Specifications

Chapter 5 - Software RS-422 And RS-485 Applications Ebook
Page 71 of 137 Manual Documentation Number: <DocNumber>
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Systems with Port Powered Converters
RS-232 to RS-422 or RS-485 converters that derive their power from
the RS-232 port are becoming more common in data systems. Power is
supplied to the converter via asserted hardware handshake lines in the
RS-232 connection. For this to work the software must assert the
required line(s) when it starts and maintain them in this state. Because
RTS (and sometimes DTR) lines are used for RS-485 driver control in
some systems, other handshake lines must be used for port powering
in those situations. Converter technical documents typically provide
information on RTS and Send Data control configuration, as well as port
powering requirements.
Figure 54: Handshake Connections to a Port Powered Converter
A good programming practice is to set unused handshake outputs to a
high voltage state in systems using any type of RS-232 to RS-422 or
RS-485 converter. This will assure the best possible operating
conditions for all converters used.