connections in
the cables you are using. (See Figure 2.)
Figure 2. PC Serial Port Used With Port Powered Converter
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 back connections,
if not necessary.
For more information on serial ports, we recommend Serial
Port Complete and C
Programmer's Guide to Serial Communication.
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