Port Powered Serial Converter FAQ
How do these converters
derive their power?
These converters steal their power from the connected RS-232
device's output signals. As an example, a PC normally has
outputs on the Transmit Data (TD), Request to Send (RTS),
and Data Terminal Ready (DTR) pins.
What are handshake lines?
Handshake lines are communication lines that do not send or
receive data but are normally used to control the flow of
data in and out of the communication device. The lines are
held high or low to identify the state of readiness of the
sending or receiving device to transmit or receive data.
The most common of these handshake lines are Request to Send
(RTS), and Clear to Send (CTS). Other lines used for communicating
readiness are Data Set Ready (DSR), Data Terminal Ready (DTR),
and Carrier Detect (CD).
How do I tell if my converter
is being powered?
By measuring the voltage across the input lines of an RS-422/485
port powered converter, you can determine if the port is supplying
power to the device. There are bias resistors on the RS-422
and RS-485 differential inputs. These bias resistors tie one
side of the differential input high and tie one side to ground.
On a port powered converter measuring the voltage across the
Receive Data A and B lines (B is positive) will indicate if
the device is getting power from the port. The typical measurement
of slightly more than 4 V dc will indicate the device is being
powered from the RS-232 port.
Will my port powered converter
work in a terminated network?
A port powered converter will generally work in a terminated
network. However, termination resistors add heavy DC loading
to a system and may overload port powered converters. The
type of components used in the RS-232 port and the state of
the handshake lines determine the amount of power available
for port power. For more detailed information, please review
the article "System
Design With Port Powered Converters" found in B&B's
technical library.
Why won't my converter work
with my new computer?
Some new computers use low power RS-232 Com Ports. The type
of RS-232 driver components used in the computer will determine
the amount of power available for port powering. There are
three basic categories of RS-232 driver components that determine
the amount of power available. The drivers that require a
(+) and (-) power supply to function, the components that
use an internal charge pump, and those that use low power
components often found in laptops or notebook computers. If
your computer uses low power Com Ports it will be unable to
port power a converter.
How far can I communicate
with port power?
Typical distance for RS-422 and RS-485 communication is 4000
feet. If the RS-232 port supplies sufficient power to port
power the converter, this distance can be obtained. Other
factors that absorb power from the RS-232 port can reduce
the power available to port power much more than the RS-422/485
distance. If quality 24 AWG twisted pair cable is used on
an unterminated connection the effect of distance is minimal.
For more detailed information, please review the article "System
Design With Port Powered Converters" found in B&B's
technical library.
Are there port powered converters
that provide an option for external power?
There are a number of port powered converters that provide
an option for external power. All of our port powered converters
with terminal blocks have the option of external power being
added on the terminal block. Some converters, such as the
485LPCOR,
422LP25N,
and 422LP25R
are port powered and can also have external power added to
pins on the 25 pin connector.
Can I bring in external
power on one of the handshake lines?
External power can be brought in on handshake lines to power
the port powered converters where there are no handshake lines
available. The power must be current limited to approx. 35
mA by adding a resistor in series with the power connection.
For more information on serial ports, we recommend Serial
Port Complete and C
Programmer's Guide to Serial Communication.
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