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5. Communications Operation
5.1 General
The PM295 communications port is optically isolated and supports EIA RS-232 and
RS-422/485 standard interfaces (user selectable), allowing connection to a
computer, PLC or a printer. It can operate at baud rates up to 38,400 baud.
The communications port can operate in either Computer Mode, where a host
computer polls the instrument in order to receive the data, or to read or program the
Powermeter configuration parameters, or in Print Mode, where values are output
directly in printable format.
In Computer Mode, the instrument uses a two -way communications protocol . The
communications works on a master - slave basis where the instrument is the slave, i.e.
the PM295 responds to host computer requests, but does not transmit information on
its own initiative. The PM295 supports two communications protocols: the ASCII
and Modicon's Modbus RTU protocols. Both protocols are open protocols that can
be used by a third-party host-based software to access all data and configuration
registers of the Powermeter. Information on the serial communications protocols is
found in the documents shipped on diskette with your PM295: "System 295
Powermeter and Harmonic Analyzer - Communications - ASCII Communications
Protocol User's Guide" and "System 295 Powermeter and Harmonic Analyzer -
Communications - Modbus Communications Protocol User's Guide".
With the ASCII protocol, the PM295 is capable of connection to a master computer
via a modem. The instrument has special communications mode allowing a modem
to be set to auto answer mode at power-up of the instrument.
In Print Mode, the PM295 can be connected to a serial printer to output a fixed
format printed report at user -defined intervals.
5.2 Eia Interface Standards
5.2.1 EIA RS-232 Standard
RS-232 is a serial interface standard that may be used for distances up to 15 meters.
It may be possible to extend this range using lower transmission rates, shielded
cabling, or repeaters. One computer serial port can be connected to one instrument.
This standard is generally used when connecting one instrument to a printer. Print
mode is available only with EIA RS-232.
Communications Operation
101
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