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System Design with Port Powered Converters 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 Powered-Converters/' >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 Dissipation
Power is consumed by the following:
a)

The RS-232 side cable
b)

Termination resistors on the RS-422 or RS-485 transmission line
c)

The RS-422 or RS-485 cable
d)

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.
Figure 1. Power vs Baud Rate
D a ta R a te , Ba ud
0.00
20.00
40.00
60.00
80.00
100.00
0
10000
20000
30000
40000
50000
60000
4000' cable only
2000' cable only 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.
Termination
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 is necessary.
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.
Test Results
Unterminated RS-422 and RS-485 Converters:
Model
V
in
at 10%
Signal Loss
Power required at
10% Loss
422LPCOR, +V input
4.6 V
64 mW
422LPCOR, -V input
-10.6 V
150 mW
422PP9TB, -V input
-7.6 V
73 mW
485LP9TB, +V input
5.6 V
52 mW
485SD9TB, +V input
5.4 V
44 mW
485SD9TB, -V input
-8.0 V
69 mW
485DRJ, +V input
5.4 V
55 mW
485DRJ, -V input
-7.8 V
76 mW
RS-422 and RS-485 Converters Terminated with 120
resistor:
Model
V
in
at 10%
Signal Loss
Power required at
10% Loss
422LPCOR, +V
5.5 V
203 mW
422LPCOR, -V
-16.6 V
554 mW
422PP9TB, -V
-11.2 V
361 mW
485LP9TB, +V
5.3 V
109 mW
485SD9TB, +V
5.4 V
86 mW 485SD9TB, -V
-8.9 V
139 mW
485DRJ, +V
5.5 V
85 mW
485DRJ, -V
-9.4 V
171 mW
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 ICs 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 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 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.
Driver
Chip
Vcc (V)
Power out
(-), mW
Power out
(+), mW
TI 1488
12.00
30.0200
30
MAX 232
5.0
17.9096
20
MAX 202
4.99
19.2027
20 MAX3244
3.3
9.577
10
MAX3244
5.0
10.2305
11
MAX3245
3.3
9.434
11
MAX3245
5.0
9.8645
10
The Columns:
Vcc: This was the Vcc provided
Power out (-), mW: This is the power that was produced with a positive driver
input and negative output.
Power out (+), mW: This is the power that was produced with a negative driver
input and positive output.
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, dont complete the loop back connections in the
cables you are using
Conclusion
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 th