System Design With Port Powered Converters
RS-232 port-powered devices are often
preferred over those that require an external power supply
for matters of convenience. This is often a good choice, but
it is important to be aware of the inherent limitations that
are faced in using these port-powered devices.
Limitations arise due to two main factors.
The first is the amount of power that is available from the
RS-232 port. The second is the amount of power dissipated
in the RS-422 or RS-485 system.
Power is consumed by the following:
- The RS-232 side cable
- Termination resistors on the RS-422
or RS-485 transmission line
- The RS-422 or RS-485 cable
- The converter itself
For the remainder of this article, it
will be assumed that the RS-232 side cable is short, 6 ft
(1.8 m), so that it will dissipate an insignificant amount
of power. This is a reasonable assumption for most applications.
The effect of the RS-422 or RS-485 cable
on the power dissipated varies according to several factors.
These are the quality of the cable, the length of the cable,
and the data rate. In the examples that follow, the cable
that was used was a 1000-ft long cable that is recommended
for RS-422 and RS-485 applications. The data rate used was
19 kbps. If an application calls for a longer cable or a higher
baud rate, more power will be dissipated in the cable. Below
is a figure that shows how this power dissipation varies with
respect to baud rate.
The remainder of this article provides
examples and anecdotal evidence to illustrate the effect of
the remaining two power-dissipation factors; namely, termination
resistors and the power consumed by the converter itself.
These examples will then be used to form a set of guidelines
to help determine whether a particular port-powered converter
will be suitable for an application.
The Basic Setup
Each test was performed in a 3-node setup, with the third
node being connected through the previously mentioned 1000-ft
cable. It should be noted that the number of nodes, until
one approaches the maximum of 32 nodes, does not have a significant
affect on the power requirement of the system. A much more
significant factor is the length of cable used. If a converter
is determined to be appropriate in this setup, it should be
appropriate for a setup that requires a similar or lower cable
length, even if a few more nodes are required. The data rate
used was 19 kbps. Each converter was tested with and without
the use of a 120
terminating resistor. A more detailed description of the setup
is included in the appendix.
Handshake Lines: High or Low
Because it is not always possible for the user to raise the
handshake lines, the tests were performed with both positive
and negative voltage supplies whenever this was appropriate.
The converters tested all required less input power when a
positive voltage supply was used. If possible, it is preferable
that the handshake lines be raised.
The reason for this is the way in which
port-powered converters derive their power. The 5 V Vcc is
obtained from the handshake lines. If these lines are asserted
(high), then this voltage is obtained directly. However, if
the handshake lines are low, a charge pump must be used to
invert the voltage and provide a positive Vcc. The method
that is used to do this is not 100 % efficient and it is therefore
preferable to raise the handshake lines if possible.
Another issue that one needs to consider when deciding whether
to use a port-powered converter is the necessity of terminating
resistors. The purpose of these resistors is to greatly reduce
interfering signal reflections by terminating the cable with
an impedance that matches that of the line. This becomes important
in situations where the cable is very long, because the longer
the cable, the longer it will take the reflections to travel
back and forth and decrease significantly in amplitude. Unfortunately,
terminating resistors significantly increase the load on the
converter, drawing more current and therefore requiring more
power. In most cases, the power required to power a converter
with termination was found to be greater than the power available
from an RS-232 port. Termination of a port-powered converter
is only possible in a few cases involving the most efficient
converters and the highest voltage RS-232 drivers. Generally,
termination is not recommended with port-powered converters.
An externally powered converter would be best if termination
Converter Power Consumption
So far, the effect that external factors relating to the setup
have on power consumption has been considered. Another very
important factor that must be taken into account is the power
consumption by the converter itself. This consumption varies
greatly between types of converters. Because of this, testing
was performed on many different converters, selected to be
representative of a large number of the converters offered
by B&B Electronics. The results of these tests were documented
in tabular form and relevant observations were documented
during testing. These results follow.
RS-422 and RS-485 Converters:
||Vin at 10% Signal
||Power required at
|422LPCOR, +V input
|422LPCOR, -V input
|422PP9TB, -V input
|485LP9TB, +V input
|485SD9TB, +V input
|485SD9TB, -V input
|485DRJ, +V input
|485DRJ, -V input
and RS-485 Converters Terminated with 120
||Vin at 10% Signal
||Power required at
The initial supply voltage that was applied
to each converter was chosen as the voltage that would bring
Vcc as close as possible to 5 V. The power requirement listed
is for the point at which the output signal is 10% lower than
it was with the initial supply voltage. 10% loss is a very
strict failure criterion and the converter should be able
to operate reliably below this point.
Power Available from Serial Port
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 IC's in use in serial
ports can be divided into four general categories.
- Drivers which require plus (+) and
minus (-) voltage power supply such as the 1488 series of
interface integrated circuits. (Most desktop and tower PC's
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 power drivers which require one
+3.3 volt power supply. This type of driver also has an
internal charge pump for voltage conversion.
Ports using the lower voltage RS-232
drivers are not able to provide as much power as the full
voltage drivers. Several of these drivers were tested and
the results recorded in tabular form. Drivers were chosen
to represent the range of required supply voltages that are
available today. A PC serial port typically has three drivers,
TD, RTS, and DTR, so the power provided would be three times
that listed in the power output column.
||Power out (-), mW
||Power out (+), mW
Vcc: This was the Vcc provided
Power out (-), mW: This is the power
that was produced with a positive driver input and negative
Power out (+), mW: This is the power
that was produced with a negative driver input and positive
Loop Back Connections
Many port powered converters will have loop back connections,
such as RTS connected to CTS. These loop back connections
each dissipates 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
Port-powered converters are a very convenient choice for many
applications. It is important to keep in mind though that
there are cases in which the RS-232 port is incapable of providing
enough power and an external source must be used. In this
case, one option is buying a converter that provides the option
to either port-power or use an external power source. B&B
also offers battery-powered converters, USB converters, and
PCMCIA/PC/ISA cards, and of which may be the best choice.
Several factors should be considered when deciding to use
a port-powered converter. It is necessary to know how much
power the serial port to be used is capable of providing.
If the type of driver is known, the power output can be found
from the previous table, keeping in mind that there are three
drivers present in the serial port. If the driver type is
not known, then the power output can be determined through
a simple experiment such as the one described in the appendix.
The next consideration is the setup in
which the converter will be used. The two most important factors
regarding setup are the length of cable required and the necessity
of termination. The tests that were performed for this article
all used 1000 feet of cable. Extending the length of the cable
has a substantial effect on the amount of power consumed by
the system. Regarding termination, port powering is not a
viable option if it is necessary to use terminating resistors.
A much less important aspect of the setup is the number of
nodes that will be used. With a given cable length, additional
nodes will have little effect on power consumption until the
number used is nearing the maximum of 32 nodes.
The final major consideration is the
specific power requirements of the converter to be used. Tests
were performed on many converters that B&B offers both with
and without terminating resistors. These tests used a failure
criterion that was extremely strict. Because of that, the
required power that was determined is a very conservative
estimate and the converters should be able to function with
slightly less power. A table is provided which lists the power
requirements of many of B&B Electronics' RS-485 and RS-422
converters. These results can be generalized to cover most
of the converters that are currently available from B&B. It
is a good idea to check this table in order to determine whether
or not a particular converter can be powered with the serial
port that is available.
Insufficient power is the cause of many
of the problems that can occur with port-powered converters.
Following these guidelines will help to determine whether
port powering is a possibility for a particular application.
Setup for Converter Power Requirement Tests:
Each converter was connected to three other converters as
loads, the third being connected through a 1000-ft cable.
For the RS-422 converters the input waveform
was a 9 V p-p, 10 kHz square wave with a 50 % duty cycle and
centered at 0 V. This frequency corresponds to a baud rate
of 19 kbps. The input was applied to the RS-232 TD input of
the second converter and looped back to the converter being
tested. This was to avoid the converter being tested obtaining
extra power from the signal itself. On the RS-232 side of
the converter tested, the RD and TD pins were jumpered together
for a worst-case load where the signal was passed in both
For the RS-485 converters, the input
waveform differed in that it did not go below 0 V and the
peak to peak voltage was reduced to 3.12 V. This waveform
was applied directly to the RS-232 TD pin of the converter
being tested. All converters were set to be in two wire (half-duplex)
For both the RS-485 and RS-422: most
converters were tested with both a positive and a negative
voltage supply. Those that weren't are made to be used with
only one voltage supply polarity. The polarity of the voltage
supply used corresponds to the state of the handshake lines,
when port powering, raised handshake lines are the equivalent
of a positive voltage supply in these tests.
Setup for Driver Power Output Tests:
Each time a driver was tested, a DC power supply was used
to supply the Vcc stated in the table. One of the Tin
pins was connected alternately to Vcc and then ground in order
to test operation on the negative and positive output sides
respectively. The corresponding Tout pin was connected
through a 3 k resistor
to ground. The output power was calculated using the measured
voltage across this load resistor.
For the MAX3244 and MAX3245: the FORCEON
and FORCEOFF' pins were connected to Vcc at all times to force
the driver to remain in operation.
Note on using termination:
The 485SD9TB and 485DRJ converters have internal bias resistors
which have values that were determined assuming that termination
would not be used. These resistor values did not affect the
power requirements of these converters and therefore were
not changed for testing. However, the resistors should be
replaced when operating with termination.
Note on External Capacitors Used:
Many of the RS-232 drivers require external capacitors for
use. When this is the case, the data sheet for the driver
includes a listing of minimum capacitance values. When drivers
were tested with more than one set of capacitor values, always
at or above the minimum, it was found that the power output
was not noticeably affected. Therefore, the results listed
are those that were found when using the minimum capacitance
values, or the closest values that were readily available.